Wyx.C (b)

SUMMARY

Wyx.C (b) is a polymorphic virus that infects boot sectors on local hard disks and floppy disks. It carries no payload, but may destroy FAT32 partitions when infecting them.

Protection

• Virus Definitions (LiveUpdate™ Weekly) August 14, 2002
• Virus Definitions (Intelligent Updater) August 12, 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: Low

Distribution

• Distribution Level: Low
• Target of Infection: Floppy and Hard Disk Boot Sectors

TECHNICAL DETAILS

Once activated Wyx.C (b) reduces the total memory available to DOS applications by 2 KB and then loads itself at the top of memory (at the 638K limit).

It then infects the Master Boot Record (MBR) and the Disk Boot Sector (DBS) of the first active partition on the local hard disk.

Wyx.C (b) is a memory-resident virus that checks periodically for uninfected boot sectors (using the timer interrupt which is activated about 18.7 times per second) on any floppy disk in drive A or on the local hard disk (MBR or DBS of first active partition).

Due to bugs in the virus it may:

• Overwrite part of the display memory during execution. This may cause garbled data to appear at the top of the screen.
• Damage or destroy FAT32 partitions when infecting them.

Strings

Wyx.C (b) contains the string:

20/01/2001 WYX

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

NOTE: These instructions are for all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines. They are for systems using FAT32.

1. Update the virus definitions.
2. Reboot the computer from a clean floppy system disk.
3. Run the DOS scanner to remove the virus.

For details on how to do this, read the following instructions.

To update the virus definitions:
There are two ways to do this:

• Run LiveUpdate, which is the easiest way to obtain virus definitions. These virus definitions have undergone full quality assurance testing by Symantec Security Response and are posted to the LiveUpdate servers one time each week (usually Wednesdays) unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, look at the Virus Definitions (LiveUpdate) line at the top of this write-up.
• Download the definitions using the Intelligent Updater. Intelligent Updater virus definitions have undergone full quality assurance testing by Symantec Security Response. They are posted on U.S. business days (Monday through Friday). They must be downloaded from the Symantec Security Response Web site and installed manually. To determine whether definitions for this threat are available by the Intelligent Updater, look at the Virus Definitions (Intelligent Updater) line at the top of this write-up.
  
  Intelligent Updater virus definitions are available here. For detailed instructions on how to download and install the Intelligent Updater virus definitions from the Symantec Security Response Web site, click here.

To reboot the computer from a clean floppy system disk:
You must obtain a floppy system disk that is known to be uninfected.

1. Insert it in the floppy disk drive an shut down the computer.
2. Turn off the power and wait at least thirty seconds.
3. Restart the computer. It should boot to an A prompt.

To run the Norton AntiVirus DOS scanner:

1. At the A:> prompt, type the following command, and then press Enter after each command:
   
   C:
   dir /s /b avdx.exe
   
   This displays the path to the Norton AntiVirus DOS scanner. If NAV is installed to a different drive, then change to the root of that drive first. The default is C:Program FilesNorton AntiVirus.
   
2. Change to the folder that contains Navdx.exe. You must use short file names. For example, if NAV is installed to C:Program FilesNorton AntiVirus, then type the following:
   
   cd progra~1 orton~1
   
3. Type one of the following commands.
   
   CAUTION: This could take several hours or more on some computers. Do not attempt to stop the scan once it has started.
   
   NOTE: The DOS-based scanner can perform one of the following actions when it detects a virus:
   
   • To be prompted for any file that is detected as infected, type the following:
     
     navdx /a /doallfiles /prompt [Enter]
     
     You must press R)epair, D)elete or C)ontinue for each infected file. If you choose this option and NAV cannot repair an infected file, then you will see the message "Unable to repair the file," followed by the same three choices. In most cases you should then choose D)elete, unless you are sure that the file is not actually infected.
     
   • To delete any file that is detected as infected, type the following:
     
     navdx /a /doallfiles /delete [Enter]
     
     The disadvantage of this is that files that could be repaired will be deleted.
     
   • To repair any file that is detected as infected, type the following:
     
     navdx /a /doallfiles /repair [Enter]
     
     CAUTION: If NAV cannot repair a file and you choose this option, the file will be skipped. This means that infected files will still be on your system. If you choose this option, then you must run Navdx again, this time using the /delete switch, as shown in the previous example.
4. Allow the scan to run. This could take several hours. When finished, remove the boot disk and restart the computer. If the computer does not restart because the virus has damaged the FAT32 partition, you may need the services of a qualified computer technician