W32.Rexli.A@mm

SUMMARY

W32.Rexli.A@mm is a mass-mailing worm that is written in Visual Basic. When executed, the worm emails all contacts in the Microsoft Outlook address book. If mIRC is found, the worm modifies a file called Script.ini. This modification causes an infected user to send the worm to people over the IRC network.

NOTE: This worm is detected as Bloodhound.W32.VBWORM using definitions dated prior to February 6, 2002. The IRC script that it creates in the mIRC folder is detected as IRC.Family.Gen by all definitions.

Behavior

Symptoms

Transmission

Protection

• Virus Definitions (Intelligent Updater) February 6, 2002

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: Medium
• Payload: When executed for the first time
• Modifies Files: This worm overwrites the script.ini file in the mirc folder and all vbs files it can find on the drive.

Distribution

• Distribution Level: High
• Subject of Email: Cool Linki
• Name of Attachment: Linki.exe
• Size of Attachment: 57,344 Bytes

TECHNICAL DETAILS

W32.Rexli.A@mm is a worm that is believed to originate in Poland. The worm is written in Visual Basic, and it requires Visual Basic runtime libraries. When the worm is executed, it starts by creating two copies of itself: Rexec.exe and Linki.exe. They are located in the %System% directory.

NOTE: %System% is a variable. The worm locates the WindowsSystem folder (by default this is C:WindowsSystem or C:WinntSystem32) and copies itself to that location.

The first time the worm executes, it displays the following fake error message.



NOTE: The worm pauses its execution until you click the OK button.

Next, the worm scans the hard disk for .vbs files. If it finds one, it alters the file, which causes the worm to execute whenever the .vbs file is run.

After the worm scans the entire hard disk, it emails all users in the Microsoft Outlook address book. The email appears as follows:

Subject: Cool Linki
Body: Przesylam ci znaleziona baze danych linków. Jest tam duzo stron, których na pewno nie znasz :)
Attachment: Linki.exe
Size of Attachment: 57 KB

The worm then looks for Script.ini in the mIRC folder. If it finds the file, the worm overwrites it. This causes the file to be sent to other users over the IRC network.

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 make sure that NAV is configured to scan all files. For instructions on how to do this, read the document How to configure Norton AntiVirus to scan all files.
3. Run a full system scan.
4. Delete all files that are detected as W32.Rexli.A@mm or IRC.Family.Gen.

If the worm has been executed, all .vbs files will have to be replaced from clean backup copies.