What is the Sasser worm? and How To Fix it?

The Sasser worm infects machines via network connections. It can attack entire networks of computers or one single computer connected to the Internet. The worm exploits a known windows vulnerability that is easily patched, however few systems seem to have this patch installed. It attacks Windows 2000 and Windows XP machines along with Windows NT and Windows Server 2003.

The patch from Microsoft known as the MS04-011 Security Update fixes the following vulnerabilities:
LSASS Vulnerability
LDAP Vulnerability
PCT Vulnerability
Winlogon Vulnerability
Metafile Vulnerability
Help and Support Center Vulnerability
Utility Manager Vulnerability
Windows Management Vulnerability
Local Descriptor Table Vulnerability
H.323 Vulnerability
Virtual DOS Machine Vulnerability
Negotiate SSP Vulnerability
SSL Vulnerability
ASN.1 “Double-Free” Vulnerability


What are the Symptoms of the Sasser worm?
You'll see a screen similar to the one below when you are infected, this will countdown to zero and literally shut down the system completely. The warning will state "This shutdown was initiated by NT AUTHORITY\SYSTEM". The message will state that the system process lsass.exe terminated unexpectedly.

The message may be prefaced by another message:

You can disable this shutdown by following the steps below during the countdown

1. Click on Start, Run
2. Type in CMD and press ENTER
3. Type in the following command and press EnterSHUTDOWN -A
This will terminate the shutdown, however in most cases the system may be to unstable to try to recover and may need to be rebooted anyway.

How Does Sasser Infect My Computer?
When W32.Sasser.Worm runs, it does the following:

1) Attempts to create a mutex named Jobaka3l and exits if the attempt fails. This ensures that no more than one instance of the worm can run on the computer at any time.

2) Copies itself as to the %Windir% directory. This is usually the C:\WINDOWS or C:\WINNT directory.

3) Adds the value:"avserve.exe"="%Windir%\avserve.exe""avserve2.exe"="%Windir%\avserve2.exe""skynetave.exe"= "%Windows%\skynetave.exe"to the following registry key, so that the worm runs on Windows startup.HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run

4) Uses the AbortSystemShutdown API to hinder attempts to shut down or restart the computer.

5) Starts an FTP server on TCP port 5554. This server is used to spread the worm to other hosts.

6) Iterates through all the host IP addresses, looking for addresses without any of the following:
127.0.0.1
10.x.x.x
172.16.x.x - 172.31.x.x (inclusive)
192.168.x.x
169.254.x.x

7) Using one of these IP addresses, the worm then generates a random IP address. 52% of the time, the IP address is completely random. 23% of the time, the last three octets are changed to random numbers. 25% of the time, the last two octets are changed to random numbers.
Because the worm can create completely random addresses, any IP range can be infected. This process is made up of 128 threads, which demands a lot of CPU time. As a result, an infected computer may become so slow and barely usable.

8) Connects to the randomly generated IP address on TCP port 445 to determine if a remote computer is online.

9) If a connection is made to a remote computer, the worm will send shell code to it, which may cause it to open a remote shell on TCP port 9996.

10) Uses the shell on the remote computer to connect back to the infected computer's FTP server, running on TCP port 5554, and retrieve a copy of the worm. This copy will have a name consisting of four or five digits, followed by _up.exe. For example, 74354_up.exe.

11) The Lsass.exe process will crash after the worm exploits the Windows LSASS vulnerability. Windows will display the alert and shut down the system in 1 minute.

12) Creates a file at C:\win.log that contains the IP address of the computer that the worm most recently attempted to infect, as well as the number of infected computers.
How Can I Remove the Sasser worm?

Follow these steps in removing the Sasser worm.
1) Disconnect your computer from the local area network or Internet
2) Terminate the running program
Open the Windows Task Manager by either pressing CTRL+ALT+DEL, selecting the Processes tab or selecting Task Manager and then the process tab on WinNT/2000/XP machines.
Locate one of the following programs (depending on variation), click on it and End Task or End Process
avserve.exeavserve2.exeskynetave.exeany process running with the "_up.exe" suffix
Close Task Manager
3) Activate the Windows XP Firewall (if running Windows XP) or another firewall to prevent the worm from shutting your system down while downloading the patches.

To activate the Windows XP firewall, follow these steps.
1. Click on Start, Control Panel
2. Double-click on Networking and Internet Connections, then click on Network Connnections
3. Right-click on the connection you use to access the Internet and choose Properties
Click on the Advanced Tab and check the box"Protect my computer and network by limiting or preventing access to this computer from the Internet"
4. Click OK and close out of the Network and Control Panel

Remove the Registry entries

1. Click on Start, Run, Regedit
2. In the left panel go to
HKEY_LOCAL_MACHINE>Software>Microsoft>Windows>Current Version>Run
3. In the right panel, right-click and delete the following entry
"avserve.exe"="%Windir%\avserve.exe""avserve2.exe"="%Windir%\avserve2.exe""skynetave.exe"= "%Windows%\skynetave.exe"
Close the Registry Editor

4) Delete the infected files (for Windows ME and XP remember to turn off System Restore before searching for and deleting these files to remove infected backed up files as well)
5) Click Start, point to Find or Search, and then click Files or Folders.
Make sure that "Look in" is set to (C:\WINDOWS).
In the "Named" or "Search for..." box, type, or copy and paste, the file names:avserve.exeavserve2.exeskynetave.exeC:\win2.log
6) Click Find Now or Search Now.
7) Delete the displayed files & Empty the Recycle bin