Threat Description



Aliases: Zasil.B, Windows-Update, Critical Security Hole, Windows Update
Category: Malware
Platform: W32


Zasil.B trojan downloader appeared on 25th of June 2003. The following e-mail message was sent to a large amount of people:


IMPORTANT!! Critical security hole in Windows!


Dear Windows User!
 New Windows 9x/2000/NT/XP critical patch has been released.
 Due to security problems, your system needs to be updated as earlier as
 You can download an update patch on Windows Update site: Best regards, Windows Update Group


Automatic Disinfection

Allow F-Secure Anti-Virus to disinfect the relevant files.

For more general information on disinfection, please see Removal Instructions.

Technical Details

When a recipient clicks on the provided link, his browser connects to the fake windows update site, downloads and activates a file named UPDATE0932.EXE. That file is a downloader called Zasil.B. The downloader connects to another website and fetches the RQ.TXT file. This plain text file contains a link to another executable file. According to reports the RQ.TXT file originally contained a link to WINPWR32.EXE file which is an installation package with a lot of hacker tools and IRC trojans inside. But after some time the contents of RQ.TXT file were changed. At the moment of writing of this description the file contains a link to SVSGHOST.EXE file which is an IRC backdoor (hacker's remote access tool).

Zasil browses the contents of RQ.TXT file, downloads and activates the backdoor file mentioned there. As a result a user's computer becomes infected.

F-Secure Anti-Virus detects the backdoor generically as 'Backdoor.SdBot.gen' with the latest updates. Detection for Zasil.B downloader will be added shortly.

Description Created: F-Secure Anti-Virus Research Team; F-Secure Corp.; June 25th, 2003


Suspect a file or URL was wrongly detected? Submit a sample to our Labs for analysis

Submit Now

Give And Get Advice

Give advice. Get advice. Share the knowledge on our free discussion forum.

Learn More