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Email Attachment Filtering

In recent years the Internet has been flooded with worms, viruses, and other software programs that are spread between computers without the knowledge or consent of the user. Almost without exception these worms are targeted at computers running Microsoft Windows, and can turn these hapless PCs into virus- and spam-spewing menaces. Even if you do not use Microsoft Windows, you may have still seen the effects of these worms by the amount of junk and virus-laden emails you have been receiving.

Typically, once embedding itself in a Windows-based computer, the worm will start sending out copies of itself to a list of email addresses found in that computer's own email address book. To further complicate matters, it will pick another email address from that same computer as a forged "From" address on the virus-laden email that it sends out, resulting in bounce messages being sent to the wrong address.

As one part of our strategy to combat this problem, we are no longer allowing incoming or outgoing email with the types of file attachments most frequently associated with malicious content. If an email addressed to you comes in with one of these files attached, the email will be delivered but the attached file will be stripped off. If you send one of these types of files to someone else (through our outgoing mail server), the message will be delivered, but the file attachment will be stripped off. If you need to send such a file to someone, compress the file before sending it. After being compressed, the file will have an extension like .ZIP or .SIT. These files are allowed through our mail server.

Here is a current list of file attachments that we are stripping off from incoming and outgoing email:

  File
Extension  Description
---------  --------------------------------------------
   ADE     Microsoft Access project extension
   ADP     Microsoft Access project
   APP     MacroMedia Authorware package
   ASD     Advanced Streaming Format Description
   ASF     Active Streaming File
   ASX     Windows Media Audio / Video
   BAS     Microsoft Visual Basic class module
   BAT     Batch file
   CHM     Compiled HTML Help file
   CMD     Microsoft Windows NT Command script
   CPL     Control Panel extension
   CRT     Security certificate
   DLL     Dynamic link library
   EXE     Program
   FXP     FoxPro compiled source code file
   HLP     Help file
   HTA     HTML program
   HTO     Hierarchical Tagged Object
   INF     Setup Information
   INI     Initialization - configuration settings
   INS     Internet Naming Service
   ISP     Internet Communication settings
   JS      JScript File
   JSE     JScript Encoded Script file
   LNK     Shortcut
   MDB     Microsoft Access program
   MDE     Microsoft Access MDE database
   MSC     Microsoft Common Console document
   MSI     Microsoft Windows Installer package
   MSP     Microsoft Windows Installer patch
   MST     Microsoft Windows Installer transform, 
           Microsoft Visual Test source file
   OCX     Object Linking and Embedding (OLE) Custom Control
   PCD     Photo CD Image, Microsoft Visual compiled script
   PIF     Shortcut to MS-DOS program
   PRG     dBASE source code file
   REG     Registration entries
   SCR     Screen saver
   SCT     Windows Script Component
   SH      Shell script
   SHB     Shell Scrap object
   SHS     Shell Scrap object
   SYS     Windows System file
   URL     Internet shortcut
   VB      VBScript file
   VBE     VBScript Encoded script file
   VBS     VBScript file
   VCS     Voice Changer Software
   VXD     Virtual Device Driver
   WMD     Windows Media Download File
   WMS     Windows Media Skin File
   WMZ     Windows Media Compressed Skin File
   WSC     Windows Script Component
   WSF     Windows Script file
   WSH     Windows Scripting Host Settings file

This list comes from Microsoft Knowledge Base Article: http://support.microsoft.com/default.aspx?scid=kb;EN-US;290497 where Microsoft identifies files with these extensions as being "unsafe" for delivery in e-mail.

Despite these protections, it is still quite possible for your computer to become infected. If you are using Microsoft Windows, it is essential that you run virus protection software, and keep it updated. Also visit http://windowsupdate.microsoft.com frequently to keep your operating system up to date with the latest security patches.