Win32/Sobig is a family of mass-mailing worms that targets computers running certain versions of Microsoft Windows. The worm sends itself to e-mail addresses that it finds on the infected computer. The worm may also spread to writeable network shares.

Threat behavior

When a Win32/Sobig variant runs, it copies itself to the Windows folder, and creates an entry in the following registry keys:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run, HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
These registry entries cause the worm to run automatically whenever Windows starts. The worm then attempts to spread by sending itself as an attachment to e-mail addresses that it finds on the infected computer. It also attempts to spread through writeable network shares. Some variants also download and run files from some URLs.


There may be no readily apparent indications that your computer is infected by a variant of the Win32/Sobig family. However, your computer may be infected by this worm if you notice any of the following symptoms:
  • Significant slowdowns during normal operation.
  • Increased outbound SMTP traffic.


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.
  • Remove unneeded network shares.

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.

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.

Remove unneeded network shares

Malicious software can often spread over network shares. Remove unneeded network shares that are mapped to your computer.
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: Feb 14, 2005
This entry was updated on: Sep 19, 2006

This threat is also detected as:
  • Win32.Sobig (CA)
  • W32.Sobig@mm (Symantec)
  • Sobig (F-secure)
  • I-Worm.Win32.Sobig (Global Hauri)
  • Sobig (Panda)
  • W32/Sobig (Sophos)
  • W32/Sobig@mm (Norman)
  • WORM_SOBIG (Trend Micro)
  • W32/Sobig@MM (McAfee)