Cruel.A

SUMMARY

This virus infects the master boot records of hard drives and the boot sectors of floppy disks.

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
• Payload: The virus will reset the CMOS on an infected system. The settings will be erased.

Distribution

• Distribution Level: Low
• Target of Infection: Master Boot Record and Floppy Boot Sectors

TECHNICAL DETAILS

When a system is booted from an infected floppy disk, the virus loads itself into memory and infects the master boot record of the hard drive. After booting up from an infected hard drive, the virus infects floppy disks that are accessed by the system. It does not display any messages. It hooks interrupt 13h to infect floppy disk boot sectors.

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 repair an infected system, boot to a clean DOS floppy disk. Then, run Norton AntiVirus to scan and repair the master boot record. Also, scan all floppy disks for infection.