The Win32/Msblast family of network worms exploit a Remote Procedure Call (RPC) Distributed Component Object Model (DCOM) vulnerability on Microsoft Windows 2000 and Windows XP systems. The worm may also attempt denial of service (DoS) attacks on some server sites, or create a backdoor program that allows attackers to access the infected system.

What to do now

Recovering from recurring infections on a network
The following additional steps may need to be taken to completely remove this threat from an infected network, and to stop infections from recurring from this and other similar types of network-spreading malware:
  1. Ensure that an antivirus product is installed on ALL computers connected to the network that can access or host shares.
  2. Ensure that all available network shares are scanned with an up-to-date antivirus product.
  3. Restrict permissions as appropriate for network shares on your network. For more information on simple access control, please see:
  4. Remove any unnecessary network shares or mapped drives.
Note: Additionally it may be necessary to temporarily change the permission on network shares to read-only until the disinfection process is complete.
Removing the threat
To detect and remove this threat and other malicious software that may be installed in your computer, run a full-system scan with an up-to-date antivirus product such as the following:
For more information on antivirus software, see

Threat behavior

When Win32/MSblast variants run, they create a registry entry under the key HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run, so that the worm automatically starts when the system reboots.
The worm then attempts to spread to other systems on the Internet by selecting an IP address, either at random or based on the IP of the infected system, and incrementing that address. The worm attempts to connect to TCP port 135 on the target system and send exploit communication sequence. If the connection is established and the target computer is an unpatched Windows 2000 or Windows XP system, the worm spreads through TCP port 4444 and UDP port 69. The worm skips any IP address that has a part equal to 255.
The worm may perform other actions, including:
  • DoS attacks against Web sites, including,, or, if the day of the month is greater than 15 or the month of the year is greater than 8.
  • Changing the Internet Explorer home page.
  • Opening a backdoor program, listening at a random TCP port, that allows attackers to gain access to the infected system, and reports the port number and infected system IP address to a remote server.


Some customers whose computers have been infected may not notice the presence of the worm at all, while others who are not infected may experience problems because the worm is attempting to attack their computers. Typical symptoms include Windows XP and Windows Server2003 systems restarting every few minutes without user input, or Windows NT 4.0 and Windows 2000 systems becoming unresponsive.
If your computer is infected, you may see the following error message:
System Shutdown


Take the following steps to help prevent infection on your computer:
  • Enable a firewall on your computer.
  • Get the latest computer updates for all your installed software.
  • Use up-to-date antivirus software.
  • Limit user privileges on the computer.
  • Use caution when opening attachments and accepting file transfers.
  • Use caution when clicking on links to Web pages.
  • Avoid downloading pirated software.
  • Protect yourself against social engineering attacks.
  • Use strong passwords.
Enable a firewall on your computer
Use a third-party firewall product or turn on the Microsoft Windows Internet Connection Firewall.
Get the latest computer updates
Updates help protect your computer from viruses, worms, and other threats as they are discovered. It is important to install updates for all the software that is installed in your computer. These are usually available from vendor Web sites.
You can use the Automatic Updates feature in Windows to automatically download future Microsoft security updates while your computer is on and connected to the Internet.
Use up-to-date antivirus software
Most antivirus software can detect and prevent infection by known malicious software. To help protect you from infection, you should always run antivirus software, such as Microsoft Security Essentials, that is updated with the latest signature files. For more information, see
Limit user privileges on the computer
Starting with Windows Vista and Windows 7, Microsoft introduced User Account Control (UAC), which, when enabled, allowed users to run with least user privileges. This scenario limits the possibility of attacks by malware and other threats that require administrative privileges to run.
You can configure UAC in your computer to meet your preferences:
Use caution when opening attachments and accepting file transfers
Exercise caution with e-mail and attachments received from unknown sources, or received unexpectedly from known sources.  Use extreme caution when accepting file transfers from known or unknown sources.
Use caution when clicking on links to Web pages
Exercise caution with links to Web pages that you receive from unknown sources, especially if the links are to a Web page that you are not familiar with, unsure of the destination of, or suspicious of. Malicious software may be installed in your computer simply by visiting a Web page with harmful content.
Avoid downloading pirated software
Threats may also be bundled with software and files that are available for download on various torrent sites. Downloading "cracked" or "pirated" software from these sites carries not only the risk of being infected with malware, but is also illegal. For more information, see 'The risks of obtaining and using pirated software'.
Protect yourself from social engineering attacks
While attackers may attempt to exploit vulnerabilities in hardware or software to compromise a computer, they also attempt to exploit vulnerabilities in human behavior to do the same. When an attacker attempts to take advantage of human behavior to persuade the affected user to perform an action of the attacker's choice, it is known as 'social engineering'. Essentially, social engineering is an attack against the human interface of the targeted computer. For more information, see 'What is social engineering?'.
Use strong passwords
Attackers may try to gain access to your Windows account by guessing your password. It is therefore important that you use a strong password – one that cannot be easily guessed by an attacker. A strong password is one that has at least 8 characters, and combines letters, numbers, and symbols. For more information, see

Alert level: High
This entry was first published on: Nov 09, 2004
This entry was updated on: Apr 17, 2011

This threat is also detected as:
  • W32.Blaster.Worm (Symantec)
  • W32/Lovsan.worm (McAfee)
  • WORM_MSBLAST (Trend Micro)
  • Win32.Poza (CA)
  • Lovsan (F-secure)
  • Worm.Win32.Blaster (Global Hauri)
  • W32/Blaster (Norman)
  • Blaster (Panda)
  • W32/Blaster (Sophos)