Threat Description



Category: Malware
Platform: W32
Aliases: Sasser.F, W32/Sasser.F, Worm.Win32.Sasser.a


Sasser.F is a minor modification of the Sasser.A worm. It shares all of its code and functionality, although it uses a different filename when copying itself into the system and a different mutex name.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.

More scanning & removal options

More information on scanning or removal options is available in the documentation for your F-Secure security product on the Downloads section of our Home - Global site.

You may also refer to the Knowledge Base on the F-Secure Community site for further assistance.

Eliminating a Local Network Outbreak

If the infection is in a local network, please follow the instructions on this webpage:

Manual Disinfection

To manually disinfect an infected system, first apply the Microsoft patch MS04-011, then use Task Manager to kill the 'napatch.exe' process, then delete the file 'napatch.exe' from your Windows directory and reboot.

For step-by-step instructions, see Microsoft's site:

Technical Details

Sasser.F is an unpacked, edited and repacked version of Sasser.A.For full details, please refer to the Sasser.A description.

System Infection

When the worm enters the system it creates a copy of itself in the Windows Directory as 'napatch.exe'. This copy is added to the Registry as

[SOFTWARE\Microsoft\Windows\CurrentVersion\Run]   "napatch.exe" = "%WinDir%\napatch.exe"  

To ensure that only one copy of the worm is running it creates a mutex named 'billgate'.


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