Damage

• Payload:
  • Causes system instability: May cause machines to crash.
  • Compromises security settings: Opens a hidden remote cmd.exe shell.

Distribution

• Ports: TCP 135, TCP 4444, UDP 69
• Target of infection: Machines with vulnerable DCOM RPC Services running.

When W32.Blaster.Worm is executed, it does the following:

1. Creates a Mutex named "BILLY". If the mutex exists, the worm will exit.
   
   
2. Adds the value:
   
   "windows auto update"="msblast.exe"
   
   to the registry key:
   
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
   
   so that the worm runs when you start Windows.
   
   
3. Calculates a random IP address, A.B.C.0, where A, B, and C are random values between 0 and 255.
   
   NOTE: 40% of the time, if C > 20, a random value less than 20 will be subtracted from C.
   
   
4. Once the IP address is calculated, the worm will attempt to find and exploit a computer on the local subnet, based on A.B.C.0. The worm will then count up from 0, attempting to find and exploit other computers, based on the new IP.
   
   
5. Sends data on TCP port 135 that may exploit the DCOM RPC vulnerability.
   
   NOTES:
   • This means the local subnet will become saturated with port 135 requests.
   • Due to the random nature of how the worm constructs the exploit data, this may cause computers to crash if it sends incorrect data.
   • While W32.Blaster.Worm cannot spread to Windows NT or Windows 2003 Server, unpatched computers running these operating systems may crash as the result of attempts by the worm to exploit them. However, if the worm is manually placed and executed on a computer that is running these operating systems, it can run and spread.
     
     
6. Creates a hidden Cmd.exe remote shell that will listen on TCP port 4444, allowing an attacker to issue remote commands on the infected system.
   
   
7. Listens on UDP port 69. When the worm receives a request from a computer it was able to connect to using the DCOM RPC exploit, it will send that computer msblast.exe and tell it to execute the worm.
   
   
8. If the current month is after August, or if the current date is after the 15th, the worm will perform a DoS on Windows Update. The worm will activate the DoS attack on the 16th of this month, and continue until the end of the year.
   

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

• Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates.
• If a blended 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.
• 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.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate infected computers quickly to prevent further compromising your organization. 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.

Removal using the W32.Blaster.Worm Removal Tool
Symantec Security Response has developed a removal tool to clean infections of W32.Blaster.Worm. This is the easiest way to remove this threat and should be tried first.

Manual Removal
As an alternative to using the removal tool, you can manually remove this threat.

The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

• W32.Blaster.Worm exploits the DCOM RPC vulnerability. This is described in Microsoft Security Bulletin MS03-026, and a patch is available there. You must download and install the patch. In many cases, you will need to do this before you can continue with the removal instructions. If you are not able to remove the infection or prevent re-infection using the following instructions, first download and install the patch.
• Because of the way the worm works, it may be difficult to connect to the Internet to obtain the patch, definitions, or removal tool before the worm shuts down the computer. There are at least two known ways to work around this, although neither solution works 100% of the time.
  
  • If you run Windows XP, activating the Windows XP firewall may allow you to download and install the patch, obtain virus definitions, and run the removal tool. This may also work with other firewalls, although this has not been confirmed.
    
  • In many cases, on both Windows 2000 and XP, changing settings for the Remote Call Procedure (RPC) Service may allow you to connect to the Internet without the computer shutting down. Follow these steps:
    1. Do one of the following:
       • Windows 2000. Right-click the My Computer icon on the Windows desktop and then click Manage. The Computer Management window opens.
       • Windows XP. Click the Start button, right-click the My Computer icon, click Manage. The Computer Management window opens.
         
    2. In the left pane, double-click Services and Applications and then select Services. A list of services appears.
    3. In the right pane, locate the Remote Procedure Call (RPC) service.
       
       CAUTION: There is also a service named Remote Procedure Call (RPC) Locator. Do not confuse the two
       
    4. Right-click the Remote Procedure Call (RPC) service and click Properties.
    5. Click the Recovery tab.
    6. Using the drop-down lists, change First failure, Second failure, and Subsequent failures to "Restart the Service."
    7. Click Apply and then OK
       
       CAUTION: Make sure that you change these settings back when you have removed the worm

1. Disable System Restore (Windows XP).
2. Update the virus definitions.
3. End the Trojan process.
4. Run a full system scan and delete all the files detected as W32.Blaster.Worm.
5. Reverse the changes that the Trojan made to the registry.
   

• "How to disable or enable Windows Me System Restore"
• "How to turn off or turn on Windows XP System Restore"

2. Updating 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.

To end the Trojan process:
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 msblast.exe.
6. If you find the file, click it, and then click End Process.
7. Exit the Task Manager.
   

1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
   • For Norton AntiVirus consumer products: Read the document, "How to configure Norton AntiVirus to scan all files."
   • For Symantec AntiVirus Enterprise products: Read the document, "How to verify that a Symantec Corporate antivirus product is set to scan all files."
2. Run a full system scan.
3. If any files are detected as infected with W32.Blaster.Worm, click Delete.

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:
   
   "windows auto update"="msblast.exe"
   
   
5. Exit the Registry Editor.

Revision History:

August 13, 2003:

• Added download location.

• Upgraded to Category 4 from Category 3, based on increased rate of submissions.
• Added additional aliases.
• Updated Technical Description.
• Added information to removal on changing settings for RPC.

• 

Write-up by: Douglas Knowles