Discovered: April 30, 2004
Updated: February 13, 2007 12:22:21 PM
Also Known As: W32/Sasser.worm.a [McAfee], WORM_SASSER.A [Trend], Worm.Win32.Sasser.a [Kaspersky, W32/Sasser-A [Sophos], Win32.Sasser.A [Computer Assoc, Sasser [F-Secure], W32/Sasser.A.worm [Panda]
Type: Worm
Systems Affected: Windows
CVE References: CAN-2003-0533



W32.Sasser.Worm is a worm that attempts to exploit the vulnerability described in Microsoft Security Bulletin MS04-011 . It spreads by scanning the randomly selected IP addresses for vulnerable systems.


Notes:
  • Rapid Release virus definitions, version 30/04/04 rev 70 (20040430.070) and greater, detect this threat.
  • This worm has an MD5 hash value of 0xA73C16CCD0B9C4F20BC7842EDD90FC20.


W32.Sasser.Worm can run on (but not infect) Windows 95/98/Me computers. Although these operating systems cannot be infected, they can still be used to infect the vulnerable systems to which they are able to connect. In this case, the worm will waste a lot of resources so that programs cannot properly run, including our removal tool. (On Windows 95/98/Me computers, run the tool in Safe mode.)

Security Response has provided some information to aid network administrators in ongoing efforts to track down W32.Sasser.Worm infected machines on their respective network. Please see the document, "Detecting traffic due to LSASS worms " for additional information.

Antivirus Protection Dates

  • Initial Rapid Release version May 01, 2004
  • Latest Rapid Release version September 28, 2010 revision 054
  • Initial Daily Certified version May 01, 2004
  • Latest Daily Certified version September 28, 2010 revision 036
  • Initial Weekly Certified release date May 01, 2004

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Writeup By: Takayoshi Nakayama

Discovered: April 30, 2004
Updated: February 13, 2007 12:22:21 PM
Also Known As: W32/Sasser.worm.a [McAfee], WORM_SASSER.A [Trend], Worm.Win32.Sasser.a [Kaspersky, W32/Sasser-A [Sophos], Win32.Sasser.A [Computer Assoc, Sasser [F-Secure], W32/Sasser.A.worm [Panda]
Type: Worm
Systems Affected: Windows
CVE References: CAN-2003-0533


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 %Windir%\avserve.exe.


      Note: %Windir% is a variable. The worm locates the Windows installation folder (by default, this is C:\Windows or C:\Winnt) and copies itself to that location.

    3. Adds the value:

      "avserve.exe"="%Windir%\avserve.exe"

      to the registry key:

      HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run

      so that the worm runs when you start Windows.

    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. Retrieves the IP addresses of the infected computer, using the Windows API, gethostbyname.


      Note: The worm will ignore any of the following IP addresses:
      • 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. Generates another IP address, based on one of the IP addresses retrieved from the infected computer.
      • 25% of the time, the last two octets of the IP address are changed to random numbers. For example, if A.B.C.D is the IP address retrieved in step 6, C and D will be random.
      • 23% of the time, the last three octets of the IP address are changed to random numbers. For example, if A.B.C.D is the IP address retrieved in step 6, B, C, and D will be random.
      • 52% of the time, the IP address is completely random.


        Notes:
      • Because the worm creates a completely random addresses 52% of the time, any IP address can be infected, including those ignored in step 6.
      • 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 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. Creates a ftp script file cmd.ftp on the attacked computer.

    11. Uses the shell on the remote computer to reconnect 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.

    12. 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 one minute.
    13. Deletes cmd.ftp on the attacked computer. If the exploit code process is halted, cmd.ftp will remain on the attacked computer.

    14. 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.


    Symantec Gateway Security 5400 Series and Symantec Gateway Security v1.0
    • Antivirus component: An update for the Symantec Gateway Security AntiVirus engine to protect against the W32.Sasser.Worm is now available. We advise Symantec Gateway Security 5000 Series users to run LiveUpdate.
    • IDS/IPS component: A signature for the Symantec Gateway Security 5400 Series that detects attacks against the Microsoft LSASS vulnerability was included in SU 8 released on April 14. A signature to detect the attacks against the Microsoft LSASS vulnerability on SGS v1.0 has been released. We advise Symantec Gateway Security 5000 Series users to run LiveUpdate.
    • Full application inspection firewall component: By default, Symantec's full application inspection firewall technology protects against the W32.Sasser.Worm by blocking attackers from accessing TCP/445, and the backdoor ports on infected systems (TCP/5554, TCP/9996). We urge Administrators to verify that their security policies do not allow inbound traffic to those ports.

    Symantec Enterprise Firewall 8.0
    By default, Symantec's full application inspection firewall technology protects against the W32.Sasser.Worm by blocking attackers from accessing TCP/445, and the backdoor ports on infected systems (TCP/5554, TCP/9996). We urge Administrators to verify that their security policies do not allow inbound traffic to those ports.

    Symantec Enterprise Firewall 7.0.x and Symantec VelociRaptor 1.5
    By default, Symantec's full application inspection firewall technology protects against the W32.Sasser.Worm by blocking attackers from accessing TCP/445, and the backdoor ports on infected systems (TCP/5554, TCP/9996). We urge Administrators to verify that their security policies do not allow inbound traffic to those ports.

    Symantec Clientless VPN Gateway 4400 Series
    This threat does not affect Symantec Clientless VPN Gateway v5.0. By default, the security gateway blocks access to TCP ports 445, 5554, and 9996.

    Symantec Gateway Security 300 Series
    By default, Symantec's stateful inspection firewall technology prevents an attacker from accessing TCP/445 on internal systems and the backdoor ports on infected systems (TCP 5554, 9996). We urge Administrators to verify that their security policy does not allow TCP/445, TCP/5554, TCP/9996 inbound and to use the AVpe feature of the SGS 300 series to make sure that all their antivirus clients are up-to-date with the most current virus definitions.

    Symantec Firewall/VPN 100/200 Series
    By default, Symantec's stateful inspection firewall technology prevents an attacker from accessing TCP/445 on internal systems and the backdoor ports on infected systems (TCP/5554, TCP/9996).

    Symantec Client Security
    Symantec has released a patch for Symantec Client Security 1.x and 2.0 that will identify the LSASS exploit with the MS_Windows_LSASS_RPC_DS_Request signature present in the infection attempt. If the Sasser worm application already exists on the system, all versions of Symantec Client Security with the default firewall policy will prompt the user to Permit/Block/Configure a rule for the worm when it tries to start the FTP server and send outbound data.

    Virus definitions that provide protection against the worm are available through LiveUpdate or Intelligent Updater as of May 1, 2004.

    Recommendations

    Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

    • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
    • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
    • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
    • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
    • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
    • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
    • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
    • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
    • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
    • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
    • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
    • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
    • For further information on the terms used in this document, please refer to the Security Response glossary.

    Writeup By: Takayoshi Nakayama

    Discovered: April 30, 2004
    Updated: February 13, 2007 12:22:21 PM
    Also Known As: W32/Sasser.worm.a [McAfee], WORM_SASSER.A [Trend], Worm.Win32.Sasser.a [Kaspersky, W32/Sasser-A [Sophos], Win32.Sasser.A [Computer Assoc, Sasser [F-Secure], W32/Sasser.A.worm [Panda]
    Type: Worm
    Systems Affected: Windows
    CVE References: CAN-2003-0533


    Before you begin :
    If you are running Windows 2000 or XP, and have not yet done so, you must patch for the vulnerability described in Microsoft Security Bulletin MS04-011 . If you do not, it is likely that your computer will continue to be reinfected.

    What to do if the computer shuts down before you can patch or get the tool
    This threat can cause Windows to keep shutting down and restarting. This can prevent you from installing the Microsoft patch or downloading the tool described below.


      Notes:
    • You may have to try this several times, as you only have about 20 seconds to do steps 3 to 6.
    • This will not work on Windows 2000.

    To prevent the shut down, do the following:
    1. Disconnect the computer from the network/Internet connection. (Disconnect the cable if necessary.)
    2. Restart the computer.
    3. As soon as Windows opens and you see the Windows desktop, click Start > Run.
    4. Type:

      cmd

      and press Enter.

    5. Type:

      shutdown -i

      and press Enter.

    6. In the Remote Shutdown Dialog that opens, do the following:
      1. Click Add, type your computer name into the Add Computers dialog box, and then click OK.
      2. In the "Display warning for" field, type 9999.
      3. Type the following text in the Comment box:

        Delay Lsass.exe shutdown.

      4. Click OK.

    7. Reconnect the network/Internet connection.
    8. Connect to the Internet, and get the patch. Then continue with the steps described below.

    When you have patched your computer and removed the threat, you can re-enable the 20 second default warning if you wish.


    Removal using the W32.Sasser Removal Tool
    Symantec Security Response has developed a removal tool to clean the infections of W32.Sasser.Worm. Use this tool first, as it is the easiest way to remove this threat.

    Manual Removal
    The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.
    1. End the malicious process (Windows NT/2000/XP).
    2. Disable System Restore (Windows XP).
    3. Update the virus definitions.
    4. Run a full system scan and delete all the files detected as W32.Sasser.Worm.
    5. Reverse the change made to the registry.
    For details on each of these steps, read the following instructions.

    1. To end the malicious process
    On Windows NT/2000/XP computers, you must first end the malicious process. Follow these instructions:
    1. Press Ctrl+Alt+Delete once.
    2. Click Task Manager.
    3. Click the Processes tab.
    4. Double-click the Image Name column header to alphabetically sort the processes.
    5. Scroll through the list and look for the following processes:
      • avserve.exe
      • any process with a name consisting of four or five digits, followed by _up.exe (for example, 74354_up.exe).
    6. If you find any such process, click it, and then click End Process.
    7. Exit the Task Manager.
    2. To disable System Restore (Windows XP)
    If you are running Windows XP, we recommend that you temporarily turn off System Restore. Windows XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.

    Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.

    Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.

    For instructions on how to turn off System Restore, read your Windows documentation, or "How to turn off or turn on Windows XP System Restore "

    Note: When you are completely finished with the removal procedure and are satisfied that the threat has been removed, re-enable System Restore by following the instructions in the aforementioned documents.

    3. To update the virus definitions
    Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:

    • Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
    • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).

      The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.

    4. To scan for and delete the infected files
    1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
    2. Run a full system scan.
    3. If any files are detected as infected with W32.Sasser.Worm, click Delete.

    5. To reverse the change made to the registry


    WARNING: Symantec strongly recommends that you back up the registry before making any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified keys only. Read the document, "How to make a backup of the Windows registry ," for instructions.
    1. Click Start, and then click Run. (The Run dialog box appears.)
    2. Type regedit

      Then click OK. (The Registry Editor opens.)

    3. Navigate to the key:

      HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run

    4. In the right pane, delete the value:

      "avserve.exe"="%Windir%\avserve.exe"

    5. Exit the Registry Editor.


    Writeup By: Takayoshi Nakayama