DA_Boys

SUMMARY

DA_Boys is a virus that specifically targets DOS 5.0 boot sectors on floppy disks and hard drives. The majority of DOS 6.0 and higher operating systems use a DOS 5.0 boot sector and are therefore susceptible to the virus as well.

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 0 - 49
• Number of Sites: 0 - 2
• Geographical Distribution: High
• Threat Containment: Easy
• Removal: Easy

Damage

• Damage Level: Low

Distribution

• Distribution Level: Low
• Target of Infection: DOS Boot sectors

TECHNICAL DETAILS

Before infection of a hard drive, DA_Boys checks for a two-byte signature of 0xBB, 0x78 in the boot record at offset 0x48. If it does not find a DOS 5.0 boot sector, it does not attempt to infect the drive. This means that multiple-boot systems are fairly safe from infection, as long as the non-DOS partition is physically first on the drive. On a system where a DOS 5.0 boot sector is located first on the drive, DA_Boys infects only the first partition on each physical hard drive.

Upon infection, DA_Boys makes the assumption that the boot sector is located at side 1, cylinder 0, sector 1. DA_Boys does not actually check the partition table for the proper location. If the boot sector is located elsewhere in the DOS partition, DA_Boys does not infect the host.

DA_Boys modifies the original boot sector without first saving a copy. The DOS boot sector contains code used only if the system files cannot be found during boot-up. As this code is infrequently used (hard drives rarely lose their operating systems), the virus overwrites this code and data and places itself in these areas. This keeps the viral code small and prevents it from having stealthing capabilities. DA_Boys does not contain any apparent harmful trigger code. Portions of the original boot sector remain intact and make references to some of the overwritten areas.

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.