Summary

Dasher.A is a worm that exploits a vulnerability in Microsoft Windows Distributed Transaction Coordinator (MS05-051).

Additional Details

Installation to system

The worm's dropper is a self-extracing RAR archive which drops the following files:

%windir%\Temp\SqlExp.exe
%windir%\Temp\Sqlrep.exe
%windir%\Temp\SqlScan.exe
%windir%\Temp\Sqltob.exe

The file Sqltob.exe is the actual worm main file. Sqlrep.exe is utilty called "Replace Commander". SqlScan.exe is a port scan utilty and SqlExp.exe is a component that is used in MSDTC exploiting.

When the main file is run, it adds the following registry entry to ensure that it is started when a user logs on or the system is restarted:

[HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]
"Windows Update" = "%windir%\Temp\Sqltob.exe"

The worm may also create the following files:

%windir%\Temp\Result.txt
%windir%\Temp\SqlScan.bat

These files are used in exploiting.

Spreading using MSTDC vulnerability

The worm scans for systems vulnerable to MSTDC (MS05-051) through TCP/1025. It constructs random addresses using a fixed list of A-class networks. If it finds a system responding to TCP SYN scan, it sends the exploit payload. The payload connects to remote address and waits for instructions. At the time of this writing, the remote server is not accessible. According to reports, the server may instruct the infected system to download and activate the worm's dropper.

Please see the following page for detailed information on the vulnerability:

http://www.microsoft.com/technet/security/Bulletin/MS05-051.mspx

Detection

Dasher.A is detected with the following F-Secure Anti-Virus updates:

[FSAV_Database_Version]

Version = 2005-12-15_03

Write-up: Jarkko Turkulainen

Technical Details: Jarkko Turkulainen, December 15, 2005

Description Updated: Jarkko Turkulainen, December 15, 2005

F-Secure Corporation