1. Symantec/
  2. Security Response/
  3. W32.Tidserv.G


Risk Level 2: Low

March 22, 2009
August 15, 2012 10:53:06 AM
Also Known As:
W32/TDSS.BU [F-Secure]
Infection Length:
46,592 bytes; 39,879 bytes; 10,752 bytes
Systems Affected:
When the worm executes, it copies itself as the following file:
%Windir%\Temp\[RANDOM NUMBERS].tmp

The worm spreads by copying itself to all drive letters available on the compromised computer, including removable drives and mapped network shares, as the following file:

When the above file is executed, the worm creates a mutex and also creates the following new copy of itself:

It then deletes the original file.

Next, the worm creates the following file so that it runs whenever removable drives are connected to another computer:

It then drops the following file:
%Temp%\tmp[RANDOM NUMBERS].tmp

Note: The above file is actually a .dll file.

The threat copies the legitimate file %System%\msi.dll to %Temp%\tmp[RANDOM NUMBERS].tmp. The copy of the file is then modified to include some of the worms own code.

It then modifies structures in the computer memory to redirect system calls for the MSIserver service to load the modified copy. This will result in the execution of the worm code.

The worm may then create the following registry entries:
  • HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Session Manager\"PendingFileRenameOperations" = "[RANDOM HEXADECIMAL CHARACTERS]"
  • HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Enum\Root\LEGACY_MSISERVER\0000\Control\"ActiveService" = "MSIServer"
  • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\"PendingFileRenameOperations" = "[RANDOM HEXADECIMAL CHARACTERS]"
  • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\Root\LEGACY_MSISERVER\0000\Control\"ActiveService" = "MSIServer"

The worm deletes the browser history from the following applications:
  • Firefox
  • Internet Explorer
  • Opera
  • Safari
  • Chrome

It then downloads another malicious component using a HTTP POST command to the following address:

Note: The POST data contains 45 bytes of information on how to encrypt the response. It also serves as authentication to the server so that only the malicious component of the worm can download the payload.

It saves the above file as the following file and executes it:
%Windir%\tempo-[RANDOM NUMBERS].tmp

It changes the DNS settings for all network connections to two of the following IP addresses:

The worm drops a kernel driver to the following location:

Note: The driver is loaded by creating the following registry subkey:

The kernel driver removes traces of itself when it is loaded by deleting the following registry subkey:

It also denies the following processes Internet access:
  • avp.exe
  • klif.sys
  • mrt.exe
  • spybotsd.exe
  • sasdifsv.sys
  • saskutil.sys
  • sasenum.sys
  • superantispyware.exe
  • szkg.sys
  • szserver.exe
  • mbam.exe
  • mbamswissarmy.sys
  • pctssvc.sys
  • pctcore.sys
  • mchinjdrv.sys

The worm injects the following file into the svchost.exe process:

It creates the following registry subkey to store data about the worm:

It hides files and registry subkeys that have the following prefix:

The worm modifies the DNS entries on the compromised computer. In case of an infection in a Server/Client environment, clients on a compromised network might acquire malicious DNS addresses from an infected server (without actually being infected itself), redirecting queries to an address controlled by the remote attacker.

The worm acts as a DHCP server for all computers on the compromised computer's LAN, serving the following malicious DNS addresses to redirect all DNS queries to an address controlled by the remote attacker:
  • (primary)
  • (secondary)

The worm may also download potentially malicious files on to the compromised computer.


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: Mircea Ciubotariu
Summary| Technical Details| Removal

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