Abstract:
A method for preventing activating a malicious object passing through a checkpoint, and decreasing the overall inspection delay thereof, the method comprising the steps of: (a) at the checkpoint, creating an envelope file, being an executable file comprising: the object; code for extracting the object from the envelope file; and an indicator for indicating the integrity of the object; (b) forwarding the envelope file instead of the object toward its destination, while holding at least a part of the envelope file which comprises the indicator; (c) inspecting the object; and (d) setting the indicator on the envelope file to indicate the inspection result thereof, and releasing the rest of the envelope file.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of detecting viruses and other malicious forms in a checkpoint (e.g. gateway). More particularly, the invention relates to a method for preventing activation of malicious objects while decreasing the overall inspection time. 
     BACKGROUND OF THE INVENTION 
     The term “gateway” refers in the art to a bridge between two networks. For each network, the gateway is a point that acts as an entrance to another network. From the implementation point of view, a gateway is often associated with both a router, which knows where to direct a given packet that arrives to the gateway and a switch, which provides the packet with the actual path in and out of the gateway. Due to its nature, the gateway to a local network is a proper point for checking out objects (e.g. files and email messages) that pass through it, in order to detect viruses and other forms of maliciousness (“inspection”) before reaching the user. 
     As a filtering facility, a gateway server has to deal with two contradicting objects: on the one hand, it has to hold a file that reaches the gateway in its path from a source to a destination until the inspection indicates it is harmless and thereby prevents its execution on the destination site, on the other hand holding a file at the gateway server until the inspection process terminates which results in a bottleneck to data traffic passing through the gateway. 
     Inspection activity has a substantial influence on the traffic speed through a gateway. U.S. patent application Ser. No. 10/002,407, titled as Security Router, deals with this problem by skipping the inspection of trusted files. According to this invention, since multimedia files (e.g. JPG files) do not comprise executable code (according to their definition), these files are not inspected, thereby diminishing the delay caused by the inspection process. 
     U.S. patent application Ser. No. 09/498,093, titled as “Protection of computer networks against malicious content”, deals with this problem by holding in a checkpoint (e.g. a gateway) only a part of the file, such as the last packet of the file, and releasing it once the file has been indicated as harmless. This way the majority of the file is not delayed at the gateway, but its execution at the destination site cannot be carried out until the last part reaches the destination. This solution is applicable only for files that in order to be executed or activated, the whole file has to be available on the executing platform. However, if the executing platform activates a file even in the case where only a part of the file is available, the executing platform is exposed to viruses and other malicious forms. 
     Furthermore, some inspection methods, such as CRC-based methods, require that the whole file be available during the inspection process. Files that should be fully accessible for inspection, may cause a substantial delay to the traffic through a checkpoint since the inspection can start only after the whole file is accessible to the inspection facility. Thus, in this case, the parts of a file should be accumulated and held at the inspection point until the inspection indicates that it is harmless, and only then the file may be “released” to its destination. 
     It is an object of the present invention to provide a method for preventing activation of malicious objects. 
     It is a further object of the present invention to provide a method for preventing from a checkpoint the activation of malicious objects on the executing platform. 
     It is a still further object of the present invention to provide a method for inspecting a file on a checkpoint, by which the delay thereof is decreased in comparable to the prior art. 
     Other objects and advantages of the invention will become apparent as the description proceeds. 
     SUMMARY OF THE INVENTION 
     A method for preventing activating a malicious object passing through a checkpoint, and decreasing the overall inspection delay thereof, the method comprising the steps of: (a) at the checkpoint, creating an envelope file, being an executable file comprising: the object; code for extracting the object from the envelope file; and an indicator for indicating the integrity of the object; (b) forwarding the envelope file instead of the object toward its destination, while holding at least a part of the envelope file which comprises the indicator; (c) inspecting the object; and (d) setting the indicator on the envelope file to indicate the inspection result thereof, and releasing the rest of the envelope file. 
     According to a preferred embodiment of the invention, the envelope file is an auto-executable file. According to one embodiment of the invention, the name of the envelope file is identical to the name of the inspected object. According to another embodiment of the invention, the name of the envelope file differs than the name of the inspected object. 
     The indicator may be a CRC of a part of the envelope file, a CRC of a part of the inspected object, a checksum of a part of the envelope file, a checksum of a part of the inspected object, a value stored within the envelope file, absence of a part of the envelope file, absence of a part of the inspected object, and so forth. 
     According to a preferred embodiment of the invention, the envelope file further comprises code for displaying an acknowledgment to the user for indicating the integrity of the inspected object. Typically the acknowledgment is displayed only when the inspected objects is indicated as malicious. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be better understood in conjunction with the following figures: 
         FIG. 1  schematically illustrates a system that may be used for implementing the present invention. 
         FIG. 2  schematically illustrates the parts of an envelope file, according to a preferred embodiment of the present invention. 
         FIG. 3  is a flowchart of a process for creating an envelope file, according to a preferred embodiment of the invention. 
         FIG. 4  is a flowchart of a process carried out by an envelope file on the destination site, according to a preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The term “checkpoint” refers herein to a point on a data channel or data junction in which the passing data is inspected. For example, a gateway is a suitable place for a checkpoint. 
       FIG. 1  schematically illustrates a system that may be used for implementing the present invention. The computers  21  are connected to the local area network  20 . The local area network  20  is connected to the internet  10 . The gateway server  30  is interposed between the local area network  20  and the internet  10 . The Internet server  40  hosts web sites. A browser being executed on a computer  21  that addresses to the web site hosted by the internet server  40  cause files to be transferred from the internet server  40  to the computer  21  through the gateway server  30 . 
     As mentioned above, a gateway server that inspects data that is transferred through it, has to deal with two contradicting objects: on the one hand, it has to hold an inspected object until its harmlessness is indicated, on the other hand holding the file at the gateway may cause a bottleneck to the data traffic passing through the gateway. 
     This problem is solved by the present invention. The present invention allows passing a file toward its destination while inspecting the file, and still preventing the activation of the file at the destination site until its integrity is indicated. This is carried out as follows: instead of forwarding the original file to its destination, a substitute file which comprises the original file is constructed and transmitted to the destination. The substitute file is also referred herein as envelope file. 
     According to one embodiment of the invention, the envelope file is an executable which comprises the following parts:
         the original file;   an indicator about the integrity of the original file; and   an executable part, upon which execution extracts the original file, and executes it.       

     At the destination site, the envelope file is executed instead of the original file. On this execution, if the integrity indicator indicates that the original file is harmless, the original file is extracted from the envelope file and executed. 
     The integrity indicator can be the CRC (Cyclic Redundancy Checks) or checksum of the envelope file, the CRC or checksum of the original file, a bit within the envelope file, a value stored within the envelope file (in a byte, a bit, etc.), and so forth. In order to prevent forge of the envelope file, the indicator and/or the envelope file can be secured (e.g. encrypted, digitally signed with the original or envelope file, and so forth). 
     For example, in the case where the integrity of the original file is indicated at the gateway, the gateway sends the true CRC value of the original file. In case the original file is indicated as comprising malicious content, the gateway sends a wrong CRC value of the original file. As well, other indicators can be used. 
     It should be noted that the envelope file performs two operations: on the one hand it is an executable file, which is loaded into the computer&#39;s memory for executing, but on the other hand it is treated as a data file. 
       FIG. 2  schematically illustrates the parts of an envelope file, according to a preferred embodiment of the present invention. The envelope file  50  comprises:
         an executable part  51 ;   the original file  52 ; and   an integrity indicator  53 .       
       FIG. 3  is a flowchart of a process for creating an envelope file, according to a preferred embodiment of the invention.
         The process starts at block  100 , where a packet of a file sent from a source to a destination reaches to a gateway server (or, generally, a checkpoint).   From block  101 , if the packet is the first packet of a file then the process continues with block  102 , where a new envelope file is created, and therefrom with block  103 , where an executable code is added to the created envelope file, and therefrom to block  104 . The operation of the executable code is detailed in the description of  FIG. 4 .   At block  104 , the packet is added to the envelope file.   At block  105 , a copy of the packet is sent to the inspection facility for inspection.   At block  106 , the accumulated parts of the envelope file are sent to the destination.   From block  107 , if the reached packet is the last packet of the file, then an integrity indicator is added to the envelope file, and the last accumulated parts of the envelope file are sent to the destination. The integrity indicator is received from the inspection facility which operates at the gateway. If the received packet is not the last packet of the original file, then when a new packet of the original file will reach to the gateway server, the process will continue from block  100 .       
     In this example it was assumed that the packets of a file reach the gateway in consecutive order, however, as known to a person of ordinary skill in the art, this is not necessarily true, since each packet may be redirected to its destination through a different path. Thus, when adding a packet of the original file to the envelope file, the order should be taken into consideration. However, according to a preferred embodiment of the invention, the indicator is stored within the last part of the envelope file. This way usually (but not always) the integrity indicator will be the last to reach the destination. 
       FIG. 4  is a flowchart of a process carried out by an envelope file on the destination site, according to a preferred embodiment of the invention.
         At block  200  the envelope file is executed.   At block  201 , the executed envelope program gets the integrity indicator. As mentioned above, the envelope file is also used as a data file which comprises the original file and an integrity indicator.   From block  202 , if according to the integrity indicator the original file is malicious, then at block  209  the envelope file acknowledges the user (e.g. by displaying a corresponding message), and the execution of the envelope file terminates, otherwise control continues with block  203 .   At block  203 , the envelope program extracts the original file from the envelope file.   At block  204 , the original file is executed. At this point the envelope executable can terminate.   At block  205 , since the envelope file is not required anymore, it can be replaced by the original file.       
     Typically the inspected object is an executable, however it should be noted that the invention can also be implemented for other file types, whether it is an executable or not, or whether it is attached to an email message or returned in an FTP or HTTP session. 
     It should also be noted that although the references herein are to a gateway, the invention can be implemented to any kind of checkpoint, e.g. an arbitrary point on a network, a server of an Internet service provider, a mail server, and so forth. 
     Those skilled in the art will appreciate that the invention can be embodied by other forms and ways, without losing the scope of the invention. The embodiments described herein should be considered as illustrative and not restrictive.