Linux.Simile

SUMMARY

{Win32,Linux}/Simile.D is a very complex virus that uses entry-point obscuring, metamorphism, and polymorphic decryption. It is the first known polymorphic metamorphic virus to infect under both Windows and Linux. The virus contains no destructive payload, but infected files may display messages on certain dates. It is the fourth variant of the Simile family. This variant introduces a new infection mechanism on Intel Linux platforms, infecting 32-bit ELF files (a standard Unix binary format). The virus infects Portable Executable (PE) files as well as ELFs on both Linux and Win32 systems. So far Symantec has not received any submissions of this virus from customers.

NOTE: The {Win32,Linux} reference follows the CARO (Computer Anti-virus Researchers Organization) standard naming convention. This is meant to imply that a threat can infect across multiple platforms, Win32 and Linux. Another such example would be {Win32,W97M}.

Behavior

Symptoms

Transmission

Protection

• Virus Definitions (LiveUpdate™ Weekly) June 5, 2002
• Virus Definitions (Intelligent Updater) May 30, 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 date is March 17 or September 17, and the virus host is a PE file, it displays this message:
  
  
  
• If the date is March 17 or May 17, and the virus host is an ELF file, the virus attempts to output a text message to the console. The message will be similar to the following:
  
  

The first Win32/Linux cross-infector, {Win32,Linux}/Peelf, uses two separate routines to carry out the infection on PE and ELF files. This variant of Simile shares a substantial amount of code between the two infection functions, such as the polymorphic/metamorphic engines, the only platform-specific parts being the directory traversal code and the API usage.

The virus was confirmed to infect successfully under versions 6.2, 7.0 and 7.2 of Red Hat Linux, and it very likely works on most other common Linux distributions.

Infected files will grow by about 110 KB on average, but the size increase is variable due to the shrinking and expansion capability of the metamorphic engine and to the insertion method.

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

To remove this Trojan, update the virus definitions, run a full system scan, and delete all files that are detected as W32.Simile or Linux.Simile. For details on how to do this, read the following instructions.

To scan with Norton AntiVirus and repair the infected files:

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.
   • NAV Consumer products: Read the document How to configure Norton AntiVirus to scan all files.
   • NAV Enterprise products: Read the document How to verify a Symantec Corporate antivirus product is set to scan All Files.
3. Run a full system scan.
4. Delete all files that are detected as W32.Simile or Linux.Simile. Replace deleted files from a clean backup or reinstall them.