W32.Simile

When the virus is first executed, it checks the current date. If the host file (the file that is infected with the virus) imports the Windows file User32.dll, then on the 17th of March, June, September, or December, a message is displayed. Depending on the version of the virus, it is either:



or



The case of the text is altered randomly. On May 14, a political message is displayed if the system locale matches a certain language. The virus then rebuilds itself. This process is very advanced, and is capable of both shrinking and expanding its code. This avoids the uncontrolled growth that is common for other metamorphic viruses. After the rebuild is complete, the virus searches for .exe files in the current folder, then in folders on all fixed and remote drives that exist when the virus is executed. Files will not be infected if they are located in a subfolder more than three levels deep, or if the folder name begins with the letter W. For each file that is found, there is a 50 percent chance that it will be ignored. Files will not be infected if they begin with the following:

• F-
• PA
• SC
• DR
• NO

or if the letter V appears anywhere in the file name. Due to the way in which the name matching is done, file names that contain certain other characters--for example, those that begin with FM or contain the number 6 are also not infected.

The virus contains many other checks to avoid infecting "goat" files (files that are commonly used to capture viruses). The infection process uses the structure of the host, as well as random factors, to control the placement of the virus body and the decryptor.

Recommendations

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.

Summary