Summary

Sinit is a backdoor that opens a random UDP port and allows unauthorized access to infected machine. According to reports, this port can be used as a part of peer-to-peer network enabling uncentralized distribution of malware to infected computers.

Additional Details

System installation

When run, the backdoor copies itself to the System directory as 'svcinit.exe'. It adds the following registry key to ensure it will be executed when the system is started:

 [HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon]
 "Userinit" = "%System%\userinit.exe,%System%\svcinit.exe"

If the system is running windows 9x (95, 98 or ME), it uses the following key instead:

 [HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunServices]
 "SVC Service" = "%System%\svcinit.exe"

Backdoor

Sinit opens two UDP ports, port 53 and a random port. Using these ports, the backdoor can receive updates and other programs. It should be noted that domain name protocol (DNS) also uses UDP port 53, but the protocol is completely different.

The backdoor also opens a TCP port 53 which acts as a simple HTTP server. When a HTTP GET request for 'ks.exe' or 'kx.htm' is received, the backdoor sends a copy of itself.

Detection

F-Secure Anti-Virus detects various variants of the Sinit backdoor starting with the following update:

[FSAV_Database_Version]
Version=2003-04-25_01
Technical Details: Jarkko Turkulainen, June 22th, 2005;