Discovered: May 26, 2006
Updated: February 13, 2007 12:58:27 PM
Also Known As: Troj/Hyder-B [Sophos], Troj/Small-COM [Sophos]
Type: Trojan Horse
Systems Affected: Windows

Trojan.Linkoptimizer is a detection for a family of Trojan horse programs that use rootkit and stealth techniques to hide their presence. The Trojan may download and display pop-up advertisements.

Note: Definitions dated prior to August 25th 2006 may detect this threat as Trojan Horse .

Antivirus Protection Dates

  • Initial Rapid Release version August 25, 2006
  • Latest Rapid Release version April 27, 2018 revision 037
  • Initial Daily Certified version August 25, 2006 revision 002
  • Latest Daily Certified version April 28, 2018 revision 001
  • Initial Weekly Certified release date August 28, 2006

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

Writeup By: Elia Florio

Discovered: May 26, 2006
Updated: February 13, 2007 12:58:27 PM
Also Known As: Troj/Hyder-B [Sophos], Troj/Small-COM [Sophos]
Type: Trojan Horse
Systems Affected: Windows

It has been reported that Trojan.Linkoptimizer may be installed by visiting one of the following Web sites:

  • [http://]
  • [http://]
  • [http://]
  • [http://]
  • [http://]
  • [http://]
  • [http://]
  • [http://]
  • [http://]
  • [http://]

The Trojan installs itself on the compromised computer by exploiting certain vulnerabilities in Internet Explorer and Mozilla Firefox, including:
When the Trojan is being installed, the browser may show the following prompt and ask the user to save a file with one of the following names:

The browser may also ask for confirmation to install the files FreeAccess.ocx or

Once executed, Trojan.Linkoptimizer performs the following actions:
  1. Checks for the presence of the virtual machine software, VMware. The threat will not run on any operating system running inside this environment.

  2. Checks for the presence of debuggers or monitoring tools. The threat will not run on computers running any of the following drivers:
    • SICE (Numega SoftICE Debugger)
    • SIWVIDSTART (Numega SoftICE Debugger)
    • FILEMON (Sysinternals Filemon)
    • REGMON (Sysinternals Regmon)

  3. Checks for the presence of other security tools by checking for the values:

    "core force"
    "microsoft visual c"
    "visual studio"

    in the registry entry:

    SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\[PROGRAM NAME]\"DisplayName"

    Note: The threat will not run on computers which contains one of the following strings inside the registry value.

  4. Creates the following files:
    • %Temp%\[RANDOM NAME]1.exe
    • %Windir%\[RANDOM NAME]1.dll

    • %Windir% is a variable that refers to the Windows installation folder. By default, this is C:\Windows or C:\Winnt.
    • %Temp% is a variable that refers to the Windows temporary folder. By default, this is C:\Windows\TEMP (Windows 95/98/Me/XP) or C:\WINNT\Temp (Windows NT/2000).

  5. Downloads files from the following hard coded IP addresses:

    • [http://][REMOVED]
    • [http://][REMOVED]
    • [http://][REMOVED]
    • [http://][REMOVED]

  6. Tries to resolve the following domain name:

  7. Registers the dropped DLL as a Browser Helper Object by creating the following registry subkeys:

    \Browser Helper Objects\[RANDOM CLASSID]

    This .dll file is responsible for displaying and/or clicking ads generated by the following remote scripts;
    • [http://][REMOVED]
    • [http://][REMOVED]
    • [http://][REMOVED]
    • [http://][REMOVED]
    • [http://][REMOVED]
    • [http://][REMOVED]
    • [http://][REMOVED]
    • [http://][REMOVED]

  8. The above .dll file may also contact the following domains:
    • [http://]
    • [http://]
    • [http://]
    • [http://]

  9. Adds the value:

    "AppInit_DLLs" = "[TROJAN .DLL FILE]"

    to the registry subkey:

    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows

  10. May add the value:


    to the registry subkey:

    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\URLSearchHooks

    to hijack Internet Explorer Address bar searches.

  11. May also create the following registry subkeys


  12. Downloads and installs some additional components, which includes the Rootkit component.

  13. Creates the following files:

    • %System%\[RANDOM NAME]aa.dll

      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).

  14. May store the above files inside the following Alternate Data Streams (ADS):

    • %System%:[RANDOM NAME]aa.dll
    • C:[RANDOM NAME]aa.dll

      Note: [RESERVED DOS NAME] can be one of the following reserved DOS device names:

    • com[NUMBER]
    • lpt[NUMBER]
    • tty
    • prn
    • nul
    • con
    • aux

      Where [NUMBER] is a number between 1 and 9.

  15. Uses Rootkit techniques to hide its files and registry subkeys.

  16. Adds a new administrator account on the compromised computer using a random user name.

  17. May lower the privileges of the current logged user in order to disable the functioning of some security-related software.

  18. Creates the following encrypted files associated to the new administrator account and stores them using the Windows Encrypted File System (EFS):

    • %CommonProgramFiles%\System\[RANDOM LETTERS].exe
    • %CommonProgramFiles%\Microsoft Shared\[RANDOM LETTERS].exe
    • %CommonProgramFiles%\Services\[RANDOM LETTERS].exe

      Note: %CommonProgramFiles% is a variable that refers to the Common Files folder. By default, this is C:\Program Files\Common Files.

  19. Creates a registry subkey and a system service associated with the new administrator account:


    Where [RANDOM_NAME]is a string composed by mixing a substring of a legitimate service name present on the compromised machine with other random letters.

    Example of service names generated by the Trojan:


  20. Attempt to download the following file:


    Note: %ProgramFiles% is a variable that refers to the program files folder. By default, this is C:\Program Files.

  21. Displays advertisements.

  22. May use SNMP queries to gather machine information.

  23. Functions as a dialer and attempts to dial high-cost numbers if a modem is present on the compromised machine.
  24. May prevent the execution of security and anti-rootkit programs that contain the following strings in the file description or properties:
    • avzantivirus
    • svv
    • avz
    • antihook
    • blacklight
    • gromozon
    • gmer
    • prevx
    • rootkit
    • sophosant-rootkit
    • rootkitrevealer
    • icesword
    • avganti-rootkit
    • rootkituncover
    • sophosanti-rootkit
    • clrav
    • avzanti-virus
  25. Attempts to block Internet connections to domains which contain any of the following strings:
    • wilderssecurity.
    • castlecops.
    • suspectfile.
    • antispywareremoval.
    • pctools.
    • paretologic.
    • scan-it-clean-it.
    • trojaner-board.
    • prevx.
    • pcalsicuro.
    • 2-spyware.
    • protecus.
    • hwupgrade.


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: Elia Florio

Discovered: May 26, 2006
Updated: February 13, 2007 12:58:27 PM
Also Known As: Troj/Hyder-B [Sophos], Troj/Small-COM [Sophos]
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. Update the virus definitions.
  2. Run a full system scan.
  3. Submit the files to Symantec Security Response.

For specific details on each of these steps, read the following instructions.

1. To update the virus definitions
A generic detection can often occur if the antivirus program discovers a threat, but does not have the latest definitions. In these cases, you should download the latest definitions, then run the scan again.

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 .

2. To scan for the infected files
  1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
  2. Run a full system scan.
  3. If the files previously detected as generic are now detected as another threat, please read the threats writeup for removal instructions.
  4. If the files are still detected as this generic threat, continue with the next step.

3. To submit the files to Symantec Security Response
Symantec Security Response suggests that you submit any files that are detected as generic to Symantec Security Response. For instructions on how to do this, read the following documents:

Writeup By: Elia Florio