Summary

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

Subject:

 IMPORTANT!! Critical security hole in Windows!

Body:

 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
 possible.

 You can download an update patch on Windows Update site:
 http://www.windows-update.com Best regards, Windows Update Group

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