Win32/Yaha is a family of mass-mailing network worms that targets certain versions of Microsoft Windows. The worm spreads primarily by sending a copy of itself as an attachment to e-mail addresses gathered from an infected computer. It can also spread through mapped drives and writeable network shares. The worm can terminate security-related processes and conduct denial of service (DoS) attacks against certain Web sites.

Threat behavior

When Win32/Yaha runs, it may take actions such as the following:
  • Drop multiple copies of itself with various names to locations such as <system folder>, %windir%, <recycle folder>. It also copies itself to <startup folder>.
  • Modify the registry as follows:
    • Create a value in registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
      so that the worm runs each time Windows starts.
    • Create a value in each of the following keys: 
      so that the worm runs each time an executable file runs.
    • Modify registry key HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main
      to change the Internet Explorer default home page.
  • Spread through e-mail. The worm gathers e-mail addresses from the computer and saves them to a file. The worm sends a copy of itself as an e-mail attachment to those addresses. Due to an Internet Explorer vulnerability, the e-mail attachment can run automatically when the user previews or views the e-mail containing the attachment. Installing MS01-020 or MS01-027 patches this vulnerability, but the patch does not prevent the worm from running if the user opens the attachment.
  • Terminate processes and close windows for system utilities such as Task Manager.
  • Copy itself to all mapped drives.
  • Copy itself to all writeable network shares. It may add a line in the win.ini file on the network share host computer such as:
    run = <worm copy file name>
  • Perform denial of service (DoS) attacks against certain Web sites.
  • Display several text messages in sequence or display an image of shaking words to look like a screensaver. 


There may be no readily apparent indications that your computer is infected by Win32/Yaha. However, your computer may be infected by this worm if you notice any of the following symptoms:
  • Presence of the value: "c:\recycled\msmdm" %1 %*"
    in registry key HKEY_LOCAL_MACHINE\SOFTWARE\Classes\exefile\shell\open\command
  • Several text messages are displayed in sequence.
  • An image of shaking words that looks like a screensaver is displayed.


Take the following steps to help prevent infection on your system:
  • Enable a firewall on your computer.
  • Get the latest computer updates.
  • Use up-to-date antivirus software.
  • Use caution with unknown attachments.
  • Disconnect from unneeded mapped drives.

Enable a firewall on your computer

Use a third-party firewall product or turn on the Microsoft Windows XP Internet Connection Firewall.
To turn on the Internet Connection Firewall in Windows XP
  1. Click Start, and click Control Panel.
  2. Click Network and Internet Connections, and click Network Connections. If you do not see Network and Internet Connections, click Switch to Category View.
  3. Highlight a connection that you want to help protect, and click Change settings of this connection.
  4. Click Advanced, and select Protect my computer and network by limiting or preventing access to this computer from the Internet.
  5. Click OK.

Get the latest computer updates

Updates help protect your computer from viruses, worms, and other threats as they are discovered. You can use the Automatic Updates feature in Microsoft Windows XP to automatically download future Microsoft security updates while your computer is on and connected to the Internet.
To turn on Automatic Updates in Windows XP
  1. Click Start, and click Control Panel
  2. Click Performance and Maintenance. If you do not see Performance and Maintenance, click Switch to Category View.
  3. Click System.
  4. Click Automatic Updates, and select Keep my computer up to date.
  5. Select a setting. Microsoft recommends selecting Automatically download the updates, and install them on the schedule that I specify and setting a regular update time.
  6. If you choose to have Automatic Updates notify you in step 5, you will see a notification balloon when new downloads are available to install. Click the notification balloon to review and install updates.

Use up-to-date antivirus software

Most antivirus software can detect and prevent infection by known malicious software. You should always run antivirus software on your computer that is updated with the latest signature files to automatically help protect you from infection. If you don't have antivirus software installed, you can get it from one of several companies. For more information, see 

Use caution with unknown attachments

Use caution before opening unknown e-mail or IM attachments, even if you know the sender. If you cannot confirm with the sender that a message is valid and that an attachment is safe, delete the message immediately, and run up-to-date antivirus software to check your computer for viruses.

Disconnect from unneeded mapped drives

Malicious software can often spread over mapped network drives. Disconnect from drives mapped to your computer that you are not using.
To remove network shares in Windows XP
  1. On the Start menu, click My Computer.
  2. On the Tools menu, click Disconnect Network Drives…
  3. In the Disconnect Network Drives dialog box, click the drives to disconnect and click OK.

Alert level: Severe
This entry was first published on: Jun 13, 2005
This entry was updated on: Sep 19, 2006

This threat is also detected as:
  • W32/Yaha@MM (McAfee)
  • W32.Yaha@mm (Symantec)
  • WORM_YAHA.GEN (Trend Micro)