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Discovered: November 02, 2003
Updated: February 13, 2007 12:13:15 PM
Also Known As: IRC-Worm.Fagot [Kaspersky]
Type: Trojan Horse
Systems Affected: Windows

The W32.Petch.B Trojan horse is a variant of W32.Petch that spreads by downloading an infected file through IRC. It deletes critical files, disables recovery settings, and changes the Internet Explorer start page.

The Trojan is written in Delphi and is packed with UPX.

Antivirus Protection Dates

  • Initial Rapid Release version November 03, 2003
  • Latest Rapid Release version March 03, 2008 revision 035
  • Initial Daily Certified version November 03, 2003
  • Latest Daily Certified version March 03, 2008 revision 037
  • Initial Weekly Certified release date November 05, 2003

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

Technical Description

When W32.Petch.B runs, it performs the following actions:

  1. Deletes the following critical system files:
    • %windir%\regedit.exe
    • %windir%\system32\taskman.exe
    • %windir%\system32\taskmgr.exe
    • %windir%\system32\regedt32.exe
    • %windir%\system32\regsvr32.exe
    • %windir%\TASKMAN.exe
    • C:\Windows\system32\userinit.exe

      Note: %Windir% is a variable. The Trojan locates the Windows installation folder (by default, this is C:\Windows or C:\Winnt) and copies itself to that location.

  2. Copies itself as:
    • %windir%\system32\userinit32.exe
    • %windir%\system32\ctfmon32.exe
    • %windir%\system32\autochk.exe
    • %windir%\system32\chkntfs.exe
    • %windir%\system32\chkdsk.exe
    • %windir%\system32\recover.exe
    • %windir%\NOTEPAD.exe
    • %windir%\regedit.exe

  3. Changes the value:

    "Userinit" = "%windir%\system32\userinit.exe"


    "Userinit" = "%windir%\system32\userinit32.exe"

    in the registry key:

    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon

    so that the Trojan starts when Windows NT/2000/XP starts.

  4. Adds the value:

    "SCRNSAVE.EXE" = "%windir%\system32\ctfmon32.exe"

    to the registry key:

    HKEY_CURRENT_USER\Control Panel\Desktop

    to ensure that the Trojan runs if a screen saver is configured.

  5. Resets or deletes the following registry keys (Windows NT/2000/XP only):
    • HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Desktop\SafeMode
    • HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\SafeBoot
    • HKEY_LOCAL_MACHINE\SYSTEM\ControlSet003\Control\SafeBoot
    • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SafeBoot
    • HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\ContentIndex
    • HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Biosinfo
    • HKEY_LOCAL_MACHINE\SYSTEM\LastKnownGoodRecovery

      which removes access to a variety of options, such as the ability to boot to Safe Mode, and the Last Known Good Configuration feature.

  6. Modifies the value:

    "Start Page"="www.blacksnake.com"

    in the registry key:

    HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main

    which changes the Internet Explorer Home page.

  7. Opens mIRC and runs a one line script to send a message that reads:

    www.angelfire.com/pro/pics33/jessica_alba.jpg <- wow !!

    Note: This file contains the Trojan.


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.


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

  1. Disable System Restore (Windows Me/XP).
  2. Update the virus definitions.
  3. Run a full system scan and delete all the files detected as W32.Petch.B.
  4. Delete the value that was added to the registry.
  5. Restore the Internet Explorer settings.
For specific details on each of these steps, read the following instructions.

1. 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:
Note: When you are completely finished with the removal procedure, and you are satisfied that the threat has been removed, you should re-enable 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.

2. 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: 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).
  • 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).

    The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.

3. 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 as infected with W32.Petch.B, click Delete.

4. To delete the value from the registry

WARNING: 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 keys only. Read the document, "How to make a backup of the Windows registry ," for instructions.
  1. Click Start, and then click Run. (The Run dialog box appears.)
  2. Type regedit

    Then click OK. (The Registry Editor opens.)

  3. Navigate to the key:

    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon

  4. In the right pane, double-click:


  5. In the Value data box, change the last part of the text from:




    and click OK.

  6. Navigate to the key:

    HKEY_CURRENT_USER\Control Panel\Desktop

  7. In the right pane, delete the value:

    "SCRNSAVE.EXE" = "%windir%\system32\ctfmon32.exe"

  8. Exit the Registry Editor.

    Note: Given the particularly destructive nature of this Trojan, Symantec recommends restoring the registry from a clean backup to restore the functionality of the various Windows features that the virus disables in step 5 of the "Technical Details" section.
5. To restore the Internet Explorer settings
Some of the changes that the Trojan makes to the registry will modify some Internet Explorer settings. One of the settings that it changes is to the home page.

To reset the home page:
  1. Start Microsoft Internet Explorer.
  2. Connect to the Internet and go to the page that you want to set as your home page.
  3. Click Tools, and then click Internet Options.
  4. In the Home page section of the General tab, click Use Current, and then click OK.

If you notice any other problems when using Internet Explorer, change these settings within the program. See your Internet Explorer documentation for instructions.

Writeup By: Maryl Magee