W97M.Thus.CV

SUMMARY

W97M.Thus.CV is a macro virus that infects active Microsoft Word documents and the Normal.dot template. The virus attempts to encrypt randomly chosen .exe and .vdb files.

Protection

• Virus Definitions (Intelligent Updater) May 25, 2001

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
• Modifies Files: Encrypts .exe and .vdb files.

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

When an infected document is opened, the virus checks active documents and the Normal.dot template for the comment line "NIST_32a" to determine whether the document or template is already infected. This is done to avoid multiple infections. If the comment line is not present, the document or template is then infected.
After infection, the virus changes the Macro Protection setting of some Microsoft Office programs to Off. The following programs are affected:

• Word 98/2000
• Excel 98/2000
• PowerPoint 98/2000

The virus then attempts to encrypt randomly chosen .exe and .vdb files that are greater than 12 bytes in size. If the encryption is successful, the file cannot be repaired.

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

1. Run LiveUpdate to make sure that you have the most recent virus definitions.
2. Start Norton AntiVirus (NAV) and run a full system scan, making sure that NAV is set to scan all files.
3. If any files are detected as infected by W97M.Thus.CV, click Repair.
4. Reset the Macro Protection of Word, Excel, or PowerPoint. See the program documentation or Help for instructions.

Any .exe and .vdb files that were encrypted by the virus will have to be restored from backup or reinstalled.