W32.Bagif

SUMMARY

W32.Bagif is a virus that infects files locally and also spreads to shares and network drives. It is polymorphic and entry-point obscuring.

Protection

• Virus Definitions (LiveUpdate™ Weekly) February 12, 2003
• Virus Definitions (Intelligent Updater) February 7, 2003

TECHNICAL DETAILS

W32.Bagif has two modes of propagation. It infects host files as a parasitic virus and spreads to shares and network drives as a self-contained worm.

The hosts are infected by inserting the virus code at the end of the first section and hooking the first ExitProcess() import call in order to gain control when the host has finished executing and wants to exit.

When a host file that is infected with W32.Bagif in parasitic mode is executed, the virus gets control on ExitProcess() and drops a dummy host named "backup.gif" in the temporary folder, infects it and runs it. The dummy host is configured to behave as a worm.

When a host file that is infected with W32.Bagif in worm mode is executed, it does the following:

1. Copies itself to the System folder as Ntloader.exe.
2. Hooks the (Default) value of the registry key
   
   HKEY_CLASSES_ROOTexefileshellopencommand
   
   so that W32.Bagif gets control whenever you open a program from Windows Explorer. After this happens, programs that you start from Windows Explorer have a 50% chance to become infected before execution.
   
3. Copies itself as Win32s.exe to the Startup folder of the current user.
4. Spawns a thread to spread over the network. This thread does the following:
   1. Periodically enumerates disk resources on the network and attempts to locate the Windows installation folder by checking this list of common folder names:
      • Windows
      • Winnt
      • Win95
      • Win98
      • Winme
      • Win2000
      • Win2k
      • Winxp
   2. When it locates one of these folders, W32.Bagif:
      1. Copies itself to that folder as Tsoc32.exe.
      2. Modifies the run= line in the Win.ini file so that the worm is executed each time that you start Windows on Windows 95/98/Me computers.
   3. Explores all drives from the Z drive down to the system drive, which is usually the C drive, and searches for hosts to infect in parasitic mode. Up to 5 levels of nested folders are explored and each file has a 50% chance to be considered for infection. Only files with the extensions .exe and .scr are infected.
      
      Before .exe or .scr files can be infected, they are checked to see if they meet these conditions:
      • Their names must not start with "expl" (such as Explorer.exe), "unre" (Unreal.exe) or "hl.e" (hl.exe).
      • They must be larger than 4 kilobytes and smaller than 2 megabytes
      • They must import ExitProcess() from Kernel32 as well as other structural conditions.

W32.Bagif uses the KME polymorphic engine that is used in W32.Toal.A@mm, W95.MTXII and W95.Zexam. The engine is embedded in the virus code in a compressed form.

Recommendations

Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

• Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates.
• If a blended threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
• Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services (for example, all Windows-based computers should have the current Service Pack installed.). Additionally, please apply any security updates that are mentioned in this writeup, in trusted Security Bulletins, or on vendor Web sites.
• Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate infected computers quickly to prevent further compromising your organization. Perform a forensic analysis and restore the computers using trusted media.
• Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.

REMOVAL

These instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

1. Update the virus definitions.
2. Run a full system scan and delete all the files detected as W32.Bagif.
3. Reverse the change that was made to the registry.
4. Remove the line that it added to the Win.ini file (Windows 95/98/Me).
   

For specific details on each of these procedures, read the following instructions.

1. Updating the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:

• Running LiveUpdate, which is the easiest way to obtain the virus definitions. These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate), in the "Protection" section, at the top of this writeup.
• Downloading the definitions using the Intelligent Updater. The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater), in the "Protection" section, at the top of this writeup.
  
  The Intelligent Updater virus definitions are available here. For detailed instructions on how to download and install the Intelligent Updater virus definitions from the Symantec Security Response Web site, click here.

2. Scanning for and deleting the infected files

1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
   • For Norton AntiVirus consumer products: Read the document, "How to configure Norton AntiVirus to scan all files."
   • For Symantec AntiVirus Enterprise products: Read the document, "How to verify that a Symantec Corporate antivirus product is set to scan All Files."
2. Run a full system scan.
3. If any files are detected as infected with W32.Bagi, click Delete.

3. Reversing the change that was made to the registry

CAUTION: Symantec strongly recommends that you back up the registry before you make any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified keys only. Read the document, "How to make a backup of the Windows registry," for instructions.

1. Click Start, then click Run. (The Run dialog box appears.)
2. Type regedit, then click OK. (The Registry Editor opens.)
3. Navigate to and select the key:
   
   HKEY_CLASSES_ROOTexefileshellopencommand
   
   NOTE: The HKEY_CLASSES_ROOT key contains many subkey entries that refer to other file extensions. One of these file extensions is .exe. Changing this extension can prevent any files ending with an .exe extension from running. Make sure that you completely browse throughout this path until you reach the command subkey.
   
   Modify the HKEY_CLASSES_ROOTexefileshellopencommand subkey, shown in the following figure:
   
   <<=== NOTE: Modify this key.
   
4. In the right pane, double-click the (Default) value.
5. Delete the current value data, and then type: "%1" %* (That is, type the characters: quote-percent-one-quote-space-percent-asterisk).
   
   NOTES
   • Under Windows 95/98/Me/NT, the Registry Editor automatically encloses the value within quotation marks. When you click OK, the (Default) value should look exactly like this:
     
     ""%1" %*"  
     
   • Under Windows 2000/XP, the additional quotation marks will not appear. When you click OK, the (Default) value should look exactly like this:
     
     "%1" %*
     
   • Make sure that you completely delete all the value data in the command key before typing the correct data. If you leave a space at the beginning of the entry, any attempt to run the program files will result in the error message, "Windows cannot find .exe." If this occurs, restart the entire process from the beginning of this document and make sure that you completely remove the current value data.
     
6. Exit the Registry Editor.

4. Editing the Win.ini file
If you are running Windows 95/98/Me, follow these steps:

1. The function you perform depends on your operating system:
   • Windows 95/98: Go to step b.
   • Windows Me: If you are running Windows Me, the Windows Me file-protection process may have made a backup copy of the Win.ini file that you are to edit. If this backup copy exists, it will be in the C:WindowsRecent folder. Symantec recommends that you delete this file before you continue with the steps in this section. To do this:
     • Start Windows Explorer.
     • Browse to and select the C:WindowsRecent folder.
     • In the right pane, select the Win.ini file and delete it. The Win.ini file will be regenerated when you save your changes to it in step f.
       
2. Click Start, and then click Run.
3. Type the following, and then click OK.
   
   edit c:windowswin.ini
   
   (The MS-DOS Editor opens.)
   
   NOTE: If Windows is installed in a different location, make the appropriate path substitution.
   
4. In the [windows] section of the file, look for an entry similar to one of the following:
   
   run= <file name of worm>
   
5. Select the entire line. Make sure that you have not selected any other text, and then press Delete.
6. Click File, and then click Save.
7. Click File, and then click Exit.