Worm:Win32/Zotob.C is a network worm that exploits the Plug-and-Play vulnerability fixed in Microsoft Security Bulletin MS05-039. The worm targets computers running Microsoft Windows 2000 that do not have MS05-039 installed. The worm can also infect computers running other versions of Windows operating systems if it is delivered through e-mail, instant messaging, or other routes.

What to do now

To manually recover from infection by Worm:Win32/Zotob.C, follow these steps:
  1. Install security update MS05-039.
  2. Disconnect from the Internet.
  3. End the worm process.
  4. Delete the worm files from your computer.
  5. Delete the worm registry entries.
  6. Clean the system host file.
  7. Restart your computer.
  8. Take steps to prevent re-infection.
  9. Run an antivirus scan.
Install security update MS05-039
To install MS05-039 using Windows Update
  1. Go to the Windows Update Web site at
  2. On the Windows Update site, click Scan for Updates. Windows Update scans your computer and returns a list of critical updates, including service packs.
  3. In the Pick updates to install list, click Critical Updates and Service Packs. Windows Update creates a list of the updates appropriate for your computer, including MS05-039 if it is not installed. Critical updates are selected for download automatically.
  4. Click Review and install updates, and then click Install Now. You may need to restart your computer after installing the updates.
Disconnect from the Internet
To help ensure that your computer is not actively infecting other computers, disconnect it from the Internet before proceeding. Print this Web page or save a copy on your computer; then unplug your network cable and disable your wireless connection. You can reconnect to the Internet after completing these steps.
End the worm process
Ending the worm process will help stop your computer from infecting other computers as well as resolve the crashing, rebooting, and performance degradation issues caused by the worm.
To end the worm process
  1. Press CTRL+ALT+DEL once and click Task Manager.
  2. Click Processes and click Image Name to sort the running processes by name.
  3. Select the process per.exe, and click End Process.
Delete the worm files from your computer
To delete the worm files from your computer
  1. Click Start, and click Run.
  2. In the Open field, type the name of the system folder, for example, C:\Winnt\system32\
  3. Click OK.
  4. Click Name to sort files by name.
  5. If per.exe is in the list, delete it.
  6. On the Desktop, right-click the Recycle Bin and click Empty Recycle Bin.
  7. Click Yes.
If deleting the files fails, follow these steps to verify that per.exe is not running:
  1. Press CTRL+ALT+DEL once and click Task Manager.
  2. Click Processes and click Image Name to sort the running processes by name.
  3. Confirm that per.exe is not in the list.
Delete the worm registry entries
Worm:Win32/Zotob.C creates entries in the Windows registry that attempt to run the worm every time your computer restarts. These entries should be deleted.
To delete the worm registry entries
  1. On the Start menu, click Run.
  2. Type regedit and click OK.
  3. In the left pane, navigate to the key: HKLM\Software\Microsoft\Windows\CurrentVersion\Run
    In the right pane, right-click the following value, if it exists:
  4. Click Delete and click Yes to delete the values.
  5. Repeat steps 3-4 for HKLM\Software\Microsoft\Windows\CurrentVersion\RunServices.
  6. Close Registry Editor.
Clean the system host file
The worm makes changes to the system host file to prevent access to certain Web sites.
To clean the system host file.
  1. On the Start menu, click Run.
  2. Type notepad.exe and click OK.
  3. On the File menu, click Open…
  4. In the File name text box, type the name of the Windows directory folder and \system32\drivers\etc\hosts, for example, C:\winnt\system32\drivers\etc\hosts.
  5. Search for text that begins with "Botzor2005 Made By…"
  6. Select this text and all text that follows. Delete the selected text and save the file.
  7. Close Notepad.
Restart your computer
To restart your computer
  1. On the Start menu, click Shut Down.
  2. Select Restart from the drop-down list and click OK.
Take steps to prevent re-infection
Do not reconnect your computer to the Internet until the computer is protected from re-infection. See the "Preventing Infection" section for more information.
Run an antivirus scan
To detect and remove this threat and other malicious software that may have been installed, run a full-system scan with an up-to-date antivirus product such as Microsoft Security Essentials, or the Microsoft Safety Scanner. For more information about using antivirus software, see
Additional remediation instructions for Worm:Win32/Zotob.C
This threat may make lasting changes to a computer’s configuration that are NOT restored by detecting and removing this threat. For more information on returning an infected computer to its pre-infected state, please see the following article/s: 

Threat behavior

Worm:Win32/Zotob.C takes the following actions:
  • Creates mutex B-O-T-Z-O-R. If this mutex already exists, an error occurs and Win32/Zotob.C exits.
  • Modifies the Windows system hosts file, %windir%\system32\drivers\etc\hosts, to block access to certain antivirus and shopping Web sites.
  • Copies itself to <system folder>\per.exe.
  • Makes the following registry key changes. These changes cause the worm to run each time Windows starts.
    • Adds value: WINDOWS SYSTEM
      with data: per.exe
      in registry key: HKLM\Software\Microsoft\Windows\CurrentVersion\Run
    • Adds value: WINDOWS SYSTEM
      with data: per.exe
      in registry key: HKLM\Software\Microsoft\Windows\CurrentVersion\RunServices
  • Sets data to: 4
    in value: Start
    of registry key: HKLM\SYSTEM\CurrentControlSet\Services\SharedAccess
    This disables the Internet Connection Firewall/Internet Connection Sharing service.
  • Creates a thread that monitors FTP connections on port 33333.
  • Creates 200 threads that scan randomly generated IP addresses on port 445. If a thread connects to a computer over port 445, the thread sends exploit code to the computer. If the computer is running Windows 2000 and does not have MS05-039 installed, the exploit code connects to the attacking computer on port 8888. The attacking computer then sends a command that causes the exploited computer to download a copy of the worm using FTP to a file called lol.exe. The worm then runs on the exploited computer.
  • Sends a message to an IRC server whenever a computer is successfully infected. This message includes the IP address of the exploited computer.
  • Connects to an IRC server to receive commands, which can include:
    • Updating itself using ftp.
    • Retrieving system information such as CPU speed, memory usage, Windows operating system,  connection type, IP address, and Windows logon information.
    • Downloading and running files using http.
    • Removing the worm.


There are no readily apparent indications that your computer is infected with Worm:Win32/Zotob.C. However, your computer may be infected with Worm:Win32/Zotob.C if you notice any of the following symptoms:
  • Presence of <system folder>/per.exe
  • Presence of value: WINDOWS SYSTEM
    with data: per.exe
    in the following registry keys:
  • Excessive network traffic over port 445


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: Severe
This entry was first published on: Aug 15, 2005
This entry was updated on: Apr 17, 2011

This threat is also detected as:
  • Zotob.C (Trend Micro)
  • W32/Zotob.worm.c (McAfee)
  • W32.Zotob.C (Symantec)
  • W32/Zotob.C (F-secure)
  • W32/Zotob-C (Sophos)
  • Zotob.C (CA)