Zasil trojan appeared in the beginning of November 2002. It was sent to many users in e-mail with one of the following attachment names:
Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.
Detailed instructions for F-Secure security products are available in the documentation found in 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.
When the trojan's file is run, it shows an x-rated picture, drops an executable file to temporary folder and runs it. This file copies itself as REGISTRY.EXE file into Windows folder and creates a startup key for itself in Windows Registry:
[HKLM\Software\Microsoft\Windows\CurrentVersion\Run] "Registry Services" = "%windir%\registry.exe"
On some operating system versions the trojan creates a different run key:
[HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer] Run = %windir%\registry.exe
The REGISTRY.EXE file is packed with UPX file compressor. When this file is run during every Windows session, it waits until a user connects to Internet and downloads a text file from an account at Geocities.com site. The downloaded file contains one or more URLs that the trojan tries to connect to. When the trojan connects to an URL, it adds information about user's IP address and the '<br>Second,email_zasil' string to the URL. This way the author of the trojan knows the IPs of computers where his trojan is installed.
Disinfection solution for this trojan is to delete its file and also to delete the dropper that came in e-mail.
Technical Details:Alexey Podrezov; F-Secure Corp., November 5th, 2002