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Discovered: October 01, 2003
Updated: February 13, 2007 12:08:18 PM
Also Known As: W32/Zezer.worm.gen [Mcafee], I-Worm.Zezer [AVP]
Type: Worm
Systems Affected: Windows

W32.Zezer.Worm is a worm that sends itself by email to all the Hotmail addresses that it finds on your MSN Messenger contact list.

It steals cached network passwords and can terminate the processes of some security software.

The worm is written in Microsoft Visual Basic 6 and is packed with UPX. The Mswinsck.ocx file must be on the computer for the worm to run.

Antivirus Protection Dates

  • Initial Rapid Release version October 01, 2003
  • Latest Rapid Release version September 28, 2010 revision 054
  • Initial Daily Certified version October 01, 2003
  • Latest Daily Certified version September 28, 2010 revision 036
  • Initial Weekly Certified release date October 02, 2003

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

Writeup By: Kaoru Hayashi

Discovered: October 01, 2003
Updated: February 13, 2007 12:08:18 PM
Also Known As: W32/Zezer.worm.gen [Mcafee], I-Worm.Zezer [AVP]
Type: Worm
Systems Affected: Windows

When W32.Zezer.Worm is executed, it does the following:

  1. Copies itself as the following files:
    • %Startup%\Msnexec.exe
    • %Windows%Mscsgs.exe
    • %System%Mscsgs32.exe

  2. Sends the following email:

    To: The worm obtains Hotmail addresses from your MSN Messenger contact list.

    From: The "From: field will contain one of the following:

    Subject: The subject will be one of following:
    Windows Update
    MSN Messenger Update
    MSN Messenger vulnerability

    Attention All Microsoft Users: A patch has been issued to correct a vulnerability in MSN Messenger which can be performed by a malicious user in order to gain unauthorized access to compromised computers. Windows users who have MSN Messenger 4.x and higher versions are affected by this vulnerability and must download and install the patch labeled Msn_inst.exe , which is attached to this email message. For any support regarding this patch please contact support@microsoft.com for more information.

    Attachment: Msn_inst.exe

  3. Steals cached network passwords and sends them to a specific email address.

  4. Adds the value:


    to the registry key:


    and the value:

    "Mscsgs32"="%SYSTEM%MSCSGS32.EXE "

    to the registry key:


  5. In the registry key:


    changes the value to:


    so that you can no longer open the registry editor.

  6. Terminates the process of the following security software:
    • _AVP.EXE
    • _AVP32.EXE
    • _AVPCC.EXE
    • _AVPM.EXE
    • ACKWIN32.EXE
    • AVE32.EXE
    • AVNT.EXE
    • AVP.EXE
    • AVP32.EXE
    • AVPDOS32.EXE
    • AVPM.EXE
    • AVPTC32.EXE
    • AVWIN95.EXE
    • AVWUPD32.EXE
    • CFINET32.EXE
    • CLAW95.EXE
    • CLAW95CF.EXE
    • CLAW95CT.EXE
    • DV95.EXE
    • DV95_O.EXE
    • DVP95.EXE
    • DVP95_0.EXE
    • EFINET32.EXE
    • F-AGNT95.EXE
    • F-PROT.EXE
    • FPROT95.EXE
    • F-PROT95.EXE
    • FP-WIN.EXE
    • FRW.EXE
    • ICLOAD95.EXE
    • ICSUPP95.EXE
    • IOMON98.EXE
    • JED.EXE
    • JEDI.EXE
    • KPF.EXE
    • KPFW32.EXE
    • LOCKDOWN2000.EXE
    • N32SCAN.EXE
    • N32SCANW.EXE
    • NAVAPW32.EXE
    • NAVLU32.EXE
    • NAVW.EXE
    • NAVW32.EXE
    • NVC95.EXE
    • PAVW.EXE
    • PCCWIN98.EXE
    • RAV7.EXE
    • SCAN32.EXE
    • SCAN95.EXE
    • SERV95.EXE
    • SMC.EXE
    • SWEEP95.EXE
    • TCA.EXE
    • TDS2-98.EXE
    • TDS2-NT.EXE
    • VET32.EXE
    • VET95.EXE
    • VET98.EXE
    • VSCAN40.EXE
    • VSHWIN32.EXE
    • VSSCAN40.EXE
    • WFINDV32.EXE


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: Kaoru Hayashi

Discovered: October 01, 2003
Updated: February 13, 2007 12:08:18 PM
Also Known As: W32/Zezer.worm.gen [Mcafee], I-Worm.Zezer [AVP]
Type: Worm
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. Disable System Restore (Windows Me/XP).
  2. Update the virus definitions.
  3. Restart the computer in Safe mode or VGA mode.
  4. Run a full system scan and delete all the files detected as W32.Zezer.Worm.
  5. Delete the value that was added to the registry.
For specific details on each of these steps, read the following instructions.

1. Disabling 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:
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. 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 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.

Note: If the worm has stopped the processes, you will not be able to start you Symantec program or run LiveUpdate. In this case, download the Intelligent Updater files as described in the previous instructions.

3. Restarting the computer in Safe mode or VGA mode

Shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in Safe mode or VGA mode.
  • For Windows 95, 98, Me, 2000, or XP users, restart the computer in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."
  • For Windows NT 4 users, restart the computer in VGA mode.

4. 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.
  2. Run a full system scan.
  3. If any files are detected as infected with W32.Zezer.Worm, click Delete.

4. Deleting the value from the registry

CAUTION: 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:


    and click OK. (Notepad opens a text file.)

  3. Type, or copy and paste, the following text into the text file:




  4. Save the file as:


    in the root folder (usually C:\).

  5. Click Start, then Run.

  6. Type:

    regedit -s \repair.reg

    and then click OK.

  7. Click Start, and then click Run. (The Run dialog box appears.)

  8. Type regedit

    Then click OK. (The Registry Editor opens.)

  9. Navigate to the key:


  10. In the right pane, delete the value:


  11. Navigate to the key:


    Note: This key is not found on all the systems.

  12. In the right pane, delete the value:

    "Mscsgs32"="%SYSTEM%MSCSGS32.EXE "

  13. Exit the Registry Editor.

Writeup By: Kaoru Hayashi