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Discovered: November 10, 2003
Updated: February 13, 2007 12:13:42 PM
Also Known As: Trojan.Win32.DiskMaster.e [Kas, Nanif [McAfee], Win32.Dister.A [Computer Assoc, BKDR_DISTER.A [Trend]
Type: Trojan Horse
Systems Affected: Windows

Backdoor.Dister is a Trojan horse that periodically contacts a server for instructions and configuration data, and then sends batches of email as the server instructs. This functionality could be used to anonymously distribute malware or spam from an infected computer. This Trojan may be found as a file named Diskserv.exe.

Antivirus Protection Dates

  • Initial Rapid Release version November 11, 2003
  • Latest Rapid Release version March 23, 2017 revision 037
  • Initial Daily Certified version November 11, 2003
  • Latest Daily Certified version March 23, 2017 revision 041
  • Initial Weekly Certified release date November 12, 2003

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Writeup By: Heather Shannon

Discovered: November 10, 2003
Updated: February 13, 2007 12:13:42 PM
Also Known As: Trojan.Win32.DiskMaster.e [Kas, Nanif [McAfee], Win32.Dister.A [Computer Assoc, BKDR_DISTER.A [Trend]
Type: Trojan Horse
Systems Affected: Windows

When Backdoor.Dister is executed, it performs the following actions:

  1. Adds the value:

    "Disk Master"=<the current filename of the Trojan>

    to the registry key:


    so that the Trojan runs when you start Windows.

  2. Periodically downloads instructions from a randomly selected remote server, as described in the next paragraph. The reply contains details about the email to be sent, the recipients of the mail, and which SMTP server to use.

    Server selection
    The Trojan randomly selects a server from a hard-coded list of IP addresses. It can update the list when instructed to do so. The current list of addresses is saved to a file in the %Windir% folder. (By default, this is C:\Windows or C:\Winnt). Known variants have used the following file names for the address list:
    • Finance.dat
    • Blacklist.ini
    • Internet.dat
    • Winmail.ini
    • Explorer.ini
    • Winhelp.bmp
    • Winsock32.ini
    • Initfile.dat

      The remote port is computed from the IP address as:

      If the address is A.B.C.D, the Trojan will connect to ports of the form N+A+B, where N is a large port number, which depends on the variant. (Known variants have used 17000 and 27000, or 18000 and 28000.) For example, if the list contained the address, the Trojan might connect to and

  3. Contacts an SMTP server on port 25, and then sends email.


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.

Writeup By: Heather Shannon

Discovered: November 10, 2003
Updated: February 13, 2007 12:13:42 PM
Also Known As: Trojan.Win32.DiskMaster.e [Kas, Nanif [McAfee], Win32.Dister.A [Computer Assoc, BKDR_DISTER.A [Trend]
Type: Trojan Horse
Systems Affected: Windows

The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

  1. Disable System Restore (Windows Me/XP).
  2. Update the virus definitions.
  3. Restarting the computer in Safe mode or VGA mode.
  4. Run a full system scan and delete all the files detected as Backdoor.Dister.
  5. Reverse the changes made to the registry.
For details on each of these steps, read the following instructions.

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: 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 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 Virus Definitions (LiveUpdate).
    • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). 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 Virus Definitions (Intelligent Updater).

      The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.

3. To restart the computer in Safe mode or VGA mode
Shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in Safe mode or VGA mode.
    • For Windows 95, 98, Me, 2000, or XP users, restart the computer in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."
    • For Windows NT 4 users, restart the computer in VGA mode.

4. To scan for and delete the infected files
5. To reverse the changes made to 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 keys only. Read the document, "How to make a backup of the Windows registry," for instructions.
    1. Click Start > Run.

    2. Type regedit

      Then click OK.

    3. Navigate to the key:


    4. In the right pane, delete the value:

      "Disk Master"

    5. Exit the Registry Editor.

Writeup By: Heather Shannon