Threat Explorer

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13 February 2007

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.

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.


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.