Discovered: May 15, 2005
Updated: February 13, 2007 12:38:50 PM
Also Known As: Win32.Mytob.DA [Computer Assoc, Net-Worm.Win32.Mytob.t [Kasper, W32/Mytob.gen@MM [McAfee], W32/Mytob-EM [Sophos], WORM_MYTOB.EM [Trend Micro]
Systems Affected: Windows
W32.Mytob.CF@mm is a mass-mailing worm that uses its own SMTP engine to send an email to addresses that it gathers from the compromised computer. The worm also opens a back door and spreads through the network by exploiting the Microsoft Windows DCOM RPC Interface Buffer Overrun Vulnerability (described in Microsoft Security Bulletin MS03-026 ) and the Microsoft Windows Local Security Authority Service Remote Buffer Overflow (as described in Microsoft Security Bulletin MS04-011 ).
Antivirus Protection Dates
- Initial Rapid Release version May 15, 2005
- Latest Rapid Release version October 27, 2017 revision 055
- Initial Daily Certified version May 15, 2005
- Latest Daily Certified version October 28, 2017 revision 002
- Initial Weekly Certified release date May 16, 2005
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.
When W32.Mytob.CF@mm is executed, it performs the following actions:
- Copies itself as the following files:
Note: %System% is a variable that refers to the System folder. By default this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).
- Drops the following file:
C:\hellmsn.exe (A copy of W32.Mytob.L@mm.)
- Adds the value:
"MPR MSG" = "mprmsg32.exe"
to the registry subkeys:
so that it runs every time Windows starts.
Note: The worm continually recreates these registry subkeys if they are deleted.
- Gathers email addresses from files with the following extensions:
The worm will not send itself to email addresses that contain any of the following strings:
The worm will not send itself to email addresses that contain any of the following strings as part of domain names:
The worm may append the following prefixes to domain names in an attempt to find Simple Mail Transfer Protocol (SMTP) servers:
- Uses its own SMTP engine to send itself to the email addresses that it finds. The email has the following characteristics:
One of the following:
The worm may also spoof an address from one of those found on the computer
One of the following:
- Good day
- Mail Delivery System
- Mail Transaction Failed
- Server Report
One of the following:
- Mail transaction failed. Partial message is available.
- The message contains Unicode characters and has been sent as a binary attachment.
- The message cannot be represented in 7-bit ASCII encoding and has been sent as a binary attachment.
- The original message was included as an attachment.
- Here are your banks documents.
One of the following:
with one of the following extensions:
- Spreads by exploiting the following vulnerabilities:
- Opens a back door by connecting to IRC channel #hb on the irc.secsnet.org domain, and listens for commands that allow the remote attacker to perform any of the following actions:
- Download and execute files.
- Obtain a version of the worm.
- Remove, terminate, or update the worm.
- May also set up an FTP server on a random port
- Adds the following text to the Hosts file to block access to several security-related Web sites:
Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":
- Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
- Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
- Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
- Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
- Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
- Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
- If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
- Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
- Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
- Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
- Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
- If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
- For further information on the terms used in this document, please refer to the Security Response glossary.
Removal using the W32.Mytob@mm Removal Tool
Symantec Security Response has developed a removal tool to clean the infections of W32.Mytob.CF@mm. Use this removal tool first, as it is the easiest way to remove this threat.
The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.
- Disable System Restore (Windows Me/XP).
- Remove entries added to the hosts file.
- Update the virus definitions.
- Run a full system scan and delete all the files detected.
- Delete any values added to the registry.
1. To disable System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.
Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.
Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.
For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:
- How to disable or enable Windows Me System Restore
- How to turn off or turn on Windows XP System Restore
Note: When you are completely finished with the removal procedure and are satisfied that the threat has been removed, reenable System Restore by following the instructions in the aforementioned documents.
For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article: Antivirus Tools Cannot Clean Infected Files in the _Restore Folder (Article ID: Q263455).
2. To remove all the entries that the risk added to the hosts file
- Navigate to the following location:
- Windows 95/98/Me:
- Windows NT/2000/XP:
- The location of the hosts file may vary and some computers may not have this file. There may also be multiple copies of this file in different locations. If the file is not located in these folders, search your disk drives for the hosts file, and then complete the following steps for each instance found.
- %Windir% is a variable that refers to the Windows installation folder. By default, this is C:\Windows (Windows 95/98/Me/XP) or C:\Winnt (Windows NT/2000).
- Windows 95/98/Me:
- Double-click the hosts file.
- If necessary, deselect the "Always use this program to open this program" check box.
- Scroll through the list of programs and double-click Notepad.
- When the file opens, delete all the entries added by the risk. (See the Technical Details section for a complete list of entries.)
- Close Notepad and save your changes when prompted.
3. To update the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:
- Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the document: Virus Definitions (LiveUpdate).
- Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted daily. You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the document: Virus Definitions (Intelligent Updater).
The latest Intelligent Updater virus definitions can be obtained here: Intelligent Updater virus definitions. For detailed instructions read the document: How to update virus definition files using the Intelligent Updater.
4. To scan for and delete the infected files
- Start your Symantec antivirus program and make sure that it is configured to scan all the files.
- For Norton AntiVirus consumer products: Read the document: How to configure Norton AntiVirus to scan all files.
- For Symantec AntiVirus Enterprise products: Read the document: How to verify that a Symantec Corporate antivirus product is set to scan all files.
- Run a full system scan.
- If any files are detected, click Delete.
Important: If your Symantec antivirus product reports that it cannot delete a detected file, Windows may be using the file. To fix this, run the scan in Safe mode. For instructions, read the document: How to start the computer in Safe Mode . Once you have restarted in Safe mode, run the scan again.
After the files are deleted, restart the computer in Normal mode and proceed with the next section.
Warning messages may be displayed when the computer is restarted, since the threat may not be fully removed at this point. You can ignore these messages and click OK. These messages will not appear when the computer is restarted after the removal instructions have been fully completed. The messages displayed may be similar to the following:
Title: [File path]
Message body: Windows cannot find [file name]. Make sure you typed the name correctly, and then try again. To search for a file, click the Start button, and then click Search.
5. To delete the value from the registry
Important: Symantec strongly recommends that you back up the registry before making any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified subkeys only. For instructions refer to the document: How to make a backup of the Windows registry.
- Click Start > Run.
- Type regedit
- Click OK.
Note: If the registry editor fails to open the threat may have modified the registry to prevent access to the registry editor. Security Response has developed a tool to resolve this problem. Download and run this tool, and then continue with the removal.
- Navigate to the subkeys:
- In the right pane, delete the value:
"MPR MSG" = "mprmsg32.exe"
- Exit the Registry Editor.
Writeup By: Kaoru Hayashi