BW.770.B

BW.770.B does not go memory resident. Instead, it does a recursive search on the system and infects any DOS .exe or .com files that it finds. The virus will also infect Portable Executable (PE) files. However, when running an infected PE file, the virus will not execute.

When infecting .com files, the virus overwrites the first 3 bytes with a jump to end-of-file (EOF). It then appends itself to the EOF. As a result, the jump at the beginning of the .com file is pointing to the start of the viral body, and the virus receives immediate control when an infected .com file is executed. After the virus has finished its malicious activities on the system, it restores the 3 bytes that it overwrote, and then executes the original host.

The infection for executable files is carried out slightly differently than the infection of .com files. First the virus determines if a file is a DOS executable file. This is done by checking for the letters MZ at the start of the file. It is possible, even though it is not common, for DOS executable files to have ZM instead of MZ as the first two bytes of the file. In this case, BW.770.B will not infect the file. Once the virus has determined that the file is an MZ executable, it will append itself at the end and change the entry point to point to itself. The virus will store the original entry point within itself, and once it is done executing, it will jump to the original entry point. An executable file infected with this virus will be detected as BW.770.B (x).

Recommendations

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.

Summary