W32.Simile

SUMMARY

W32.Simile is a very complex virus that uses entry-point obscuring, metamorphism, and polymorphic decryption. It infects files in folders on all fixed and remote drives that are mapped at the time that the virus is executed. The virus contains no destructive payload, but infected files may display messages on certain dates.

Protection

• Virus Definitions (LiveUpdate™ Weekly) March 6, 2002
• Virus Definitions (Intelligent Updater) March 6, 2002

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 0 - 49
• Number of Sites: 0 - 2
• Geographical Distribution: Low
• Threat Containment: Easy
• Removal: Easy

Damage

• Damage Level: Low

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

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":

• Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates.
• If a blended 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 (for example, all Windows-based computers should have the current Service Pack installed.). Additionally, please apply any security updates that are mentioned in this writeup, in trusted Security Bulletins, or on vendor Web sites.
• 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.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate infected computers quickly to prevent further compromising your organization. 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.

REMOVAL

1. Obtain the most recent virus definitions. There are two ways to do this:
   • Run LiveUpdate. LiveUpdate 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. For instructions on how to do this, read the document How to configure Norton AntiVirus to scan all files.
3. Run a full system scan.
4. Delete all files that are detected as W32.Simile. Replace deleted files from a clean backup or reinstall them.