Alert level

Win32/Sasser


Encyclopedia entry
Updated: Sep 19, 2006  |  Published: Nov 11, 2004

Aliases
  • W32/Sasser.worm (McAfee)
  • W32.Sasser.Worm (Symantec)
  • WORM_SASSER (Trend Micro)
  • Win32.Sasser (CA)
  • Sasser (F-secure)
  • Sasser (Panda)
  • W32/Sasser (Sophos)
  • W32/Sasser (Norman)

Alert Level (?)
Severe

Antimalware protection details
Microsoft recommends that you download the latest definitions to get protected.

Summary

Win32/Sasser is a family of network worms that exploit the Local Security Authority Subsystem Service (LSASS) vulnerability fixed in Microsoft Security Update MS04-011. The worm spreads by randomly scanning IP addresses for vulnerable machines and infecting any that are found.

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Symptoms

Your computer may be infected with Win32/Sasser if you experience one or more of the following symptoms:
  • You see an LSA Shell crash dialog box similar to the following screenshot:
  • Your computer restarts every few minutes without user interaction. You may see a system shutdown dialog box, similar to the following screenshot:
  • Your computer performance is decreased or your network connection is slow.

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Technical Information (Analysis)

When Win32/Sasser runs on a computer, it copies itself to the %WINDOWS% folder. In most cases, it adds a value to the registry key HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run. This value causes the worm to start when Windows is started.
Win32/Sasser acts as an FTP server listening on TCP port 5554. For each connection made on this port, the worm sends a copy of itself to that connected host using the file name <random number>_up.exe.
 
The worm generates random IP addresses using a certain logic and then sends the exploit shell code to these IP addresses on TCP port 445. If the exploit is successful, a command line shell listens on a TCP port of the remote infected machine. To complete the infection, the worm executes a remote shell script that instructs the newly infected machine to connect to the infecting host and download and execute the worm through FTP. The worm records the count of successful infections to a file on the C: drive.
 
Win32/Sasser also attempts to abort any unexpected system shutdown by calling AbortSystemShutdown every several seconds in a continuous loop.
Later variants of the worm may drop a variant of Netsky worm. Later variants may not infect Windows 2000 because they import IcmpSendEcho from IPHlpAPI.dll, which is not present in Windows 2000.

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Prevention

Take the following steps to prevent infection from Win32/Sasser:
  • Enable a firewall on your computer
  • Install the MS04-011 security update
  • Use up-to-date antivirus software

Enable a firewall on your computer

Win32/Sasser cannot infect computers running a firewall that blocks traffic on port 445. Use a third-party firewall product or turn on the Microsoft® Windows® XP Internet Connection Firewall.
To turn on the Internet Connection Firewall in Windows XP
  1. Click Start, and click Control Panel.
  2. Click Network and Internet Connections, and click Network Connections. If you do not see Network and Internet Connections, click Switch to Category View.
  3. Highlight a connection that you want to help protect, and click Change settings of this connection.
  4. Click Advanced, and select Protect my computer and network by limiting or preventing access to this computer from the Internet.
  5. Click OK.

Install the MS04-011 security update

Win32/Sasser exploits a Windows vulnerability on computers that do not have the MS04-011 security update installed. Installing the security update helps to prevent infection by Win32/Sasser. You can install the MS04-011 security update using Windows Update.
To install the MS04-011 security update using Windows Update
  1. Go to the Windows Update Web site at windowsupdate.microsoft.com.
  2. On the Windows Update site, click Scan for Updates. Windows Update scans your computer and returns a list of critical updates, including service packs.  
  3. In the Pick updates to install list, click Critical Updates and Service Packs. Windows Update creates a list of the updates appropriate for your computer, including MS04-011 if it is not installed. Critical updates are selected for download automatically.
  4. Click Review and install updates, and then click Install Now. You may need to restart your computer after installing the updates.
You should also turn on Automatic Updates so that your computer is automatically updated with the latest critical security updates.
To turn on Automatic Updates
  1. Click Start, and click Control Panel. 
  2. Click Performance and Maintenance. If you do not see Performance and Maintenance , click Switch to Category View.
  3. Click System.
  4. Click Automatic Updates, and select Keep my computer up to date.
  5. Select a setting. Microsoft recommends selecting Automatically download the updates, and install them on the schedule that I specify and setting a regular update time.
  6. If you choose to have Automatic Updates notify you in step 5, you will see a notification balloon when new downloads are available to install. Click the notification balloon to review and install updates.

Use up-to-date antivirus software

Most antivirus software can detect and prevent infection by known malicious software. You should always run antivirus software on your computer that is updated with the latest signature files to automatically help protect you from infection.

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