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 security response

W32.Sasser Removal Tool

Discovered on: May 1, 2004
Last Updated on: May 2, 2004 11:05:03 AM GDT

Symantec Security Response has developed a removal tool to clean the infections of the following variants of the W32.Sasser worm:



What the tool does

The W32.Sasser Removal Tool does the following:
  1. Terminate the W32.Sasser viral processes
  2. Delete the W32.Sasser files
  3. Delete the registry values that the worm adds

Available command-line switches for this tool


Switch
Description

/HELP, /H, /?
Displays the help message.

/NOFIXREG
Disables the registry repair (We do not recommend using this switch).

/SILENT, /S
Enables the silent mode.

/LOG=<path name>
Creates a log file where <path name> is the location in which to store the tool's output. By default, this switch creates the log file, FxSasser.log, in the same folder from which the removal tool was executed.

/MAPPED
Scans the mapped network drives (We do not recommend using this switch. See the following Note).

/START
Forces the tool to immediately start scanning.

/EXCLUDE=<path>
Excludes the specified <path> from scanning (We do not recommend using this switch. See the following Note).

/NOFILESCAN
Prevents the scanning of the file system.

Note: Using the /MAPPED switch does not ensure the complete removal of the virus on the remote computer, because:
    • The scanning of the mapped drives scans only the mapped folders. This may not include all the folders on the remote computer, which can lead to missed detections.
    • If a viral file is detected on the mapped drive, the removal will fail if a program on the remote computer uses this file.

Therefore, you should run the tool on every computer.

The /EXCLUDE switch will only work with one path, not multiple. An alternative is the /NOFILESCAN switch followed by a manual scan with AntiVirus. This will let the tool alter the registry. Then, scan the computer with AntiVirus and with the current virus definitions. You should be able to clean the file system after completing these steps.

The following example command line can be used to exclude a single drive:

>"C:\Documents and Settings\user1\Desktop\FxSasser.exe" /EXCLUDE=M:\ /LOG=c:\FxSasser.txt


Alternatively, the command line below will skip the scanning of the file system, but will repair the registry modifications. Run a regular scan of the system with the proper exclusions:

>"C:\Documents and Settings\user1\Desktop\FxSasser.exe" /NOFILESCAN /LOG=c:\FxSasser.txt

Notes:
  • The greater than symbol (>) is not part of the path.
  • The name of the log file can be whatever you select. The name listed is for the sole purpose of this example.

Obtaining and running the tool
Note: You must have administrative rights to run this tool on Windows NT 4.0, Windows 2000, or Windows XP.

WARNING: For network administrators. If you are running MS Exchange 2000 Server, we recommend that you exclude the M drive from the scan by running the tool from a command line with the Exclude switch. For more information, read the Microsoft knowledge base article, "XADM: Do Not Back Up or Scan Exchange 2000 Drive M" (Article 298924).

  1. Download the FxSasser.exe file from: http://securityresponse.symantec.com/avcenter/FxSasser.exe. Note: Version 1.0.1 (As shown in removal tool dialog title bar) provides support for both W32.Sasser.B.Worm and W32.Sasser.Worm.
  2. Save the file to a convenient location, such as your downloads folder or the Windows desktop, or removable media known to be uninfected.
  3. To check the authenticity of the digital signature, refer to the "Digital signature" section later in this writeup.
  4. Close all the running programs before running the tool.
  5. If you are on a network or if you have a full-time connection to the Internet, disconnect the computer from the network and the Internet.
  6. If you are running Windows Me or XP, then disable System Restore. Refer to the "System Restore option in Windows Me/XP" section later in this writeup for further details.

    Caution    : If you are running Windows Me/XP, we strongly recommend that you do not skip this step.
  7. Double-click the FxSasser.exe file to start the removal tool.
  8. Click Start to begin the process, and then allow the tool to run.
  9. Restart the computer.
  10. Run the removal tool again to ensure that the system is clean.
  11. If you are running Windows Me/XP, then re-enable System Restore.
  12. Run LiveUpdate to make sure that you are using the most current virus definitions.

Note: The removal procedure may not be successful if Windows Me/XP System Restore is not disabled as previously directed, because Windows prevents outside programs from modifying System Restore.

When the tool has finished running, you will see a message indicating whether W32.Sasser infected the computer. In the case of a removal of the worm, the program displays the following results:
  • Total number of scanned files
  • Number of deleted files
  • Number of repaired files
  • Number of terminated viral processes
  • Number of fixed registry entries

Digital signature
FxSasser.exe is digitally signed. Symantec recommends that you use only copies of FxSasser.exe, which have been directly downloaded from the Symantec Security Response Web site. To check the authenticity of the digital signature, follow these steps:
  1. Go to http://www.wmsoftware.com/free.htm.
  2. Download and save the chktrust.exe file to the same folder in which you saved FxSasser.exe (for example, C:\Downloads).
  3. Depending on your operating system, do one of the following:
    • Click Start, point to Programs, and then click MS-DOS Prompt.
    • Click Start, point to Programs, click Accessories, and then click Command Prompt.

  4. Change to the folder in which FxSasser.exe and Chktrust.exe are stored, and then type: chktrust -i FxSasser.exe.

    For example, if you saved the file to the C:\Downloads folder, you would enter the following commands:

    cd\
    cd downloads
    chktrust -i FxSasser.exe

    Press Enter after typing each command. If the digital signature is valid, you will see the following:

    "Do you want to install and run "W32.Sasser Removal Tool" signed on 05/01/2004 10:248 PM and distributed by: Symantec Corporation"

    Note
    • The date and time displayed in this dialog box will be adjusted to your time zone, if your computer is not set to the Pacific time zone.
    • If you are using Daylight Saving time, the displayed time will be exactly one hour earlier.
    • If this dialog box does not appear, there are two possible reasons:
      • The tool is not from Symantec: Unless you are sure that the tool is legitimate and that you downloaded it from the legitimate Symantec Web site, you should not run it.
      • The tool is from Symantec and is legitimate: However, your operating system was previously instructed to always trust content from Symantec. For information on this and on how to view the confirmation dialog again, read the document, "How to restore the Publisher Authenticity confirmation dialog box."

  5. Click Yes to close the dialog box.
  6. Type exit, and then press Enter. (This will close the MS-DOS session.)

System Restore option in Windows Me/XP
Users of Windows Me and Windows XP should 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 onto 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.

Revision History:

  • May 2, 2004: Updated fixtool ( version 1.0.1 ) to remove infections of W32.Sasser.B.Worm.