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Discovered: March 24, 2004
Updated: February 13, 2007 12:20:12 PM
Also Known As: I-Worm.Snapper [Kaspersky], W32/Snapper@MM [McAfee], Snapper [F-Secure]
Type: Worm
Systems Affected: Windows

W32.Snapper.A@mm is a worm that spreads to all the contacts in the Windows Address Book. It does not send itself as an email attachment. Instead, it exploits the Internet Explorer Object Tag Vulnerability that is described in Microsoft Security Bulletin MS03-032. This vulnerability allows W32.Snapper.A@mm to automatically download and install the worm when the email is opened.

The email has the following characteristics:

From: <Spoofed>
Subject: Re:
Message: The message body consists of the following HTML code, which will appear to be a blank message when loaded by most mail clients:

<HTML><BODY><IFRAME src='http://<omitted>/banner.htm' style='display:none'></IFRAME></HTML></BODY>

Antivirus Protection Dates

  • Initial Rapid Release version March 24, 2004
  • Latest Rapid Release version August 08, 2016 revision 023
  • Initial Daily Certified version March 24, 2004
  • Latest Daily Certified version August 09, 2016 revision 001
  • Initial Weekly Certified release date March 24, 2004

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

Technical Description

W32.Snapper.A@mm consists of a .dll file, which may be found in the following locations:

    • %Windir%\ieload.dll
    • %System%\ieload.dll

When the .dll is loaded, it does the following:
    1. Copies itself as %System%\ieload.dll.

    2. Registers itself as a Browser Helper Object. This lets the worm run when Internet Explorer is started, by creating the following registry keys:

      Explorer\Browser Helper Objects\<random clsid>HKEY_CLASSES_ROOT\CLSID\<random clsid>

      where <random clsid> is a random value of the form "{########-####-####-####-############}".

    3. Creates the following registry entries to store configuration information:

      Internet Settings\PopupsLoaded
      Internet Settings\TimerTicks

    4. Stops the following processes:
      • NAVAPW32.EXE
      • CCAPP.EXE

    5. Periodically contacts a Web server on TCP port 80.

    6. Uses its own SMTP engine to send a message to all the contacts in the Windows Address Book.

      The message has the following format:

      From: <Spoofed>

      Subject: Re:

      Message: The message body consists of the following HTML code, which will appear to be a blank message when loaded by most mail clients:

      <HTML><BODY><IFRAME src='http://(omitted)/banner.htm' style='display:none'></IFRAME></HTML></BODY>
      When an email client loads this message, it downloads and displays the file Banner.htm.

    At the time of this writing, Banner.htm is a Web page that appears to be blank, but actually contains links to the worm. This page uses the Internet Explorer Object Tag Vulnerability described in Microsoft Security Bulletin MS03-032 to automatically download a malicious html file, Htmlhelp.cgi.

    This is an HTML file containing an encoded copy of the worm .dll. This file contains a VBScript that installs the worm as %Windir%\ieload.dll.


    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. Close all open browser windows.
    4. Run a full system scan and delete all the files detected as W32.Snapper.A@mm.
    5. Modify 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:
    Note: When you are completely finished with the removal procedure and are satisfied that the threat has been removed, 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. 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.

    3. Closing all the open browser windows

    As W32.Snapper.A@mm functions as a Microsoft Internet Explorer plugin, it is necessary to close all the open browser windows to remove it. If you are reading this writeup in Internet Explorer, print this writeup using our printer-friendly option at the top of the page, or note the following instructions, and then close all the open browser windows.

    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.Snapper.A@mm, click Delete.

    5. Modifying the registry

    Note: This worm adds several registry entries that are not harmful once the infected files are deleted. If you do not feel comfortable editing the registry as described below, consider this step optional.

    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:

      Internet Settings

    4. In the right pane, delete the following entries:


    5. Search the registry for the keys that load the worm as a Browser Helper Object:

    6. Click Edit > Find.

    7. In the "Find what" box, type:


    8. Make sure that the Data box is selected, and then click Find Next.

    9. Allow the search to run until it finds an entry similar to the following:


      in a registry key similar to the following:


      If you find such a key, note the subkey, and then delete it.

      For example, if you find an entry:

      (Default) = "C:\WINNT\System32\IELOAD.DLL"

      in the key:


      then delete this key:


    10. Navigate to the key:

      Explorer\Browser Helper Objects

      In the left pane, look for any subkeys that match the random CLSID you found in step e.

      For example: {3FFA7212-1CD4-4899-AF06-C356CC21EBBE}. If you find such a key, delete it.

    11. Exit the Registry Editor.

    Writeup By: Heather Shannon