XM.Delta

SUMMARY

XM.Delta is a macro virus which infects Microsoft Excel workbooks. The virus contains the following macro functions: Auto_Open, Auto_Close, Tim, Ky, Ghst, Trl, Dstr, Thc1, Chk1, Chk2, Thc2, ins, txt, Hdn, Waw, Hlt, Icn_1, Icn_3, bt_2, bt_1, and prt. However, the Ghst and Trl functions are never called. The macros reside on a hidden VBA worksheet named Module1, but you can see them by looking in the macro listing under the Tools menu.

Protection

• Virus Definitions (Intelligent Updater) January 8, 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
• Payload Trigger: 5th of any month, after January

Distribution

• Distribution Level: Low
• Target of Infection: Microsoft Excel documents

TECHNICAL DETAILS

When an infected file is opened, the virus first looks in the C:MSOFFICEEXCELXLSTART directory for a file named EXCELVBA.XLA. If this file does not exist or its size is less than 22,000 bytes (which means the file was not created by XM.Delta), the virus creates it and copies the viral code into it. Thus it overwrites any file already named EXCELVBA.XLA in that directory. XM.Delta then writes out the text of the VBA module to the XLSSTART directory as a hidden file named EXCELVBS.TXT. Another copy of the virus is created in the same way but named BOOK1.XLS.

Next the virus’s first payload is released. If the date is the 5th of any month after January, the virus deletes all *.ini files in C:WINDOWS and all files on the A: drive. A dialog box then appears with the following text:

You are prompted to input a password—"Delta." If you enter the wrong password, another message box opens with the text "Sorry ..!, My Project Name is Delta," and Excel closes. However, if you enter the correct password, the virus continues executing and Excel opens normally.

The virus next sets up its second payload. Fifteen minutes after Excel starts, XM.Delta deletes the contents of cells C5, C6, and G12-H13 on the first worksheet. Finally the virus sets up its infection routines. It changes the functions of the New and Save buttons on the Tool bar to insert the viral worksheet into a new or saved document, respectively.

Because the paths are hard-coded, the virus may not spread unless Excel was installed into the default directory (C:MSOFFICEEXCEL).

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.