Abstract:
Computer-implemented methods, systems, and computer-readable media for detecting the presence of malicious computer code in an e-mail sent from a client computer ( 1 ) to an e-mail server ( 2 ). An embodiment of the inventive method comprises the steps of: interposing ( 41 ) an e-mail proxy server ( 31 ) between the client computer ( 1 ) and the e-mail server ( 2 ); allowing ( 42 ) the proxy server ( 31 ) to intercept e-mails sent from the client computer ( 1 ) to the e-mail server ( 2 ); enabling ( 43 ) the proxy server ( 31 ) to determine when a file ( 30 ) is attempting to send itself ( 30 ) as part of an e-mail; and declaring ( 44 ) a suspicion of malicious computer code when the proxy server ( 31 ) determines that a file ( 30 ) is attempting to send itself ( 30 ) as part of an e-mail.

Description:
RELATED APPLICATION 
   This patent application claims priority upon commonly owned U.S. provisional patent application Ser. No. 60/397,922 filed Jul. 22, 2002 entitled “Proactive Prevention of SMTP Mass Mailing Worms”, which provisional patent application is hereby incorporated by reference in its entirety into the present patent application. 

   TECHNICAL FIELD 
   This invention pertains to the field of preventing malicious attacks to computers, and, in particular, preventing e-mail propagation of malicious computer code. 
   BACKGROUND ART 
   As used herein, “malicious computer code” is any code that enters a computer without an authorized user&#39;s knowledge and/or without an authorized user&#39;s consent. Malicious computer code that propagates from one computer to another over a network, e.g., via e-mail, is often referred to as a “worm”. Most worms that spread from one computer to another are spread via e-mail over the Internet. The most common way to send e-mail over the Internet is using the SMTP (Simple Mail Transfer Protocol). SMTP is part of TCP/IP (Transfer Control Protocol/Internet Protocol). SMTP was originally designed to send only that e-mail that consists solely of text and that is encoded using the ASCII character set, which is limited. It soon became apparent that computer users wished to send other than straight ASCII characters as e-mail, and so encoding schemes such as UUencode and MIME were developed. These encoding schemes are capable of encoding any type of file, including a binary graphics file, into ASCII so that it can be sent as an e-mail attachment. 
     FIG. 1  illustrates a common system by which a client computer  1  can send e-mail to a recipient computer  5  over an open network  4  such as the Internet. In  FIG. 1 , it is assumed that there are a plurality N of client computers  1  located within an enterprise  3 . Enterprise  3  may be a company, a university, a government agency, etc. Computers  1  are coupled to each other and to an e-mail server computer  2  over a Local Area Network (LAN)  6 . E-mail server  2  collects and formats e-mails sent from computers  1  and sends them to the designated recipients  5  using the SMTP protocol. It is assumed that there are a plurality J of recipient computers. 
     FIG. 2  illustrates a similar network in which client computers  1  are not associated with the same enterprise  3 , but rather may be more geographically dispersed and are subscribers to an Internet Service Provider (ISP). In this case, computers  1  communicate with the ISP&#39;s e-mail server  2  via the Public Switched Telephone Network (PSTN)  6 . In other respects, the functioning of the networks illustrated in  FIGS. 1 and 2  are the same. 
   DISCLOSURE OF INVENTION 
   Computer-implemented methods, systems, and computer-readable media for detecting the presence of malicious computer code in an e-mail sent from a client computer ( 1 ) to an e-mail server ( 2 ). An embodiment of the inventive method comprises the steps of: interposing ( 41 ) an e-mail proxy server ( 31 ) between the client computer ( 1 ) and the e-mail server ( 2 ); allowing ( 42 ) the proxy server ( 31 ) to intercept e-mails sent from the client computer ( 1 ) to the e-mail server ( 2 ); enabling ( 43 ) the proxy server ( 31 ) to determine when a file ( 30 ) is attempting to send itself ( 30 ) as part of an e-mail; and declaring ( 44 ) a suspicion of malicious computer code when the proxy server ( 31 ) determines that a file ( 30 ) is attempting to send itself ( 30 ) as part of an e-mail. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other more detailed and specific objects and features of the present invention are more fully disclosed in the following specification, reference being had to the accompanying drawings, in which: 
       FIG. 1  is a system level diagram of a conventional network for sending e-mail from within an enterprise  3 . 
       FIG. 2  is a system level diagram of a conventional network for sending e-mail via an Internet Service Provider (ISP) computer  2 . 
       FIG. 3  is a block diagram illustrating an embodiment of the present invention. 
       FIG. 4  is a flow diagram illustrating an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Nefarious persons sending malicious computer code via e-mails have resorted to many tricks to spread their malicious messages. A typical e-mail may look something like this:
         IP x.y.z.1:25 (SMTP)   HELLO someone   RCPT to: edGXYZ.com   FROM: XYZGx17as.com   SUBJECT: HELLO   DATA   MIME-encoded attachments       

   One of the tricks employed by authors of malicious code is to falsify the “FROM” field so that the recipient of the e-mail will be lulled into thinking that the e-mail was sent from a known, reputable source. 
   Sometimes the malicious code will be encrypted, making it difficult for a conventional anti-virus scanner to analyze it. 
   Modern worms such as Klez self-activate simply by the user clicking open the e-mail message itself: the user doesn&#39;t even have to click on the e-mail attachment containing the worm. Klez has operated through the popular e-mail software known as Microsoft Outlook. Klez contains its own SMTP client embedded in the worm; it does not rely on Outlook. 
   The present invention thwarts the propagation of malicious computer code being sent in an email from a client computer  1  to an e-mail server  2 , by means of interposing (step  41  of  FIG. 4 ) an e-mail proxy server  31  (hereinafter referred to as “proxy”) between the client computer  1  and the e-mail server  2 . The client computer  1  thinks that the proxy  31  is the real e-mail server  2 . The proxy  31  may be associated with the client computer  1 , e.g., it may reside within computer  1 . 
     FIG. 3  illustrates the basic architecture of proxy  31 . Redirector  36  intercepts e-mail messages, and tricks client file  30  into thinking that redirector  36  is e-mail server  2 . Scan manager  32  is coupled to re-director  36 , and contains intelligence for examining the contents of e-mails. Decomposer  33  is coupled to scan manager  32 ; and unpacks (e.g., unzips) objects and sends the decomposed objects back to scan manager  32  one by one. Decomposer  33  is invoked when the e-mail being analyzed by scan manager  32  contains many objects, e.g., an e-mail body and several e-mail attachments that are zipped or otherwise combined. In that case, decomposer  33  unzips the objects and presents them to scan manager  32  one by one for further analysis. API  34  such as Norton Antivirus Application Programming Interface (NAVAPI)  34  is coupled to scan manager  32 , and presents scan manager  32  with ready access to conventional antivirus software. Extensions  35  such as Norton Antivirus Extensions (NAVEX)  35  are coupled to NAVAPI  34  and contain all of the scanning engines, virus signatures, and virus names used in conventional antivirus scanning. Modules  31 - 36  may be implemented in hardware, software, and/or firmware, or any combination thereof. 
   In the embodiment where e-mail server  2  adheres to the SMTP protocol, proxy  31  adheres to the SMTP protocol as well. Generally speaking, proxy  31  adheres to the same protocol adhered to by e-mail server  2 . 
   At step  42  of  FIG. 4 , proxy  31  is enabled to intercept e-mail sent from the client computer  1  to the e-mail server  2 . The enabling may be accomplished by the user of computer  1  clicking on a “e-mail scanning” feature on antivirus software (such as Norton Antivirus manufactured by Symantec Corporation of Cupertino, Calif.) that has been installed on the user&#39;s computer  1 . Such an enabling may, for example, serve to activate proxy  31  every time a client file  30  within client computer  1  attempts to access the computer&#39;s port  25 , which is the conventional port used in personal computers for sending e-mail over the Internet. 
   At step  43 , scan manager  32  determines whether file  30  is attempting to send itself, either as part of the e-mail body or as an e-mail attachment. The determination that is made in step  43  can vary based upon the type of file  30 . The name of the file  30  is ascertained by redirector  36  and given to scan manager  32 . In the WIN32 API of Microsoft Corporation, scan manager  32  determines whether file  30  is a file in the PE (portable executable) format. The PE header identifies file  30  as a PE file. Section headers determine the type of the section, e.g., code sections, data sections, resource sections, etc. For a PE file in the WIN 32 API, scan manager  32  examines the entire code section or code sections. Scan manager  32  performs a compare between two versions of file  30 : the version that has been intercepted and that now resides within proxy  31  versus the version that resides in client computer  1 . In one embodiment, scan manager  32  declares a suspicion of malicious code in step  44  when the two versions are nearly identical. If the two versions are not nearly identical, scan manager  32  declares in step  45  that no malicious code is present in file  30 . “Nearly identical” is defined throughout this patent application to mean that no more than one byte out of a preselected threshold number of bytes varies between the two versions. In one embodiment, the preselected threshold number of bytes is 512. Other preselected threshold numbers can be selected based on the application. The reason for not insisting upon perfect matching between the two versions of the file is that the malicious code occasionally modifies a byte of the file. 
   Once a suspicion of malicious code is declared in step  44 , one or more optional steps  46 ,  47 , and  48  can be invoked. Steps  46  and  48  serve to reduce the number of unwanted false positives (declaring a file  30  to be contaminated when it isn&#39;t). 
   In optional step  46 , the user of computer  1  is given a set of choices when a suspicion of malicious code has been declared in step  44 . These choices may be presented to the user via a dialog box which pops up on the user&#39;s monitor. Such a dialog box may look like the following: 
   Malicious Worm Alert 
   Filename: readme.exe 
   Norton AntiVirus has detected a malicious worm on your computer that is trying to e-mail itself to other computers. If this Malicious Worm Alert appeared when you were not sending an e-mail message, the worm is trying to spread itself by e-mail, and you should select the “Quarantine this worm (Recommended)” option from the following drop down list. You can get more information about the worm from the Symantec Security Response virus encyclopedia. 
   Select one of the following actions:
         Stop this worm from e-mailing itself. This stops the worm from e-mailing itself at this time, but does not quarantine the worm. This action leaves the worm on your computer, where it can possibly be activated again. Select this option only if you are sure you want to leave the worm on your computer.   Quarantine this worm (Recommended). This permanently stops the worm by putting it in the Norton AntiVirus Quarantine. While in Quarantine, the worm will not be able to spread itself. This is the safest action.   Allow this application to send e-mail attachments. This sends the e-mail containing a potential worm. Such a worm could infect the recipient&#39;s computer. Select this option only if you are sure the e-mail is not infected with a worm.   Always allow this application to send e-mail attachments. In the future, Norton AntiVirus will not check this file for worms. This is the riskiest action, because such a worm could e-mail itself from your computer without your knowledge.       

   Note that the file name of the suspicious file  30  is given to the user, along with four choices. If the second choice is selected (quarantining the worm), file  30  is encrypted and sent to the headquarters of the antivirus company (in this case, Symantec) for analysis. 
   It is expected that the user would rarely select choices three or four (allowing the application to send e-mail attachments). Such a choice might be selected when the user is attempting to e-mail the entire e-mail software program to a recipient  5 . 
   In optional step  47 , an alert is sent to every client computer  1  associated with the enterprise  3 . The alert serves to warn other users of possible problems. 
   In optional step  48 , scan manager  32  checks to see whether a digital signature has been affixed to file  30 , and, if so, verifies the digital signature with a trusted source in a conventional manner. If the digital signature is present and is verified by the trusted third party, scan manager  32  then rescinds the declaration of suspected malicious code found in step  44 , and deems the file  30  to be clean after all. 
   The above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the present invention.