AntiEXE

SUMMARY

AntiEXE is a virus that infects the master boot record (MBR) and DOS boot sectors (DBS). AntiEXE spreads only when there is an attempt to boot the system from an infected floppy disk.

Protection

• Virus Definitions (Intelligent Updater) March 1, 1995

TECHNICAL DETAILS

During the boot process, AntiEXE loads the MBR into memory and checks for infection. If it determines that the MBR is not infected, then AntiEXE stores the uninfected MBR at cylinder 0, side 0, sector 13 on the hard disk. AntiEXE then places its virus code into the MBR and writes the infected MBR back to the hard disk at cylinder 0, side 0, sector 1.

After the boot process is complete and AntiEXE is active in memory, then AntiEXE displays its stealthing capabilities by redirecting any disk reads of the infected MBR or DBS to their clean counterparts. (On floppy disks, the original DBS is stored in the last sector of the root directory.) Along with stealthing the reads of the MBR or DBS during all disk-read operations, AntiEXE searches for a specific .exe file, whose identity remains unknown, and corrupts the file if it is found.

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 the AntiEXE virus:
To remove AntiEXE, you must boot the computer using a uninfected external source, such as a bootable floppy disk or bootable CD, and then run a virus scan. For this purpose, you can use Norton AntiVirus Rescue disks or Norton AntiVirus Emergency Disks.

NOTE: The Norton AntiVirus Emergency Disk set is available as a free download. For instructions on how to obtain and use it, see the document How to create Norton AntiVirus Emergency Disks.