Discovered: September 01, 2003
Updated: February 13, 2007 12:06:38 PM
Also Known As: W32/Lovsan.worm.f [McAfee], W32/Blaster-F [Sophos], WORM_MSBLAST.G [Trend], Win32.Poza.F [CA]
Type: Worm
Systems Affected: Windows
CVE References: CAN-2003-0352

W32.Blaster.F.Worm is a worm that exploits the DCOM RPC vulnerability (described in Microsoft Security Bulletin MS03-026 ) using TCP port 135. The worm targets only Windows 2000 and Windows XP computers. While Windows NT and Windows 2003 Servers are vulnerable to this exploit (if not properly patched), the worm is not coded to replicate to those systems. This worm attempts to download the Enbiei.exe file into the %Windir%\System32 folder, and then execute it.

W32.Blaster.F.Worm does not have a mass-mailing functionality.

Additional information is available in the Microsoft article, "What You Should Know About the Blaster Worm and Its Variants ."

We recommend that you block access to TCP port 4444 at the firewall level, and then block the following ports, if you do not use the following applications:

  • TCP Port 135, "DCOM RPC"
  • UDP Port 69, "TFTP"

The worm also attempts to perform a Denial of Service (DoS) on At the time of writing this description, resolves to a blank address.

Click here for more information on the vulnerability that this worm exploits, and to find out which Symantec products can help mitigate risks from this vulnerability.

NOTE: Beta Definition build #24246 or later will detect this variant.

Antivirus Protection Dates

  • Initial Rapid Release version September 02, 2003
  • Latest Rapid Release version September 28, 2010 revision 054
  • Initial Daily Certified version September 02, 2003
  • Latest Daily Certified version September 28, 2010 revision 036
  • Initial Weekly Certified release date September 03, 2003

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

Technical Description

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

  1. Checks to see whether a computer is already infected and whether the worm is running. If so, the worm will not infect the computer a second time.

  2. Adds the value:


    to the registry key:


    so that the worm runs when you start Windows.

  3. Generates an IP address and attempts to infect the computer that has that address. The IP address is generated according to the following algorithms:
    • For 40% of the time, the generated IP address is of the form A.B.C.0, where A and B are equal to the first two parts of the infected computer's IP address.

      C is also calculated by the third part of the infected system's IP address; however, for 40% of the time the worm checks whether C is greater than 20. If so, a random value less than 20 is subtracted from C. Once the IP address is calculated, the worm will attempt to find and exploit a computer with the IP address A.B.C.0.

      The worm will then increment the 0 part of the IP address by 1, attempting to find and exploit other computers based on the new IP address, until it reaches 254.

    • With a probability of 60%, the generated IP address is completely random.

  4. Sends data on TCP port 135 that may exploit the DCOM RPC vulnerability. The worm sends one of two types of data: either to exploit Windows XP or Windows 2000.

    For 80% of the time, Windows XP data will be sent; and for 20% of the time, the Windows 2000 data will be sent.

    • The local subnet will become saturated with port 135 requests.
    • While W32.Blaster.F.Worm cannot spread to the Windows NT or Windows Server 2003, unpatched computers running these operating systems may crash as a result of the worm's attempts to exploit them. However, if the worm is manually placed and executed on a computer running these operating systems, it can run and spread.
    • Due to the random nature of how the worm constructs the exploit data, this may cause the RPC service to crash if it receives incorrect data. This may manifest as svchost.exe, generating errors as a result of the incorrect data.
    • If the RPC service crashes, the default procedure under Windows XP and Windows Server 2003 is to restart the computer. To disable this feature, see step one of the "Removal Instructions" below.

  5. Uses Cmd.exe to create a hidden remote shell process that will listen on TCP port 4444, allowing an attacker to issue remote commands on an infected system.

  6. Listens on UDP port 69. When the worm receives a request from a computer to which it was able to connect using the DCOM RPC exploit, it sends Enbiei.exe to that computer and tells it to execute the worm.

  7. If the current date is the 16th through the end of the month for the months of January to August, or if the current month is September through December, the worm will attempt to perform a DoS on However, the attempt to perform the DoS will succeed only if one the following conditions is true:
    • The worm runs on a Windows XP computer that was either infected or restarted during the payload period.
    • The worm runs on a Windows 2000 computer that was infected during the payload period and has not been restarted since it was infected.
    • The worm runs on a Windows 2000 computer that has been restarted since it was infected, during the payload period, and the currently logged in user is Administrator.

  8. The DoS traffic has the following characteristics:
    • Is a SYN flood on port 80 of
    • Tries to send 50 HTTP packets every second.
    • Each packet is 40 bytes in length.
    • If the worm cannot find a DNS entry for, it uses a destination address of
    Some fixed characteristics of the TCP and IP headers are:
      • IP identification = 256
      • Time to Live = 128
      • Source IP address = a.b.x.y, where a.b are from the host ip and x.y are random. In some cases, a.b are random.
      • Destination IP address = dns resolution of ""
      • TCP Source port is between 1000 and 1999
      • TCP Destination port = 80
      • TCP Sequence number always has the two low bytes set to 0; the two high bytes are random.
      • TCP Window size = 16384

Mitigating the DoS Payload
Network administrators can use the following recommendations to mitigate the DoS payload:
  • Reroute to a special internal IP address: This will alert you to the infected machines if you have a server listening to catch the SYN flood.
  • Configure anti-spoofing rules on the routers, if this has not already been implemented: This will prevent a high percentage of packets from leaving the network. Using uRPF or egress ACLs would be effective.


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.


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

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.

Important Notes:
  • W32.Blaster.F.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 continuing 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 completely all 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 action may also work with other firewalls, although this has not been confirmed.
    • In many cases, on both Windows 2000 and XP, changing the settings of 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 then 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 click 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.F.Worm.
  5. Reverse the changes that the Trojan made to the registry.
For specific details on each of these steps, read the following instructions.

1. Disabling System Restore (Windows XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/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 one of the following articles:
For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article, "Antivirus Tools Cannot Clean Infected Files in the _Restore Folder ," Article ID: Q263455.

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.

3. Ending the Worm process
    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 Enbiei.exe.
    6. If you find the file, click it, and then click End Process.
    7. Exit the Task Manager.
4. Scanning for and deleting 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.Blaster.F.Worm, click Delete.

5. Reversing the changes made to the registry

: 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:


  4. In the right pane, delete the value:


  5. Exit the Registry Editor

Writeup By: Douglas Knowles