Printer Friendly Page

Discovered: December 05, 2002
Updated: February 13, 2007 11:41:26 AM
Also Known As: Backdoor.Skun.001 [KAV]
Type: Trojan Horse
Systems Affected: Windows

Backdoor.Skun is a Backdoor Trojan that gives an attacker unauthorized access to an infected computer. It attempts to log keystrokes to capture confidential information. Backdoor.Skun is packed with UPX v1.20.

Antivirus Protection Dates

  • Initial Rapid Release version December 05, 2002
  • Latest Rapid Release version August 20, 2008 revision 017
  • Initial Daily Certified version December 05, 2002
  • Latest Daily Certified version August 20, 2008 revision 016
  • Initial Weekly Certified release date December 06, 2002

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

Technical Description

When Backdoor.Skun runs, it performs the following actions:

It copies itself as %system%\Task.tsk.

NOTE: %system% is a variable. The Trojan locates the System folder and copies itself to that location. By default this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).

The Trojan creates the registry key


and adds the following values to this key:

(Default)         exefile
Content Type      application/x-msdownload

Then, the Trojan creates the value

Microsoft Task Control    %system%\task.tsk

in the registry key


so that the Trojan starts when you start Windows.

In addition, the Trojan adds the value

run    %system%\task.tsk

to the registry key

HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows

If the operating system is Windows 95/98/Me, the Trojan registers itself as a service process so that it continues to run after you log off. Backdoor.Skun attempts to obtain access to the password cache that is stored on the local computer. It installs hook procedures into a hook chain to monitor the system for keyboard and mouse messages. This permits Backdoor.Skun to intercept keystrokes.

After Backdoor.Skun is installed, it waits for commands from the remote client. The commands allow the hacker to perform the following actions:

  • Perform various annoying actions
  • Intercept confidential information by hooking keystrokes
  • Alter the mIRC program's Script.ini file


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.


NOTE: These instructions are for all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

  1. Update the virus definitions.
  2. Do one of the following:
    • Windows 95/98/Me: Restart the computer in Safe mode.
    • Windows NT/2000/XP: End the Trojan process.
  3. Run a full system scan, and delete all files that are detected as Backdoor.Skun.
  4. Reverse the changes that the Trojan made to the registry.
For details on how to do this, read the following instructions.

To update the virus definitions:
All virus definitions receive full quality assurance testing by Symantec Security Response before being posted to our servers. There are two ways to obtain the most recent virus definitions:
  • Run LiveUpdate, which is the easiest way to obtain virus definitions. These virus definitions are posted to the LiveUpdate servers once 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 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.

To restart the computer in Safe mode or end the Trojan process:
    Windows 95/98/Me
    Restart the computer in Safe mode. All Windows 32-bit operating systems, except for Windows NT, can be restarted in Safe mode. For instructions on how to do this, read the document How to start the computer in Safe Mode.

    Windows NT/2000/XP
    To end the Trojan process:
    1. Press Ctrl+Alt+Delete once.
    2. Click Task Manager.
    3. Click the Processes tab.
    4. Double-click the Image Name column header to sort the processes alphabetically.
    5. Scroll through the list, and look for Task.tsk
    6. If you find the file, click it, and then click End Process.
    7. Exit the Task Manager.
To scan for and delete the infected files:
  1. Start your Symantec antivirus program, and make sure that it is configured to scan all files.
  2. Run a full system scan.
  3. If any files are detected as infected with Backdoor.Skun, click Delete.

To reverse the changes that the Trojan made to the registry:

: 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 and then delete the registry key

  4. Navigate to the key

  5. In the right pane, delete the value

    Microsoft Task Control    %system%\task.tsk
  6. Navigate to the key

    HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows
  7. In the right pane, delete the value

    run    %system%\task.tsk
  8. Exit the Registry Editor.

Writeup By: Serghei Sevcenco