Linux.Svat

When a file that is infected with Linux.Svat runs, the first time control flow reaches a call to the close() function, the body of the virus is executed. Linux.Svat then places a partially encrypted copy of its source code into the file.

/usr/local/include/stdio.h

If this file existed previously, it is overwritten by the virus.

/usr/local/include is the location where the GCC compiler first looks for .h files that are referenced by #include directives in C programs. As a result, applications that are compiled after the virus is installed will use the viral stdio.h instead of the real copy that is located in the /usr/include/ directory.

In stdio.h, Linux.Svat redefines the close() function, which is used by applications to close file descriptors. The new close() function runs the installation routine of the virus and then makes use of the appropriate syscall (under Linux) or interrupt 21h function (under DOS) to close the file descriptor.

Linux.Svat also spreads to DOS partitions that are mounted on the Linux system. It parses the list of mounted file systems in /proc/mounts and looks for an MS-DOS partition. If it finds one, it installs itself to \usr\local\include\stdio.h on this partition. In the unlikely event that GCC is installed on the DOS partition, Linux.Svat attempts to replicate in a similar way under DOS as it does under Linux.

Because of its peculiar infection method, Linux.Svat can be considered both an entry-point obscuring and a polymorphic virus. It relies on the compiler itself to mutate the virus body and bury the virus entry point.

Linux.Svat is not dangerous and is very unlikely to spread. On most Linux systems the /usr/local/include is only writeable by root; therefore, an infected file would have to be run as root for the installation routine of the virus to work.

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