XM.Sofa

SUMMARY

The XM.Sofa virus is the first member of the second family of Excel macro viruses. First discovered by SARC in early December of 1996 (west coast USA), XM.Sofa, like other Excel macro viruses, is written in MS Visual Basic and it spreads by copying its own viral macros to other MS Excel spreadsheet hosts. Unlike XM.Laroux (the first known, viable Excel macro virus) which infects by creating a file called PERSONAL.XLS in the default startup directory, XM.Sofa spreads by way of a file called BOOK.XLT placed in the alternate startup directory.

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
• Target of Infection: Microsoft Excel spreadsheets

TECHNICAL DETAILS

This virus contains 4 macro functions: Auto_Open, Auto_Range, Current_Open and Auto_Close. When an infected file is opened, the virus takes control and changes the caption at the top of the screen to “Microsofa Excel� instead of the normal “Microsoft Excel.�

Next the virus checks to see if the system is already infected. The virus looks in the alternate startup directory for a file named BOOK.XLT (or defaults to C:MSOFFICEEXCELXLSTART if this directory is not defined). If the file does not exist in the in the target directory, the virus displays the following message box and infects the system:

Microsoft Excel has detected a corrupted add-in file.

Click ‘OK’ to repair this file.

After the OK button is clicked, the file BOOK.XLT is created in the target startup directory and the virus is ready to infect other spread sheets.

NOTE: If the alternate startup directory is defined, but does not exist, the virus cannot create the BOOK.XLT file, will not be infectious upon startup and the message box will not be displayed.

When infecting, the virus creates two sheets, one with a name of 12 blank spaces and the other with 13 blank spaces. Both sheets contain the text of the macros, however only one of them is specified as a Visual Basic module, while the other is defined as a normal worksheet.

XM.Sofa does not contain any deliberately harmful payloads.

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.