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Discovered: June 21, 2002
Updated: February 13, 2007 11:53:25 AM
Type: Trojan Horse
Systems Affected: Windows

Backdoor.Mirab allows a hacker to remotely control an infected computer. It is written in the Borland Delphi programming language and compressed with UPX. By default, it uses port 4912 for direct control and port 6430 for file transfer.

Antivirus Protection Dates

  • Initial Rapid Release version June 24, 2002
  • Latest Rapid Release version March 03, 2008 revision 035
  • Initial Daily Certified version June 24, 2002
  • Latest Daily Certified version March 03, 2008 revision 037
  • Initial Weekly Certified release date June 26, 2002

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

Writeup By: Yana Liu

Discovered: June 21, 2002
Updated: February 13, 2007 11:53:25 AM
Type: Trojan Horse
Systems Affected: Windows

When Backdoor.Mirab runs, it does the following:

It displays the following messages:

It then copies itself to the \%System% folder as C:\%System%\WinKer.exe.

NOTE : %System% is a variable. The Trojan locates the \Windows\System folder (by default this is C:\Windows\System or C:\Winnt\System32) and copies itself to that location.

It adds the value

WinKernel C:\%System%\WinKer.exe

to the registry key


NOTE : The messages that the Trojan displays and the name of the file that it drops may vary, these are determined by the hacker who creates this Trojan.

The Trojan repeatedly opens TCP/UDP ports. This may cause a degradation of computer performance.

The hacker who creates the Backdoor.Mirab server file, can add many functions. For example, it can be programmed to:

  • Choose the ports that are used by the Trojan to communicate with the hacker. By default, it uses port 4912 for direct control and port 6430 for file transfer.
  • Use different notification methods to send information to the hacker about the compromised computer. For example, it attempts to open an HTTP connection to a Web server of the hacker's choice, and post the victim's information to a script file on that Web server. The information may include:
    • Notification that the victim is online
    • The connection port
    • The upload/download port
    • The computer's system date and time

The Trojan also uses ICQ pager to send the compromised computer's information to the hacker.

If Backdoor.Mirab runs, it allows the hacker to remotely take control over the compromised computer, and can include the ability to:
  • Steal information from the host computer.
  • Take full control over the file system.
  • Upload to and download from the host computer
  • Rename, delete, list, and run files of the hacker's choice
  • Delete folders
  • Display messages
  • View the screen
  • Log keystrokes
  • Shut down the host computer
  • Perform annoying actions, such as:
    • Change various desktop settings (wallpaper, resolution, etc.)
    • Manipulate the mouse
    • Close, hide, minimize, and maximize windows
    • Turn the monitor on and off


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: Yana Liu

Discovered: June 21, 2002
Updated: February 13, 2007 11:53:25 AM
Type: Trojan Horse
Systems Affected: Windows

Update the virus definitions, run a full system scan, and delete all files that are detected as Backdoor.Mirab.

  1. Delete the value

    WinKernel C:\%System%\WinKer.exe

    NOTE: This value may vary, depending on how the Trojan was programmed.

    from the registry key


For details on how to do this, read the following instructions.

To scan with Norton AntiVirus and delete the infected files:
  1. Obtain the most recent virus definitions. There are two ways to do this:
    • Run LiveUpdate, which is the easiest way to obtain virus definitions. These virus definitions have undergone full quality assurance testing by Symantec Security Response and are posted to the LiveUpdate servers one time each week (usually Wednesdays) unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, look at the Virus Definitions (LiveUpdate) line at the top of this write-up.
    • Download the definitions using the Intelligent Updater. Intelligent Updater virus definitions have undergone full quality assurance testing by Symantec Security Response. They are posted on U.S. business days (Monday through Friday). They must be downloaded from the Symantec Security Response Web site and installed manually. To determine whether definitions for this threat are available by the Intelligent Updater, look at the Virus Definitions (Intelligent Updater) line at the top of this write-up.

      Intelligent Updater virus definitions are available here. For detailed instructions on how to download and install the Intelligent Updater virus definitions from the Symantec Security Response Web site, click here.
  2. Start Norton AntiVirus (NAV), and make sure that NAV is configured to scan all files.
  3. Run a full system scan.
  4. If any files are detected as Backdoor.Mirab, write down their names and then delete them.

To remove the value from the registry:

CAUTION : Symantec strongly recommends that you back up the registry before you make any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify only the keys that are specified. Read the document How to make a backup of the Windows registry for instructions.
  1. Click Start, and click Run. The Run dialog box appears.
  2. Type regedit and then click OK. The Registry Editor opens.
  3. Navigate to the following key:

  4. In the right pane, delete any values that refer to the file name that NAV detected as infected; for example,

    WinKernel         C:\%System%\WinKer.exe
  5. Click Registry, and click Exit.

Writeup By: Yana Liu