Tremor.4000.A

SUMMARY

The Tremor.4000.A virus is a complex virus that uses several advanced techniques to avoid detection. Built into this virus is the ability to bypass both the CPAV and MSAV antivirus programs, along with the ability to hide any visible signs of file infection from either a DIR command or through the use of a disk editing program.

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

An interesting set of routines contained within this virus allow the normal execution and/or disinfection of infected files under specific conditions. When an infected file is first run, a system version check is performed. If the operating system is older than 3.30, the file is allowed to execute normally. If the virus is active in memory and the copy function is performed on an infected file, the virus will disinfect the destination file.

As this virus is attempting to load into memory it will check for both XMS and UMB usage. If found, the virus will move itself into either extended or high memory; if unavailable, the virus code is moved to the upper part of conventional memory.

If the virus triggers, the following message is displayed to the screen (after the screen has been cleared):

-=>T.R.E.M.O.R was done by NEUROBASHER…

After the above message is displayed, the screen is returned to normal.

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.