Zasil.B trojan downloader appeared on 25th of June 2003. The following email 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 possible. You can download an update patch on Windows Update site: http://www.windows-update.com Best regards, Windows Update Group
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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.