Win32/Mydoom is a family of mass-mailing worms that spread through e-mail. Some variants also spread through peer-to-peer networks. The worm acts as a backdoor Trojan, which allows an attacker to access the infected system. This backdoor may be used to distribute other malicious software. Some variants of Win32/Mydoom launch denial of service (DoS) attacks against specific Web sites. 

What to do now

To detect and remove this threat and other malicious software that may be installed on your computer, run a full-system scan with an appropriate, up-to-date, security solution. The following Microsoft products detect and remove this threat:

For more information on antivirus software, see

Threat behavior

When Win32/Mydoom worm is executed, it copies itself to the %system% or %temp% directory. The worm also creates a registry value in one of the following keys:
This value causes the worm to start when Windows is started.
Win32/Mydoom creates a backdoor Trojan in %system% or %windows% directory. The backdoor Trojan allows unauthorized access to the infected system. The worm may load and execute the backdoor Trojan. The worm may modify the default values of the following registry keys to reference the backdoor Trojan; this causes Explorer.exe to load and execute the Trojan when the system restarts:
Win32/Mydoom may copy itself to the share folder of the Kazaa P2P application, in order to spread through P2P networks. The worm obtains the location of the share folder from the value DlDir0 in the registry key HKEY_CURRENT_USER\Software\Kazaa\Transfer.
Win32/Mydoom may copy itself to random directories on an infected system.
Win32/Mydoom collects e-mail addresses from files on an infected system and sends e-mail with an attached copy of the worm to the addresses. This function is the primary propagation method the worm uses.


If your computer is infected by Win32/Mydoom, you may notice one or more of the following symptoms:
  • Some variants create a text file containing random data that looks similar to the following screenshot:
  • Some variants overwrite the hosts file, which may block access to some Microsoft and antivirus vendor Web sites. The overwritten hosts file may look similar to the following screenshot:


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.
  • 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
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 in order to compromise a computer, they also attempt to exploit vulnerabilities in human behavior in order to do the same. When an attacker attempts to take advantage of human behavior in order 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: Severe
This entry was first published on: Nov 10, 2004
This entry was updated on: Aug 16, 2011

This threat is also detected as:
  • W32.Mydoom@mm (Symantec)
  • W32/Mydoom@MM (McAfee)
  • WORM_MYDOOM (Trend Micro)
  • Win32.Mydoom (CA)