Patent Publication Number: US-11023088-B2

Title: Composing the display of a virtualized web browser

Description:
RELATED APPLICATION DATA 
     This application is a continuation in part of, and claims priority to, U.S. Pat. No. 9,201,850, application Ser. No. 13/526,373, issued on Dec. 1, 2015, entitled “Composing the Display of a Virtualized Web Browser,” the contents of which are hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the invention relate to composing the display of a virtualized web browser. 
     BACKGROUND 
     Ensuring the security of computer networks and network connected devices is one of the grand challenges facing us today. The current state of affairs is very problematic, as our cyber-security infrastructure is routinely subverted by cyber criminals, resulting in great economic loss, invasion of privacy, and loss of productivity. 
     Certain web browsers have attempted to provide a level of security by isolating downloaded code. However, present approaches for doing so are fairly primitive in their ability to deal with the full gamut of security issues that arise during the course of a typical user&#39;s web experience. For example, the sandboxing capability of many web browsers do not address safety issues arising from downloaded browser plugins and various types of native executables, and therefore, possesses a vulnerability to a zero day exploit attack that via Adobe Flash or Microsoft Word. 
     Web browser vendors rely upon monetizing the web browsing habits of their users within their own business processes and with their industry partners. This monetization relies, at least in part, on data about users&#39; browsing habits which is stored in the web cookies that are set and later provided to web servers during the course of web sessions. Companies such as Google and Microsoft have a great interest in learning as much as possible about a person&#39;s browsing habits and typically arrange the default privacy settings of web browsers to be advantageous to them, but less than optimal from a security and privacy standpoint. The default privacy settings may cause web browsers to transfer large amounts of sensitive information from users&#39; machines to Internet related businesses, such as Google, Microsoft, Apple, etc., thereby allowing such businesses to better monetize their customer base by offering appropriate products and services and serving targeted ads. These same settings, however, can be leveraged by malicious parties to exploit security vulnerabilities. While all web browsers provide some level of control to the sophisticated user to tune his or her web browser privacy settings, the vast majority of users never change the default settings. 
     According to some current approaches for enhancing the security of a computer, the computer runs multiple independent operating systems using multiple virtual machines (VMs) within the computer. Multiple virtual machines may be created using a hypervisor, such as from VMware of Palo Alto, Calif. or Virtual PC, available from Microsoft Corporation of Redmond, Wash. When client virtualization is used to achieve improved security, different VMs are used to run different types or classes of applications. For example, an operating system in one VM may be dedicated for accessing the corporate network that the user may be part of and running corporate applications (local and web). Another operating system in a second VM might be used by the user to run his or her personal programs and store personal documents. Finally, a different operating system in a third VM may be used for general web browsing on the wider Internet and running native executables that may have been downloaded from the Internet. An example of such a solution is XenClient, which is made by Citrix Systems of Ft Lauderdale, Fla. 
     This use of classical client virtualization suffers from several drawbacks. One drawback is that there is too much management overhead for the end-user. The end-user has the onus of making the decision as to what VM to use for each activity. Any mistake, intentional or accidental, may subvert the integrity of the system. While many safeguards can be added as a layer on top of the core virtualization technology to help prevent the user from making mistakes, this has not yet been demonstrated to work in a practical and robust fashion. 
     Another drawback is that any VM that is used for general web browsing is just as vulnerable to a security problem as any monolithic system running a single VM while accessing web sites on the general Internet. Therefore, it is quite likely that any VM dedicated to web browsing described in the arrangement above will be subverted by malware eventually. Any subsequent activities in that VM, then, will be compromised. 
     For these and other reasons, client virtualization has not been used widely to improve the security of computer systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is a block diagram of system according to one embodiment of the invention; 
         FIG. 2  is a flowchart illustrating the steps of retrieving a web page using a virtualized web browser according to an embodiment of the invention; 
         FIG. 3  is a block diagram illustrating which virtual machines in an illustrative system are responsible for retrieving and rendering portions of a single requested web page according to an embodiment of the invention; 
         FIG. 4  is an illustration of injecting state data into a virtual machine according to an embodiment of the invention; 
         FIG. 5  is an illustration of an application downloading a file to an intermediate location according to an embodiment of the invention; 
         FIG. 6  is an illustration of an application uploading a file to an intermediate location according to an embodiment of the invention; and 
         FIG. 7  is a block diagram that illustrates a computer system upon which an embodiment of the invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Approaches for composing the display of a virtualized web browser are presented herein. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the invention described herein. It will be apparent, however, that the embodiments of the invention described herein may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form or discussed at a high level in order to avoid unnecessarily obscuring teachings of embodiments of the invention. 
     Virtualized Web Browser 
     Embodiments of the invention are directed towards a virtualized web browser. A virtualized web browser refers to a plurality of components that cooperate to provide a secure web browsing experience. A characteristic of a virtualized web browser is that a portion of the components implementing the virtualized web browser resides in the host while another portion of the components implementing the virtualized web browser resides in one or more virtual machines. While the portion of the virtualized web browser that resides in the host is responsible for displaying content, that portion need not retrieve the displayed content from across the network. Instead, the content displayed by a virtualized web browser is typically retrieved by components in one or more virtual machines, although the component(s) of the virtualized web browser residing in the host may retrieve and render content if desired. 
     A virtualized web browser presents the same user experience as a traditional (or “native”) web browser. A person using a virtualized web browser may, in fact, not even realize that they aren&#39;t using a traditional web browser. Therefore, a user of a virtualized web browser need not alter their usage patterns and may use a virtualized web browser in the same way as a traditional web browser. 
     As used herein, the term “web browser” will be used to refer to both traditional web browsers and virtualized web browsers. When specific reference is made to traditional web browsers (i.e., those web browsers which are not virtualized web browsers), the term “traditional web browsers” or “native web browsers” will be used herein. 
     The types of operations that are typically performed on a web browser include, without limitation, browsing to various web sites, entering information and logging into web pages, navigating back and forth between visited web pages, uploading and downloading content through a web page, and saving and using bookmarks. To support such activity, web browsers maintain temporary and persistent state information to ensure such operations are properly performed. 
     The desire to ensure that a virtualized web browser offers the same user experience as a traditional web browser requires that virtualized web browsers manage the temporary and persistent state information necessary to perform expected web browser operations across the underlying micro-virtualization environment in which these operations are performed. Many approaches will be presented herein for synchronizing resources of a virtualized web browser so that the virtualized web browser provides a familiar user experience. Prior to doing so however, a more detailed description of how a virtualized web browser of an embodiment operates will be presented. 
     Anatomy of a Virtualized Web Browser 
     To better appreciate how a virtualized web browser operates, consider  FIG. 1 , which is a block diagram of system  100  according to one embodiment of the invention. System  100  includes a virtualized web browser that has components that are implemented on the host as well as on different virtual machines.  FIG. 1  depicts host  110  and virtual machines  120 A,  120 B, . . .  120 N. Host  110  corresponds to the host operating system installed upon a device. Host  110  may be embodied by a wide variety of operating systems, either presently available or subsequently developed. 
     In an embodiment (not depicted in  FIG. 1 ), the functional components residing in host  110  may instead reside in a persistently maintained virtual machine. 
     Virtual machines  120 A,  120 B, . . .  120 N each represent any type of virtual machine. In an embodiment, virtual machines  120 A,  120 B, . . .  120 N may each correspond to an Untrusted Code Virtual Machine (UCVM) discussed in U.S. Pat. No. 8,972,980. While  FIG. 1  only specifically identifies three virtual machines, namely virtual machines  120 A,  120 B, and  120 N, the graphical notations of  FIG. 1  are meant to convey that embodiments of the invention may employ any number of virtual machines. Moreover, during the normal course of operation, virtual machines may be periodically instantiated or destroyed, and so the particular number of virtual machines within system  100  will always be changing during normal operation. 
     Virtual machines  120 A,  120 B, . . .  120 N may reside on the same physical machine as host  110 , but they need not. In fact, one or more of virtual machines  120 A,  120 B, . . .  120 N may reside on a different physical machine than on which host  110  resides and may communicate with host  110  over a network. For example, host  110  may be implemented on a cell phone, laptop computer, tablet computer, PC, or other device or computer, and virtual machines  120 A,  120 B, . . .  120 N may be implemented on one or more separate physical machines located over a network. To facilitate the retrieval of web content, virtual machines  120 A,  120 B, . . .  120 N may have a network connection. Note that the particular network settings and network configuration of virtual machines  120 A,  120 B, . . .  120 N may be different and specifically tailored to suit the activity assigned thereto. 
     In a virtualized web browser of an embodiment, different virtual machines may be used to perform user initiated actions such as retrieving requested content from the Internet. Each of virtual machines  120 A,  120 B, . . .  120 N is completely isolated from each other and from host  110 . As shall be explained below in greater detail, the determination of which virtual machine a particular action should be performed in may be based on policy, as reasons will be discussed for wanting to perform certain actions together in the same virtual machine as well as reasons for wanting to perform various actions in separate virtual machines. 
     As shown in  FIG. 1 , host  110  comprises web browser  112  while virtual machines  120 A,  120 B, . . .  120 N each comprise web browsers  122 A,  122 B, . . .  122 N respectively. For clarity, web browsers  122 A,  122 B, . . .  122 N shall be termed VM web browsers to signify that the web browsers execute within a virtual machine in system  100 . By the same token, web browser  112  shall be termed host web browser  112  to signify that it executes within host  110 . 
     A virtualized web browser of an embodiment is comprised of host web browser  112 , one or more VM web browsers, such as VM web browsers  122 A,  122 B, and  122 N, host module  114 , and one or more guest modules, such as guest modules  124 A,  124 B, and  124 N. Host module  114  and guest modules  124 A,  124 B, and  124 N are components which assist in synchronizing resources of the virtualized web browser. 
     VM web browsers  122 A,  122 B, and  122 N and other applications that are run on behalf of the virtualized web browser in each virtual machine store and update the state of a user&#39;s web browser session. However, since each VM web browser is running in an isolated virtual machine, the state changes made to the web browser session made by a particular virtual machine are local to that virtual machine. This provides complete isolation between various user actions, which assists in promoting security; however, isolating virtual machines from one another frustrates certain interfaces and behavior of a web browser that the user expects. 
     Therefore, host module  114  and guest modules  124 A,  124 B, and  124 N cooperate to share information about certain changes made to the web browser session and synchronize resources when appropriate. The guest module within each virtual machine tracks the resources that are requested and updated by the VM web browser within that virtual machine. For example, guest module  124 A within virtual machine  120 A tracks the resources that are requested and updated by web browser  122 A. Host module  114  coordinates resource synchronization between host  110  and virtual machines  120 A,  120 B, . . .  120 N. Host module  114  and the guest modules within each virtual machine communicate over secure communication channels that are not accessible to possible insecure code running in virtual machines  120 A,  120 B, . . .  120 N or in host  110 . 
     Host module  114  and guest modules  124 A,  124 B, and  124 N provide an interface to host web browser  112  so that information about the activity in virtual machines  120 A,  120 B, . . .  120 N is shared as needed and reflected in the user experience provided by the virtualized web browser. As a result of the activity of host module  114  and guest modules  124 A,  124 B, . . .  124 N, the user is not responsible for coordinating the activity of the underlying micro-virtualization of the virtualized web browser. 
     To illustrate the role played by host module  114  and guest modules  124 A,  124 B, . . .  124 N, consider that the VM web browser in each virtual machine maintains its own list of history entries reflecting where that particular VM web browser navigated. However, the user expects that any navigations previously performed by him to show up in the history entries of the virtualized web browser regardless of which particular VM web browser was involved in the navigation. Therefore, operations involving a user&#39;s past navigation history should reflect the browsing history of the user regardless of which particular virtual machine was responsible for retrieving the web pages. Host module  114  and guest modules  124 A,  124 B, . . .  124 N help ensure that relevant information about the user&#39;s browsing activity is shared appropriately between host  110  and virtual machines  120 A,  120 B, . . .  120 N so that browsing history behaves as expected. 
     Another illustration of how host module  114  and guest modules  124 A,  124 B, . . .  124 N synchronize resources of a virtualized web browser may be shown by their coordinated support for the use of APIs from popular web sites such as Facebook and Google on third-party web sites. When one visits a third party web site that is integrated with Facebook, the web browser ideally uses secure mechanisms to show content related to your Facebook account on the third party web site. For example, a news web site can show stories that your friends from Facebook “liked” on that news web site. These types of web page features typically use cookies or web protocols such as OAuth. Cookies are resources that are exchanged between the web browser and web sites being visited by the user. If VM web browser  122 A executing in virtual machine  120 A and the web site being browsed by VM web browser  122 A exchange cookies, then the cookies will likely be needed in host  110  (and potentially in other virtual machines as explained below) to ensure that the display of the web page properly integrates with Facebook. Therefore, as shall be explained in more detail below, host module  114  and guest modules  124 A,  124 B, . . .  124 N make sure that each virtual machine has the cookies necessary so that the VM web browser executing therein can properly display and render any web page retrieved by that VM web browser. 
     Operation of a Virtualized Web Browser 
     Embodiments of the invention seamlessly synchronize resources of a virtualized web browser to present a completely native browser experience to the end user. A wide variety of resources may be coordinated between host  110  and virtual machines  120 A,  120 B, . . .  120 N including without limitation: URLs, screen data for all or at least a portion of a web page retrieved by VM web browser, user input (such as keyboard and mouse actions) directed against a web page displayed by a web browser, cookies, navigation history, bookmarks, configuration settings for the web browser, cache files of various types, DOM storage, and files saved and uploaded. Any type of data that indicates a change in state in the user&#39;s web browsing experience (“state data”) may be synchronized as appropriate between host  110  and virtual machines  120 A,  120 B, . . .  120 N according to embodiments. 
     Prior to discussing in detail how these types of resources are synchronized, it will be helpful to understand how a virtualized web browser retrieves and displays a requested web page.  FIG. 2  is a flowchart illustrating the steps of retrieving a web page using a virtualized web browser according to an embodiment of the invention. 
     In step  210 , a user instructs a virtualized web browser to display a web page. As previously explained, the user may not know that the web browser he or she is using is a virtualized web browser, as the virtualized web browser will exhibit the appearance and behavior of a traditional web browser. However, in all actuality, when a user interacts with a virtualized web browser, the user is actually directly interacting with host web browser  112  executing in host  110 . 
     The user actions taken with respect to host web browser  112  are identified by host module  114  in cooperation with host web browser  112 . Embodiments may inform host module  114  of the user actions taken with respect to host web browser  112  using a variety of different techniques. Different types of web browsers and/or host operating system may provide different options for informing host module  114  of user actions directed against host web browser  112 , and embodiments of the invention may employ any appropriate manner to do so. Non-limiting, illustrative mechanisms for enabling communication between host web browser  112  and host module  114  include APIs, callbacks, and web browser plug-ins. To recite one specific example, in an embodiment where host web browser  112  is a version of Microsoft Internet Explorer, then Browser Helper Objects (BHO) may be used to inform module  114  of user actions directed against host web browser  112 . As another example, host web browser  112  may be augmented with a plug-in that causes host module  114  to be informed of any user input submitted to host web browser  112 . 
     Once host module  114  identifies a user action directed against host web browser  112 , host module  114  either performs the request action itself or forwards notice of the requested action to an appropriate virtual machine within system  100  for processing. Host module  114  may consult policy data to determine whether the user action should be processed within host  110  or within a particular virtual machine. 
     To provide a concrete example, assume the user action is a request to display a web page. Further assume that host module  114 , in consultation with policy data, determines that virtual machine  120 A is the appropriate virtual machine responsible for retrieving that web page. 
     In an embodiment, the particular virtual machine selected by host module  114  to process any user input or perform any requested user action will possess characteristics appropriate for the type of activity involved in doing so. In this example, virtual machine  120 A should be configured to possess appropriate characteristics for retrieving the type of web page being requested. If the requested web page is a web page on an internal corporate intranet site, then virtual machine  120 A should have appropriately tailored configuration settings to do so in a manner that addresses any security concerns. Similarly, if the requested web page is a web page associated with the user&#39;s personal bank or other such financial institution, then virtual machine  120 A may have a different set of configuration settings to address the particular type of activity and level of risk involved 
     Virtual machine  120 A need not be instantiated at the time host module  114  determines that virtual machine  120 A should retrieve the web page, as virtual machine  120 A may be instantiated on-the-fly. Host module  110  may instruct a hypervisor to create a particular virtual machine using a template that causes the particular virtual machine to have characteristics chosen for particular task at hand, namely retrieving a particular web page in this example. Any instantiated virtual machine will comprise a guest module and may comprise a VM web browser. Additional description of how virtual machines may be implemented on-the-fly with specifically tailored characteristics is described in the previously mentioned U.S. Pat. No. 8,972,980. 
     To inform virtual machine  120 A to retrieve the requested web page, host module  114  communicates an instruction to guest module  124 A within virtual machine  120 A to retrieve the requested web page. If VM web browser  122 A is not instantiated at the time guest module  124 A receives the instruction to retrieve the requested web page, then guest module  124 A instantiates VM web browser  122 A at that time. 
     In step  220 , guest module  124 A instructs VM web browser  122 A to retrieve the requested web page. In response, VM web browser  122 A retrieves the requested web page and renders the web page. Note that as VM web browser  122 A is executing within virtual machine  120 A, the rendering of the requested web page by VM web browser  122 A will not cause the rendered web page to be visually displayed to the user by VM web browser  122 A, but instead, the retrieved web page is merely interpreted without being shown upon a connected display device. 
     Instead, host web browser  112  is responsible for displaying the retrieved web page on a connected physical display device. To make this so, the visual appearance of the web page rendered by VM web browser  122 A is shown as the web page displayed by host web browser  112 . In other words, the content that was to be shown by VM web browser  122 A is instead shown by host web browser  112 . 
     There are many ways in which the screen of a VM web browser may be displayed to the user by host web browser  112 . One way involves guest module  124 A generating or obtaining a graphical representation (termed “screen data”) of the rendered web page. Screen data is a graphical depiction of at least a portion of the appearance of the web page. Screen data may represent the visual presentation of the entire web page or, as is performed by some embodiments, only a portion of the web page. For example, an uninterpreted web page may be represented as an HTML file, whereas screen data for that web page may be an image file, such as a file in a JPEG, TIF, PNG, or GIF format. Screen data may be embodied using any type of format for defining how to display content, such as a graphical representation. 
     In an embodiment, after guest module  124 A obtains screen data for the requested web page, guest module  124 A sends the screen data for the requested web page to host module  114 . 
     In another embodiment, once screen data is generated for the requested web page, host module  114  retrieves the screen data for the requested web page from virtual machine  120 A. For example, host module  114  may retrieve the contents from a predetermined location in memory at which the screen data is to be stored. 
     In an embodiment, other data besides screen data may be obtained by host module  114  in step  220 . If guest module  124 A retrieves a file or graphic in virtual machine  120 A, then the graphic or file may be made available to host module  114 . After host module  114  possesses screen data for the requested web page, processing proceeds to step  230 . 
     In step  230 , the virtualized web browser processes the data received in step  220  and displays the requested web page. In an embodiment, step  230  is performed by host module  114  instructing host web browser  112  to display the requested web page using the screen data received from guest module  124 A. After doing so, the user may physically view the requested page upon a display device, as the visual representation of the web page may be displayed on a physical display device by host web browser  114 . 
     Note that host web browser  112  may display the requested web page using the screen data with or without any visual indicators that identify that the requested web page, or any portion thereof, was not retrieved within host  110  or by host web browser  112 . If the use of visual indicators is desired, as an example, a specifically-colored border or a window decoration of a special color may be used to denote the host windows or tabs that are being rendered in an untrusted virtual machine. 
     Using Multiple Virtual Machines to Retrieve a Single Requested Web Page 
     The description of  FIG. 2  was presented in terms of a retrieving a requested web page using a single web browser executing on a single virtual machine. However, this need not be the case in all embodiments, as other embodiments may employ multiple virtual machines to retrieve different portions of a single requested web page. In such an embodiment, host web browser  112  would display the requested web page by displaying the assembled portions of screen data received from the two or more virtual machines involved in retrieving the different portions of the web page. 
     To further illustrate such an embodiment, consider  FIG. 3 , which is a block diagram illustrating which virtual machines in system  100  are responsible for retrieving and rendering portions of a single requested web page according to an embodiment of the invention.  FIG. 3  depicts web page  310  that comprises two bounded areas of content  312  and  314 . Bounded areas of content  312  and  314  may be identified using a HTML construct called an “iFrame.” The iFrame HTML tag is supported by all major web browsers and is a way to demarcate different portions of a web page. Within an iFrame, content may originate from a different source than the web page in which it resides. In other words, web page  310 , content within iFrame  312 , and content within iFrame  314  may each originate from a different source on the Internet. 
     When host module  114  receives a request to display a web page, host module  114  may instruct multiple virtual machines to return screen data for certain portions of the requested web page. For example, as depicted in  FIG. 3 , host module  114  may instruct virtual machine  120 A to retrieve screen data for the content web page  310  without any content for iFrame  312  or iFrame  314 . Also in this example, host module  114  may instruct virtual machine  120 B to retrieve screen data for the content associated with iFrame  312  and may instruct virtual machine  120 N to retrieve screen data for the content associated with iFrame  314 . The determination of which virtual machine should retrieve screen data for which portion of a requested web page may be made based on policy, as explained in more detail in a later section. 
     When host module  114  receives screen data from two or more guest modules, host module  114  assembles the graphical images represented by the sets of screen data and instructs host web browser  112  to display the assembled web page. In this way, the web page depicted in  FIG. 3  may be shown to the user without the user being informed that content within iFrame  312  and content within iFrame  314  were retrieved in a different virtual machine than web page  310 . 
     It may be advantageous to retrieve different portions of a web page from different virtual machines for security purposes. Content contained within an iFrame may contain malicious code. It is common for advertisements and the like to be included within an iFrame. Advertisements and the like may be served by a third party service. As a result, it is difficult for the operator of a web site to monitor content embedded within a web page of their web site for malicious code. By retrieving the content associated with an iFrame in a separate virtual machine, any malicious code that attempts to access unauthorized resources will be unable to access other portions of the web page, let alone access to sensitive resources outside of the virtual machine in which it executes. 
     Communication Between the Host Module and the Guest Module During a Browsing Session 
     The steps of  FIG. 2  describe how the display of a single requested web page may be performed using a virtualized web browser. During the course of a normal web browsing session, a user will, naturally, interact with a displayed web page, either by clicking a link on the web page, entering and submitting text via a text box, and other more complex click and type interactions. Since different virtual machines may be responsible for retrieving different web pages, or even different portions of the same web page, any interaction of the user with a particular portion of a web page should be handled by the appropriate virtual machine. To do so, host module  114  and one or more guest modules will communicate with each other to support a user&#39;s interaction with a displayed web page. 
     When a user selects a link on web page, clicks a button, submits character data entered via the keyboard, and the like, host module  114  is notified that a user is submitting user input relative to a particular location on a web page. Host module  114  identifies the portion of a displayed web page to which the user input is directed. For example, host module  114  may identify a particular iFrame associated with the submitted character data or a particular link that was selected. In certain embodiments, the host module  114  also identifies where the position of a mouse cursor or any other input, such as input from a light pen, a finger on a display, and the like, relative to the web page. In this way, host module  114  may identify where the mouse pointer is relative to the web page to facilitate the performance of a mouse hover operation or any other such operation that requires the location of the mouse pointer or input mechanism relative to the web page. 
     Host module  114  identifies, based on policy and results of prior operation of the system, the specific virtual machine responsible for the portion displayed web page to which the user input was directed. Host module  114  then notifies the guest module within the identified virtual machine of the user input. For example, if host module  114  determines that virtual machine  120 B is responsible for a web page, then host module forwards the user input to guest module  124 B for processing. 
     Once guest module  124 B receives the user input, guest module  124 B will instruct VM web browser  122 B to act on the user input by performing the action associated with the user input. For example, if the user input identifies a particular location on a web page over which the user clicked a mouse button, then VM web browser  122 B will navigate the mouse pointer to the same location, perform a mouse click operation, and process the results. Note that VM web browser  122 B will be displaying the same web page as host web browser  114 , and so the user input may be processed against the web page displayed by host web browser  114  similarly as if host web browser  114  was actually processing the user input. Once VM web browser  122 B has processed the user input (such as, for example, obtaining a requested web page from a remote location in response to the user&#39;s mouse click), guest module  124 B either sends screen data for the resulting appearance of the content displayed by VM web browser  122 B to host module  114 , or host module  114  retrieves such screen data directly from virtual machine  120 B, so that host module  114  may instruct host web browser  112  to display the screen data to the user. 
     Guest module  124 B may send a request to host module  124 B for any information about the user&#39;s web browsing session that is pertinent to the processing of the user input. If in this example guest module  124 B requires one or more resources from host module  114  to properly process the user input, then guest module  124 B may contact host module  114  to obtain such information. For example, if, in order to properly display a web page, guest module  124 B determines that a particular cookie is needed, then guest module  124 B may obtain the cookie by issuing a request for the cookie to host module  114 . 
     If the user clicks a link on the displayed web page, guest module  124 B may send a request to host module  124 B to determine if guest module  124 B can process a subsequent browsing action. If host module  114  allows this action, guest module  124 B will instruct VM web browser  122 B to retrieve the web page indicated by the selected link. On the other hand, if host module  114  denies guest module  124 B permission to continue processing the web processing requests, host module  114  will instruct another virtual machine to retrieve the web content identified by the selected link. The decision of the host module  114  on whether to allow guest module  124 B permission to continue processing browsing requests will be made based on policy data. 
     Persistently Storing Data and Injecting State Data 
     Generally, absent special considerations, virtual machines  120 A,  120 B, . . .  120 N are not intended to be persisted ad infinitum. While one or more of virtual machines  120 A,  120 B, . . .  120 N may be persistently stored in certain cases, more commonly such virtual machines will be destroyed or de-allocated when they are no longer required. Thus, the typical use case of virtual machines  120 A,  120 B, . . .  120 N is to instantiate them as needed and de-allocate them when their use is concluded. Consequently, data stored within one of virtual machines  120 A,  120 B, . . .  120 N may not be persisted. 
     As a result, in an embodiment, when a guest module determines that data within the virtual machine should be persistently stored, the guest module sends a request to host module  114  to persistently store the data within host  114 . To persistently store data describing state changes made to the web browsing session (“state data”) at the virtual machine  120 A, guest module  124 A may send a request to persistently store the state data. Upon receipt of the request, host module  114  stores the data persistently on host  110 . Host module  114  may persistently store data at host  110  at state data storage  130 . State data storage  130  may be implemented by any mechanism capable of storing data, such as a database or a file system. 
     In some embodiments, certain state data may be persisted by updating host web browser  112 . For example, if the state data which guest module  124 A is requesting host module  114  to persistently store is a configuration setting change to the virtualized web browser, a change to the user&#39;s bookmarks, or a cookie, then such changes may be stored by updating host web browser  112 . Host web browser  112  will thereafter persist such state changes. 
     Alternately, rather than persistently storing state data at host  110 , data may be persistently stored in a virtual machine that is persisted. In such an embodiment, a guest module may communicate with an entity in a virtual machine to store state data within the virtual machine. This embodiment may be useful if it is desired to separate state data from host  110 . For example, if there is a possibility of a malicious program residing in the host, by persistently storing the state data in a virtual machine, the malicious program will be unable to inspect the state data, thereby rendering the state data private and secure. In such an embodiment, rather than state data storage  130  residing in host  110  as depicted in  FIG. 1 , state data storage  130  will reside in a virtual machine, such as one of virtual machines  120 A,  120 B, . . .  120 N, which is configured to be persisted. 
     The guest module running in each virtual machine has explicit and full visibility into the activity occurring within the virtual machine. The guest module can decide, based on and within the limits of policies contained within policy data, when the guest module should send resources or state data to host  110  to assist in servicing activity performed by the VM web browser. When a VM web browser within a virtual machine decides to update or save new resources, the guest module within that virtual machine may determine, based on the policies within the policy data, to send the information back to host module  114 . Host module  114  may decide to either update or add to the data stored in state data store  130  using the information sent by the guest module. Host module  114  may also determine to send updates to any currently running virtual machines that are performing various other navigations or other browser actions based on policy settings and/or knowledge of what is the activity happening in each of the virtual machines. This resource coordination may also make use of the knowledge of what all the virtual machines are doing with applications and resources other than the virtualized web browser. 
     It will, on occasion, be necessary to inform a virtual machine about activities performed in a web browsing session that occurred outside of that virtual machine. To do so, state data for a web browsing session is “injected” into that virtual machine.  FIG. 4  is an illustration that will be referenced in a discussion of injecting state data into a virtual machine according to an embodiment of the invention. Initially, with reference to  FIG. 4 , assume that VM web browser  122 A is instructed to perform an action by guest module  124 A that causes VM web browser  122 A to change the state of the web browsing session of the user. Guest module  124 A will be cognizant of any change in the web browsing state as guest module  124 A monitors the activity performed by VM web browser  122 A. 
     After guest module  124 A determines that a state change in the web browsing session for the user has occurred in virtual machine  120 A, guest module  124 A consults policy data to determine whether such change should be persisted within system  100 . The policy data that is consulted by guest module  124 A in this regard may be implemented in a variety of ways. The policy data may reside within guest module  124 A or may reside elsewhere, such as a persisted virtual machine dedicated to storing policy data. Policy data refers to data that defines one or more policies that are used to shape the behavior of one or more components of the virtualized web browser. Policy data will be described in greater detail below. 
     Alternately, guest module  124 A may be implemented such that guest module  124 A causes any state data describing a local change in the state of the web browsing session to be automatically sent to host module  114  rather than doing so based on a consultation with policy data. In such an embodiment, host module  114  may be designed to consult policy data to determine how to process any state data received from a guest module in such an embodiment. 
     In an embodiment, if guest module  124 A determines that the change in state in the web browsing session should be persisted, then the guest module  124 A causes state data that describes this change in state to be persistently stored.  FIG. 4  depicts this as action  410  embodied as a request to persistently store the state data sent from guest module  124 A to host module  114 . While, as described above, embodiments may persistently store data using a variety of different mechanisms, the embodiment of  FIG. 4  stores state data persistently at host  110  within state data storage  130 . When host module  114  receives instruction  410  from guest module  124 A, host module  114  stores the state data within state data storage  130  as shown in action  420 . This act of host module  114  storing the state data within state data storage  130  need not be automatic, but instead, may be based upon consultation with policy data. For example, policies may be defined to determine whether state data should be persisted at all, whether the state data should be persisted using state data storage  130  or host web browser  112 , etc. 
     The purpose of virtual machine  120 A providing host  110  state data to persistently store is to ensure that the necessary information is available to notify components of the virtualized web browser of any relevant activity (or changes in state) that occur in other components of the virtualized web browser. Therefore, after persistently storing relevant state data concerning the activities of the web browsing session occurring in each virtual machine, embodiments send out (or “inject”) relevant information to virtual machines as necessary so the VM web browsers executing therein have all the necessary information to provide a seamless user experience. 
     Injecting state data into a virtual machine is illustrated in  FIG. 4  in action  430 . As shown by action  430 , host module  114  is providing guest module  124 B in virtual machine  120 B state data. The state data provided to virtual machine  120 B identifies information about how the user previously interacted with the virtualized web browser that will be used by VM web browser  122 B in retrieving and/or rendering any content. 
     Note that not all state data for a user need be injected into each virtual machine. For security reasons, it is desirable to only expose the minimum amount of state data to a virtual machine necessary for the VM web browser executing therein to retrieve and render a requested web page properly. By “properly,” what is meant is that the requested web page looks and behaves as the user expects based on his or her past history interacting with web site. Said differently, a properly rendered web page takes into account all pertinent state data necessary to resolve the display all content therein. Policies defined by the policy data, therefore, will inject into a virtual machine only the state date which is deemed pertinent to the assigned responsibilities for that virtual machine. 
     To provide a concrete example, assume that virtual machine  120 B is tasked with the responsibility of interacting with social media web sites. Further assume that policies defined within the policy data consider such social media web sites as posing a risk of infection of malicious code. Therefore, it would not be advisable to expose any malicious code that is inadvertently introduced into virtual machine  120 B to any cookies associated with the user&#39;s sensitive information, such as bank account information, passwords, and the like. Consequently, in an embodiment, when host module  114  injects state data into a virtual machine, host module consults policy data, either stored within host  110  or within a virtual machine that is persistently stored, to identify state data that is pertinent to the assigned responsibilities of the virtual machine, so that only pertinent state data is injected therein. 
     Injecting state data into a virtual machine may also be performed to warm up a cache within the virtual machine. For example, if a user previously visited a particular web site, cache files for the web site may be persistently stored. If the user visits that web site again and a new virtual machine is instantiated to handle the navigation, the cache files associated with the particular web site may be injected into the newly instantiated virtual machine. 
     When the state information associated with a virtual machine is to be persisted beyond the lifetime of the virtual machine (as per host module  114 ), it is convenient to persist this information with an identifier related to the activity being executed in the virtual machine. For example, the top-level domain name (TLD) of the URL being rendered inside the virtual machine may be a convenient identifier. The identifier allows this information to be retrieved when it is needed, such as when this information is injected into a virtual machine created to render a URL with the same TLD as of the virtual machine being destroyed. 
     Policy Based Decision Making 
     In embodiments of the invention, the operation of host module  114  and guest modules  124 A,  124 B, . . .  124 N is based, at least in part, upon policies defined in policy data. For example, host module  114  may consult one or more policies defined in policy data to (a) identify which virtual machine should be tasked with the responsibility of servicing a request to retrieve a web page, and/or (b) determine how to instruct a guest module, within the virtual machine designated responsible, to retrieve the web page. Policies may consider a host of factors in determining which virtual machine should service a request to retrieve a web page. For example, the web address, knowledge of any scripts executing on the parent web page, and whether the requested web page is in a new or existing web domain may all play a factor in determining which virtual machines retrieves the requested web page. 
     When host module  114  receives a subsequent request to display another web page, host module  114  determines, based on policy data accessible to host module  114 , whether the same virtual machine that handled the retrieving and rendering of a previous requested web page should retrieve and render the newly requested web page. For example, the policy data may indicate that a newly requested web page has characteristics similar to a previous web page retrieved and rendered by virtual machine  120 A. In such as case, host module  114  may task guest module  124 A with retrieving and rendering the newly requested web page. In this way, the policy data may treat web pages within the same web page similarly so that all the web pages of a particular domain are serviced within the same virtual machine. 
     Alternately, the policy data may indicate that the newly requested web page can be processed within the same virtual machine but using a new web browser. In such a case, host module  114  may task guest module  124 B with retrieving and rendering the newly requested web page using a newly instantiated web browser in virtual machine  120 A other than VM web browser  122 A. 
     Alternately, the policy data may indicate that the newly requested web page should be processed within a different virtual machine than any currently instantiated. In such a case, host module  114  may instantiate a new virtual machine and instruct a guest module, executing within the newly instantiated virtual machine, to retrieve and render the newly requested web page. 
     Deallocating Virtual Machines in a Controlled Manner 
     In an embodiment, when a particular virtual machine is no longer required, that virtual machine may be de-allocated. As a result, when host module  114  receives notification that the user is closing the virtualized web browser, host module  114  may, without human intervention, de-instantiate any virtual machine associated with the web browsing session. Thus, any virtual machine that assisted in providing content to the virtualized web browser will be automatically de-instantiated in an embodiment as a result of the user closing the virtualized web browser. 
     Retrieving Content Securely Based on Policy 
     A virtualized web browser of an embodiment will have the same visually appearance and support the same features as a traditional web browser. Therefore, like traditional web browsers, a virtualized web browser will support the use of tabs in an embodiment. However, a virtualized web browser may retrieve content to be displayed in a tab in a more secure manner than prior approaches. 
     In an embodiment, upon the virtualized web browser being instructed to display a new tab, the virtualized web browser displays the new tab without instantiating a new virtual machine. Thus, if a user were to repeated press “Control-T” or a similar mechanism to launch a new tab, the new tab may be created and displayed by the virtualized web browser without instantiating or assigning any virtual machines to retrieve content for the new tab. This advantageously minimizes memory usage of the virtualized web browser. 
     Once a user instructs the virtualized web browser to display a web page within a tab, host module  114  may consult policy data to determine how the requested web page should be retrieved and rendered. The policy data may specify, for example, that the requested web page should be retrieved and rendered in the host, an existing virtual machine, or in a new virtual machine that has yet to be instantiated. 
     In an embodiment, the policy data may specify that requested web pages from the same domain may be retrieved and rendered in the same location, e.g., within the same virtual machine. In this way, the synchronization of certain resources, such as cache files, is simplified and/or expedited. Thus, if a user requests a web page from a domain in which another web page has already been fetched and rendered within an existing virtual machine, then the newly requested web page may be fetched and rendered within that same virtual machine. On the other hand, if the user requests a web page from a domain from which no web pages have previously been fetched, for example, then a new virtual machine may be instantiated in which to fetch and render that web page. 
     In an embodiment, the policy data may identify a trigger condition which, when satisfied, switches the location from which content, displayed in a tab of a web browser, is retrieved and/or rendered. For example, the policy data may identify a trigger condition which, when satisfied, causes the host, rather than the virtual machine currently responsible for retrieving and rendering web content, to assume responsibility for retrieving and rendering the web content. As another example, the policy data may identify a trigger condition which, when satisfied, causes a virtual machine having characteristics specified by the policy data, rather than the host which is currently responsible for retrieving and rendering web content, to assume responsibility for retrieving and rendering the web content. Thus, in embodiments of the invention, a virtualized web browser may comprise a tab used to display web content which was retrieved and rendered from a variety of configurable locations; further, the location at which web content displayed in a tab of a virtualized web browser of an embodiment may be updated or changed dynamically in real time in response to a trigger condition specified by the policy data being satisfied. State data may be injected into a virtual machine as necessarily, as described below and herein, to ensure retrieving and rendering content from a new location in seamless and transparent to the user. 
     In an embodiment, a virtualized web browser may comprise two or more tabs that display web content retrieved and rendered from different locations. For example, a virtualized web browser may comprise a first tab that operates as a traditional or native web browser (i.e., the web content displayed in the first tab is retrieved and rendered in the host operating system) and a second tab that operates as a virtualized web browser (i.e., the web content displayed in the second tab is retrieved and rendered in a virtualized machine). 
     In an embodiment, as a user instructs a virtualized web browser to navigate to a different web domain, a virtualized web browser of an embodiment may consult the policy data to determine if web content associated with the different web domain should be retrieved and rendered within the host operating system or within a virtual machine (the policy data would also specify the characteristics which that virtual machine should possess). In accordance with the policy data, the virtualized web browser may transparently switch the location at which web content displayed in the tab is retrieved and rendered. 
     For purposes or providing additional security, the policy data may specify that the location at which web content is retrieved and rendered is to be switched from a first machine to a second virtual machine. For example, the second virtual machine may have configuration settings or characteristics deemed more restrictive than the first virtual machine (e.g., the second virtual machine may have access to less computational or hardware resources than the first virtual machine). The policy data can establish a variety of different policies at many levels of granularity. For example, the policy data may indicate that the web content for each web domain should be retrieved and rendered in a separate virtual machine. 
     Certain web domains might require certain privileges (such as the ability to access network resources on a local Intranet) to be retrieved and rendered correctly within a virtual machine. When a user instructs a virtualized web browser to navigate to a domain requiring a higher level privilege than possessed by the virtual machine currently retrieving and rendered the web content displayed in the tab associated with the navigation, the virtualized web browser of an embodiment may transparently start a new virtual machine (possessing a higher access privilege) and switch the responsibility for retrieving and rendering that web content to the newly instantiated virtual machine from the previously responsible virtual machine. Some embodiments of the invention may perform a check to ensure that the user possesses sufficient privileges to navigate to that web domain prior to instantiating the new virtual machine and performing the navigation. Similarly, when a user instructs a virtualized web browser to navigate to a domain needing less privilege than possessed by the virtual machine currently retrieving and rendered the web content displayed in the tab associated with the navigation, the virtualized web browser of an embodiment may transparently start a new virtual machine (possessing a lower access privilege) and switch the responsibility for retrieving and rendering that web content to the newly instantiated virtual machine from the previously responsible virtual machine. 
     Synchronizing History Data 
     Embodiments of the invention seamlessly synchronize resources of a virtualized web browser, and in particular history data, across host  110  and virtual machines  120 A,  120 B, . . .  120 N to present a completely native browser experience to the end user. History data is a type of state data. History data is data that describes a user&#39;s navigation history. History data may include the navigation history of the current session as well as web sites and resources previously visited during prior browsing sessions. By synchronizing history data across the virtualized web browser, the user is able to perform any type of operation involving history data in the same manner as in a traditional web browser even though web pages may be retrieved from a plurality of virtual machines. 
     In an embodiment, history data may include bookmarks for the user. History may also include one or more URLs visited by the user as well as titles of web pages and names of resources used. In order to perform certain operations on a web browser, such as back page and forward page, it is necessary to know the sequence and identity of visited web pages in each tab; therefore, in an embodiment, history data may also identify one or more actions that were performed with respect to a particular tab of the virtualized web browser. 
     Operations involving these types of resources will be performed seamlessly since any history data pertinent to the function of the various virtual machines will be merged correctly by host module  114 . When new history data is created locally within a particular virtual machine, host module  114  is informed of the new state data so that it may be persistently stored and injected into other virtual machines as necessary. For example, if guest module  124 A detects that new history data has been created within virtual machine  120 A, then guest module  124 A may notify host module  114  of the new history data. This may be accomplished in a variety of different ways by embodiments, e.g., guest module  124 A may send host module  114  the new history data via a secure communication channel or guest module  124 A may cause the new history data to be persistently stored and may notify host module  114  that new history data has been stored and may be retrieved as needed. In an embodiment, host module  114  causes any new history data sent from a virtual machine to be persistently stored so that host module  114  may inject the new history data, as needed, into appropriate virtual machines. 
     In an embodiment, we optionally do not update history data to identify all actions taken in a virtual machine. For example, if there is a redirect operation performed in a VM web browser, only the final or the first URL may be shared with the host and other VMs, rather than all URLs involved in the redirect. 
     In an embodiment, history data is used to update the visual appearance of host web browser. For example, back or forward buttons may be displayed as disabled (or “grayed out”) or active depending upon the available of appropriate history data to service such operations. When a user initiates the display of a menu in the host operating system that enables the user to navigate previously visited sites, history data will be used to perform the operation. For example, if the user performs a right click operation with his or her mouse to display a menu offering navigation options, or if the user clicks a back or forward button on the virtualized web browser, such navigation commands will be performed using history data obtained from the virtual machines on the system. 
     Synchronizing Cookies 
     Embodiments of the invention seamlessly synchronize resources of a virtualized web browser, and in particular cookie data, across host  110  and virtual machines  120 A,  120 B, . . .  120 N to present a completely native browser experience to the end user. Cookie data is a type of state data. Cookie data is data that describes a user&#39;s cookies. Cookie data may include cookies dropped in the current session as well as cookies dropping in prior browsing sessions. By synchronizing cookie data across the virtualized web browser, the user is able to perform any type of operation involving cookie data in the same manner as in a traditional web browser even though web pages may be retrieved from a plurality of virtual machines. 
     In an embodiment, when host module  114  instructs a guest module to coordinate with a VM web browser to retrieve and render a requested web page, host module  114  provides the guest module cookie data. The cookie data identifies one or more cookies deemed to be pertinent to the retrieval of the web page. Rather than relying solely using a push model, certain embodiments may also employ a pull model where the guest module may send a request to host module  114  to ascertain whether there are any additional cookies for the user associated with an embedded web page comprised within the web page. In this way, host module  114  may provide to a guest module any cookie data which host module  114  deems pertinent to the retrieval of a requested web page. However, upon retrieving the requested web page, if the guest module determines that possession of one or more cookies would affect how the requested web page is rendered, then the guest module may communicate with host module  114  to determine if such cookies have been established for the user, and if so, then the guest module may obtain them from host module  114 . 
     The type and amount of cookie data sent from host module  114  to a guest module may be configured using policies defined in the policy data. For example, in an embodiment, host module  114 , based on consultation with policy data, may only sent to a guest module cookie data that identifies cookies associated with the top-level domain of the requested web page and does not identify any cookies associated with the domain of any embedded web pages comprised within the requested web page. On the other hand, in other embodiment, policy data may define a different set of policies, and host module  114  in that embodiment may provide, to a guest module, cookie data that identifies cookies associated with the top-level domain of the requested web page and any domain of any embedded web pages comprised within the requested web page. Embodiments may employ policy data that defines a variety of policies for specifying which cookies should be provided to a virtual machine. 
     Embodiments of the invention may store cookie data using a variety of different mechanisms. Cookie data may be stored in any location and in any manner that state data may be stored. Cookie data may be stored at state data storage  130  in an embodiment. In another embodiment, cookie data may be persistently stored by host web browser  112 . In another embodiment, cookie data may be stored in a virtual machine that is persistently stored. 
     A guest module may employ a variety of mechanisms to determine when a VM web browser drops a cookie. For example, the guest module may detect the performance of set cookie operation via an API, a callback, or a plug-in to the VM web browser. 
     In an embodiment, a guest module may redirect processing of any set cookie instruction or get cookie instruction comprised within the web page to host module  114 . In this way, host module  114  may become a central manager of all cookies throughout the virtualized web browser, regardless of which particular virtual machine is retrieving a web page. 
     Managing Display Issues 
     Embodiments of the invention enable a web page retrieved and rendered within a virtual machine by a VM web browser to be displayed upon a physical display device by host web browser  112  executing on host  110 . Host module  114  assists in assembling the rendered content obtained from the host or from any virtual machine responsible in contributing rendered content for a web page displayed by host web browser  114 . 
     Certain types of web content may be deemed secure while other types of web content may be deemed insecure. For example, web sites accessible over a secure connection (vis-à-vis a https connection) or web pages hosted internally on an Intranet may be deemed secure (i.e., low risk of comprising malicious software). Other public web sites available on the Internet may be deemed insecure due to the higher level risk that such sites may be infected with malicious code. 
     Embodiments may treat secure web sites different than insecure web sites. A virtualized web browser of an embodiment may interact with a secure web site using a VM web browser executing with a virtual machine having characteristics specifically tailored to interact with the secure web site. In the same vein, a virtualized web browser of an embodiment may interact with an insecure web site using a VM web browser executing with a virtual machine having characteristics specifically tailored to interact with the insecure web site. 
     A single web page may contain secure portions and insecure portions. For example, a child web page may be embedded within a parent web page using a HTML construct such as an “iFrame.” The parent web page may be deemed secure while the child web page may be deemed insure. In an embodiment, when a requested web page contains both secure and insecure content, a warning may be displayed to the user. The warning may inform the user that the requested web page contains content deemed insecure. The warning may allow the user to consent to assume the risk and continue with processing the web page. If the user wishes to proceed with retrieving the web page, then a new virtual machine may be instantiated for purposes of retrieving the insecure portions of the requested web page. The newly instantiated virtual machine will contain a guest module and a VM web browser. The newly instantiated guest module and VM web browser will cooperate to retrieve and render the insecure portions of the requested web page. Once host module  114  obtains screen data for the insecure and secure portions of the requested web page, host module  114  may assemble the rendered portions of the requested web page and instruct host web browser  112  to display the assembled web page. 
     Host module  114  may obtain screen data for a rendered portion of a web page in a variety of different ways. For example, in an embodiment, host module  114  may obtain screen data by accessing a frame buffer that stores data representing a rendered portion of a web page. Host module  114  may do so for each virtual machine responsible for rendering a portion of a requested web page. After obtaining all the pieces of the requested web page, host module  114  may assemble the pieces and instruct host web browser  112  to display the assembled web page. 
     In another embodiment, host module  114  may obtain screen data using an operating system protocol for displaying the contents of one frame in another frame. In this way, the rendered content displayed by a VM web browser may be identified and displayed by host web browser  112 . 
     Note that it may be desirable in certain embodiments to configure a policy to disallow the display of certain types of content. For example, content originating from certain prohibited web sites or possessing certain undesirable characteristics, such as embedded graphics or text associated with one or more keywords, may be prevented from being displayed. In such an embodiment, policy data may be stored which defines one or more polices that demarcate when certain content should or should not be displayed by host web browser  112 . When host module  114  obtains screen data, host module  114  may consult the policy data to determine whether it is permissible to display the screen data. If it is not permissible to display a set of screen data, then host module  114  instructs host web browser  112  to display any portion of the web page which is allowed, but redacts any disallowed portions of the requested web page by causing such portions to be displayed blank or with a graphic that explains that the portion is not being displayed for violation of a policy. 
     In an embodiment, when the virtualized web browser comprises two or more tabs, each tab may be serviced by a separate VM web browser executing within a separate virtual machine. 
     Managing Downloading and Uploading Files Using a Virtualized Web Browser 
     Embodiments of the invention provide for a virtualized web browser that supports the same user experience as a traditional web browser. However, unlike traditional web browsers, a virtualized web browser retrieves and renders content within virtual machines which need not be persisted. As a result, how a virtualized web browser supports uploading and downloading files is substantially different than how a traditional web browser supports such functions. 
     In an embodiment, when the virtualized web browser receives an instruction to download one or more files external to and not an integrated part of the web page (such as when a file is downloaded from a web site), the virtualized web browser displays an interface which enables at least a portion of a file system, maintained by host  110 , to be browsed while preventing files stored within a virtual machine to be browsed. 
     For example, with reference to  FIG. 1 , assume that host web browser  112  receives an instruction to download one or more files from a web page. Host web browser  112  in turn displays an interface (such as a pop up dialogue box) which enables at least a portion of a file system, maintained by host  110 , to be browsed while preventing files stored within any of virtual machines  120 A,  120 B, . . .  120 N to be browsed. 
     Once host web browser  112  receives input that identifies a target location within the file system maintained by host  110 , host web browser  112  instructs a VM web browser to download the one or more files to the target location. Note that in certain embodiments, not all of the file system maintained by host  110  is exposed to the user. In such an embodiment, host module  114  may consult policy data to determine which portions of the file system maintained by host  110  by visible and exposed to the user. Such an embodiment may be used to protect certain files from being tampered with by the user or to hide certain areas of the file system from the user. 
     In an embodiment, policy data may define a policy that specifies that any files downloaded from outside of a particular region (such as the physical machine itself, the Intranet, one or more specified domains deemed to be trusted, etc.) are deemed untrustworthy. When host module  114  exposes an interface to a virtual machine that facilitates the download of files to a target location, host module  114  may consult policy data to determine how to treat the download files. If, as a result of consulting policy data, host module  114  determines that the downloaded files should be deemed untrustworthy, then host module  114  allows the files to be downloaded to the target location, but additionally stores data that identifies the one or more downloaded files to be untrusted documents. The download files may be subsequently processed differently (such as being opened only in a virtual machine designed to handle untrusted documents) by virtue of being labeled untrusted documents. 
     The policy data consulted by host module  114  may require that host module  114  store a record in a log file to document any files being downloaded. In this way, the virtualized web browser will produce a record of all the files it downloads. This log file may be configured to contain a variety of other supporting metadata, such as the user downloading the file, the day and time of the download, etc. 
     In an embodiment, prior to storing a copy of the downloaded file to the target location, a copy of the downloaded file may be stored in an intermediate location. To help illustrate this principle, the following description will reference  FIG. 5 , which is an illustration of downloading a file to an intermediate location according to an embodiment of the invention.  FIG. 5  will be used to describe how a file may be downloaded using VM web browser  122 A executing in virtual machine  120 A to a particular location in file system  540  on host  110 . 
     In an embodiment, policy data will instruct where a downloaded file will initially reside. Different embodiments may employ different approaches for intaking downloaded files. In an embodiment, guest module  124 A consults policy data to determine where a file downloaded by VM web browser  122 A should be stored. The policy data may indicate that the downloaded file should be stored at temporary location  510 . Temporary location  510  may be designed to only store data downloaded from virtual machine  120 A or may store data from any virtual machine in system  100 . 
     The purpose of storing a downloaded file in temporary location  510  is to allow analysis on the downloaded file prior to moving the downloaded file to its final destination. Analysis that may be performed on a downloaded file at temporary location  510  includes but is not limited to checking for malicious code, such as a virus. The purpose of running such analysis is to determine if the downloaded file should be deemed safe. Temporarily location  510  may, but need not, be located within a virtual machine. 
     In an embodiment, after a downloaded file is deemed safe as a result of analysis performed on the downloaded file at temporary location  510 , the downloaded file may be transferred to shared location  520 . Alternately, a downloaded file may immediately be stored at shared location  520  instead of temporary location  510 . In such an embodiment, the analysis that was discussed above as being performed at temporary location  510  may instead be performed at shared location  520 , or not performed at all. 
     Shared location  520  is a location where downloaded files may be stored and accessed by other components within system  100 . For example, in an embodiment, a downloaded file is never moved or copied to file system  540 . Instead, a downloaded file is copied into shared location  520 . A shadow file that links to the downloaded file in shared location  520  is stored at the target location in file system  540 . A user browsing to the location in file system  540  where he or she wanted to store a copy of the downloaded file would instead see the shadow file. The user may not be able to ascertain that the shadow file is not the downloaded file. The shadow file may be moved or copied into any location. If an operation is to be performed on the shadow file (for example, the user clicks on the shadow file to open up the file), the link between the shadow file and the download file will be traversed and the operation will be performed against the downloaded link within shared location  520  (for example, clicking on the shadow file within file system  540  will cause the downloaded file within shared location  520  to open in a virtual machine). The advantage of this embodiment is that if the downloaded file contains malicious code that is undetected, the malicious code will remain outside of host  110 , thereby minimizing or preventing infection. In another embodiment, if a downloaded file is deemed safe within shared location  520 , then it may be moved to the target location within file system  540 . 
     Shared location  520  may be implemented within a virtual machine. Alternately, downloaded files may be stored directly into a virtual machine, such as virtual machine  530  in  FIG. 5 . As shown in  FIG. 5 , a downloaded file may be stored directly into virtual machine  530  by VM web browser  122 A or the downloaded file may be transferred from either temporary location  510  or shared location  520 . Virtual machine  530  is intended to represent a secure repository for downloaded files. In an embodiment, a new virtual machine  530  may be instantiated for each downloaded file to provide a pristine environment in which analysis may be performed to determine whether the downloaded file contains any malicious code. In an embodiment, if a downloaded file is deemed safe within virtual machine  530 , then it may be moved to the target location within file system  540 . 
     Note that embodiments need not implement all of temporary location  510 , shared location  520 , and virtual machine  530 , as embodiments may implement zero or more of these features. The discussion of these components is meant to illustrate the variety of design choices embodiments of the invention enjoy in implementing a quarantined area for downloaded files to facilitate the performance of analysis upon the downloaded files. 
     Embodiments may move a downloaded file to host  110  using a variety of different approaches. Policy data may specify a particular mechanism to use in moving a downloaded file into file system  540  based on the size of the downloaded file. For example, based on the size of the downloaded file, it may be more efficient to move the downloaded file into file system  540  by way of a network connection, by way of copying to disk, or by way of copying into memory. 
     Embodiments also support uploading files.  FIG. 6  is an illustration of uploading a file according to an embodiment of the invention. In an embodiment, a user may instruct host web browser  112  to upload one or more files. Host web browser  112  will communicate this instruction to host module  114 . When host module  114  receives an instruction to upload one or more files stored locally to an external target location, host module  114  displays an interface which enables at least a portion of a file system, maintained by host  110 , to be browsed while preventing files stored within any virtual machine to be browsed. When host module  114  receives input identifying one or more files in the file system, host module  114  moves the selected files to shared location  610 . 
     Guest module  124 C is instructed by host module  114  to upload the selected files in shared location  610 . Therefore, guest module  124 C communicates with VM web browser  122 C to cause VM web browser  122 C to upload the selected files from shared location  610  to the target location. Advantageously, only the particular files to be uploaded are exposed to virtual machine  120 C, as the remainder of the files in file system  620  remain inaccessible to virtual machine  120 C. If any malicious code were to reside within a virtual machine such as virtual machine  120 C, the malicious code would be prevented from inspecting the files within file system  620  or obtaining any data therein. 
     Policy data consulted by host module  114  may require that host module  114  store a record in a log file to document any files being uploaded. In this way, the virtualized web browser will produce a record of all files uploaded using the virtualized web browser. This log file may be configured to contain a variety of other supporting metadata, such as the user uploading the file, the day and time of the upload, where the file(s) were uploaded, etc. 
     Extensions 
     Note that embodiments of the invention have been chiefly described with reference to a virtualized web browser. However, the principles presented herein are equally application to other types of applications. Indeed, the teachings described above are equally applicable to any type of virtualized application, such as a virtualized email client or virtualized email server. In such an embodiment, rather than retrieving and rendering web pages within virtual machines, each virtual machine may retrieve and render email or email attachments. Host  110  may be implemented on a cell phone, tablet PC, and the like, and virtual machines  120 A,  120 B, . . .  120 N may reside in a data center or other remote location. When a user instructs the cell phone or tablet PC to display an email or an email attachment, the request could be routed to a particular virtual machine, which retrieves the email, renders the email or email attachment to produce screen data, and sends the screen data to the cell phone or tablet PC for display. 
     Hardware Mechanisms 
     In an embodiment, host  110  and one or more of virtual machines  120 A,  120 B, . . .  120 N of  FIG. 1  may be implemented on a computer system.  FIG. 7  is a block diagram that illustrates a computer system  700  upon which an embodiment of the invention may be implemented. In an embodiment, computer system  700  includes processor  704 , main memory  706 , ROM  708 , storage device  710 , and communication interface  718 . Computer system  700  includes at least one processor  704  for processing information. Computer system  700  also includes a main memory  706 , such as a random access memory (RAM) or other dynamic storage device, for storing information and instructions to be executed by processor  704 . Main memory  706  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  704 . Computer system  700  further includes a read only memory (ROM)  708  or other static storage device for storing static information and instructions for processor  704 . A storage device  710 , such as a magnetic disk or optical disk, is provided for storing information and instructions. 
     Computer system  700  may be coupled to a display  712 , such as a cathode ray tube (CRT), a LCD monitor, and a television set, for displaying information to a user. An input device  714 , including alphanumeric and other keys, is coupled to computer system  700  for communicating information and command selections to processor  704 . Other non-limiting, illustrative examples of input device  714  include a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  704  and for controlling cursor movement on display  712 . While only one input device  714  is depicted in  FIG. 7 , embodiments of the invention may include any number of input devices  714  coupled to computer system  700 . 
     Embodiments of the invention are related to the use of computer system  700  for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system  700  in response to processor  704  executing one or more sequences of one or more instructions contained in main memory  706 . Such instructions may be read into main memory  706  from another machine-readable medium, such as storage device  710 . Execution of the sequences of instructions contained in main memory  706  causes processor  704  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement embodiments of the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software. 
     The term “machine-readable storage medium” as used herein refers to any tangible medium that participates in storing instructions which may be provided to processor  704  for execution. Such a medium may take many forms, including but not limited to, non-volatile media and volatile media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  710 . Volatile media includes dynamic memory, such as main memory  706 . 
     Non-limiting, illustrative examples of machine-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, or any other medium from which a computer can read. 
     Various forms of machine readable media may be involved in carrying one or more sequences of one or more instructions to processor  704  for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a network link  720  to computer system  700 . 
     Communication interface  718  provides a two-way data communication coupling to a network link  720  that is connected to a local network. For example, communication interface  718  may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  718  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  718  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  720  typically provides data communication through one or more networks to other data devices. For example, network link  720  may provide a connection through a local network to a host computer or to data equipment operated by an Internet Service Provider (ISP). 
     Computer system  700  can send messages and receive data, including program code, through the network(s), network link  720  and communication interface  718 . For example, a server might transmit a requested code for an application program through the Internet, a local ISP, a local network, subsequently to communication interface  718 . The received code may be executed by processor  704  as it is received, and/or stored in storage device  710 , or other non-volatile storage for later execution. 
     In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.