Threat Description

Zasil.B

Details

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

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


Removal


Automatic action

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

More

You may wish to refer to the Support Community for further assistance. You also may also refer to General Removal Instructions for a general guide on alternative disinfection actions.



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


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