Microsoft security software detects and removes this threat.
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.

What to do now

Use the following free Microsoft software to detect and remove this threat:

You should also run a full scan. A full scan might find hidden malware.

Get more help

You can also visit our advanced troubleshooting page or search the Microsoft virus and malware community for more help.

If you’re using Windows XP, see our Windows XP end of support page.

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. 


The following can indicate that you have this threat on your PC:
  • 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.


Alert level: Severe
This entry was first published on: Jun 13, 2005
This entry was updated on: Jul 16, 2015

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