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Discovered: February 13, 2006
Updated: February 13, 2007 12:51:17 PM
Also Known As: Worm.Win32.Agent.a [Kaspersky , W32/Choss [McAfee], TROJ_DROPPER.AHY [Trend Micro]
Type: Worm, Virus
Systems Affected: Windows

W32.Exvid.A@mm is a virus with keylogging capabilities that spreads using its mass-mailing functionality and through network shares.

Antivirus Protection Dates

  • Initial Rapid Release version February 13, 2006
  • Latest Rapid Release version March 23, 2017 revision 037
  • Initial Daily Certified version February 13, 2006
  • Latest Daily Certified version March 23, 2017 revision 041
  • Initial Weekly Certified release date February 15, 2006

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Writeup By: Mark McGuill

Discovered: February 13, 2006
Updated: February 13, 2007 12:51:17 PM
Also Known As: Worm.Win32.Agent.a [Kaspersky , W32/Choss [McAfee], TROJ_DROPPER.AHY [Trend Micro]
Type: Worm, Virus
Systems Affected: Windows

Once W32.Exvid.A@mm is executed, it performs the following actions:

  1. Creates the following mutexes so that only one copy of the virus runs on the compromised computer at any one time:

    • svchoss
    • this is my one mutex

  2. Copies itself as the following file:


    Note: %Windir% is a variable that refers to the Windows installation folder. By default, this is C:\Windows or C:\Winnt.

  3. Creates the following file, which is the keylogging and mass-mailing component of this virus:


    Note: %System% is a variable that refers to the System folder. By default this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).

  4. Adds the value:

    "svchost Netware Manager" = "%Windir%\svchost.exe -r"

    to the following registry subkey:


    so that it runs every time Windows starts.

  5. Adds itself as a service with the following characteristics:

    Title: svchoss

  6. Attempts to infect all .exe files it finds on logical drives and network shares by appending a copy of itself to the last section of the file, and altering the entry point.

    Any files infected in this way contain the following infection marker in the last five bytes of the file:


  7. Logs all keystrokes on the compromised computer and periodically sends the information it gathers to an email account.

  8. Attempts to send itself, using smtp.126.com as SMTP server, to email addresses it generates. These addresses will have the following format:

    • [RANDOM THREE LETTERS]@126.com
    • [RANDOM THREE LETTERS]@sohu.com
    • [RANDOM THREE LETTERS]@163.com
    • [RANDOM THREE LETTERS]@tom.com
    • [RANDOM THREE LETTERS]@263.net
    • [RANDOM THREE LETTERS]@21cn.com

      where the variable [RANDOM THREE LETTERS] represents three randomly chosen letters.

      The email message has the following characteristics:

      One of the following:

    • [STRING 1].exe
    • [STRING 1]0.exe
    • [STRING 1]1.exe
    • [STRING 1]2.exe
    • [STRING 1]3.exe
    • [STRING 1]4.exe
    • [STRING 1]5.exe
    • [STRING 1]6.exe
    • [STRING 1]7.exe
    • [STRING 1]8.exe
    • [STRING 1]9.exe

      where [STRING 1] is one of the following:

    • sexvideo
    • MyDocument
    • MyVideo
    • MyPicture
    • MyPassword

      [STRING 1] also includes one of the following Chinese characters:


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

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • 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.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a 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.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. 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.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.

Writeup By: Mark McGuill

Discovered: February 13, 2006
Updated: February 13, 2007 12:51:17 PM
Also Known As: Worm.Win32.Agent.a [Kaspersky , W32/Choss [McAfee], TROJ_DROPPER.AHY [Trend Micro]
Type: Worm, Virus
Systems Affected: Windows

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

  1. Find and stop the service.
  2. Restore critical system files
  3. Disable System Restore (Windows Me/XP).
  4. Update the virus definitions.
  5. Run a full system scan and delete all the files detected.
  6. Delete any values added to the registry.
For specific details on each of these steps, read the following instructions.

1. To find and stop the service
  1. Click Start > Run.
  2. Type services.msc, and then click OK.
  3. Locate and select the service that was detected.
  4. Click Action > Properties.
  5. Click Stop.
  6. Change Startup Type to Manual.
  7. Click OK and close the Services window.
  8. Restart the computer.

2. To restore critical system files
The following documents provide general instructions on how to extract files. This information is provided for your convenience. The exact steps may vary slightly depending on the configuration of your operation system, the location of the files, and so on. For additional information, read the Help files, contact Microsoft, or refer to the following Windows documentation:

3. To disable System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.

Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.

Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.

For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:
When you are completely finished with the removal procedure and are satisfied that the threat has been removed, reenable System Restore by following the instructions in the aforementioned documents.

For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article: Antivirus Tools Cannot Clean Infected Files in the _Restore Folder (Article ID: Q263455).

4. To update 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 virus definitions:
    • If you use Norton AntiVirus 2006, Symantec AntiVirus Corporate Edition 10.0, or newer products, LiveUpdate definitions are updated daily. These products include newer technology.
    • If you use Norton AntiVirus 2005, Symantec AntiVirus Corporate Edition 9.0, or earlier products, LiveUpdate definitions are updated weekly. The exception is major outbreaks, when definitions are updated more often.
  • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted daily. 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 Virus Definitions (Intelligent Updater).

    The latest Intelligent Updater virus definitions can be obtained here: Intelligent Updater virus definitions. For detailed instructions read the document: How to update virus definition files using the Intelligent Updater.

5. To scan for and delete the infected files
  1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
  2. Run a full system scan.
  3. If any files are detected, click Delete.

Important: If you are unable to start your Symantec antivirus product or the product reports that it cannot delete a detected file, you may need to stop the risk from running in order to remove it. To do this, run the scan in Safe mode. For instructions, read the document, How to start the computer in Safe Mode . Once you have restarted in Safe mode, run the scan again.

After the files are deleted, restart the computer in Normal mode and proceed with the next section.

Warning messages may be displayed when the computer is restarted, since the threat may not be fully removed at this point. You can ignore these messages and click OK. These messages will not appear when the computer is restarted after the removal instructions have been fully completed. The messages displayed may be similar to the following:

Title: [FILE PATH]
Message body: Windows cannot find [FILE NAME]. Make sure you typed the name correctly, and then try again. To search for a file, click the Start button, and then click Search.

5. To delete the value from the registry
Important: Symantec strongly recommends that you back up the registry before making any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified subkeys only. For instructions refer to the document: How to make a backup of the Windows registry.
  1. Click Start > Run.
  2. Type regedit
  3. Click OK.

    Note: If the registry editor fails to open the threat may have modified the registry to prevent access to the registry editor. Security Response has developed a tool to resolve this problem. Download and run this tool, and then continue with the removal.

  4. Navigate to the subkey:


  5. In the right pane, delete the value:

    "svchost Netware Manager" = "%Windir%\svchost.exe -r"

  6. Exit the Registry Editor.

Writeup By: Mark McGuill