Weed.5850.A

SUMMARY

This is the third documented version of the Weed family of viruses. The 5850.A and 1.1 versions use an identical encryption/decryption routine, with only the keys being different.

Protection

• Virus Definitions (Intelligent Updater) April 28, 1997

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

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

This is the third documented version of the Weed family of viruses. The 5850.A and 1.1 versions use an identical encryption/decryption routine, with only the keys being different. A later version has also been encountered, which is similar to 5850.A except that the encryption has different keys and instead of displaying garbage, the virus will display an understandable message in the second payload.

This virus will infect all files in all directories included in the path statement and the current directory. Files smaller than 5850 bytes have a high probability of becoming corrupt and unrepairable. The virus does not check to see if the host file has been previously infected, so will often re-infect the host multiple times. The virus is classified as a "direct infector" meaning that it does not go into memory and hide there.

Weed.5850.A has the following random displays (will not always appear) when the host program is run:

• A starfield might display, similar to a screensaver, and then lock the machine.

  If the starfield payload activates, the host file will not be re-infected during that execution, but it may still have the encrypted host body attached to the end of the file. Other infection routines will function normally (as explained above). The file will still execute properly.

• The virus will display the following line of garbage, randomly, when the original host program is run:

  hVVU1VB?D1zú1yvâv21>1[åúà1hrzà=1yvy=1zà1xvàú1svààvâ1

The virus should run on any platform that can support a DOS 5 environment. Meaning, PC emulators under a Macintosh environment may be enough for the virus to infect PC files stored there, as well as running in a DOS window under Windows95 and WindowsNT.

A detection and repair for this virus is included with the May 1997 and all later virus definition sets.

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.