W32.Yaha.Z@mm

SUMMARY

W32.Yaha.Z@mm is a variant of W32.Yaha.C@mm. It has been repacked to make it difficult for antivirus software to detect.

NOTE: Definitions dated prior to July 10, 2003 may detect this threat as W32.Yaha.V@mm.

Protection

• Virus Definitions (LiveUpdate™ Weekly) July 9, 2003
• Virus Definitions (Intelligent Updater) July 5, 2003

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 0 - 49
• Number of Sites: 0 - 2
• Geographical Distribution: Low
• Threat Containment: Easy
• Removal: Moderate

Damage

• Damage Level: Low

Distribution

• Distribution Level: High

TECHNICAL DETAILS

Refer to the W32.Yaha.C@mm writeup.

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

Sequentially perform the following instructions. We strongly recommend that you read and understand the entire procedure before proceeding.

To remove this worm, do the following:

1. Configure Windows to show all the files.
2. Copy Regedit.exe to Regedit.com, in most cases.
3. Edit the registry and remove the keys and changes that the worm made.
4. Delete the files that the worm placed on the computer.
   

For specific details, refer to the following sections.

1. Configuring Windows to show all the files
Perform this step to make sure that you can find the files that W32.Yaha.Z@mm installed.

1. Start Windows Explorer.
2. Click the View menu (Windows 95/98/NT) or the Tools menu (Windows Me/2000), and then click Options or Folder Options.
3. Click the View tab, and if necessary, uncheck "Hide file extensions for known file types."
4. Click Show all the files, and then click OK.

2. Copying Regedit.exe to Regedit.com
If you cannot start the program files, or if you see the message, "Windows cannot find wintask.exe," then you will need to copy Regedit.exe to Regedit.com.

1. Do one of the following, depending on the operating system you are running:
   • Windows 95/98 users: Click Start, point to Programs, and then click MS-DOS Prompt.
   • Windows NT/2000 users:
     • Click Start, and then click Run.
     • Click Browse, and then browse to the Winntsystem32 folder.
     • Double-click the Command.com file, and then click OK.
2. Type copy regedit.exe regedit.com, and then press Enter.
3. Type start regedit.com, and then press Enter.
4. Proceed to the following section, "Editing the registry and removing the keys and changes that the worm made."

NOTE: The Registry Editor will open in front of the DOS window. After you finish editing the registry and close Registry Editor, close the DOS window.

3. Editing the registry and removing the keys and changes that the worm made

CAUTION: We strongly recommend that you back up the system registry before making any changes. Incorrect changes to the registry can result in permanent data loss or corrupted files. Make sure to modify only the specified keys in this document.

For more information about backing up the registry, read, "How to back up the Windows registry," before proceeding with the following steps. Consult a computer technician, if you prefer to seek the assistance of a professional.

1. Start the Registry Editor if necessary.
   • If you performed the procedure in the previous section, the Registry Editor is already open. Skip to step 4.
   • If it was not necessary to perform the procedures in the previous section, proceed to step 2.
     
2. Click Start, and then click Run. (The Run dialog box appears.)
3. Type regedit, and then click OK. (The Registry Editor opens.)
   
   NOTE: If you see an error message or if the Registry Editor does not open, go back to and follow the instructions in the previous section.
   
   
4. Navigate to and select the following key:
   
   HKEY_LOCAL_MACHINESoftwareClassesexefileshellopencommand
   
   CAUTION: The HKEY_LOCAL_MACHINE key contains many subkey entries that refer to other file extensions. One of these file extensions is .exe. Changing this extension can prevent any files ending with a .exe extension from running. Make sure that you browse all the way along this path until you reach the command subkey.
   
   Modify the HKEY_LOCAL_MACHINESoftwareClassesexefileshellopencommand subkey that is shown in the following figure:
   
   
   <<=== NOTE: This is the key that you need to modify.
   
   
5. In the right pane, double-click the (Default) value.
6. Delete the current value data, and then type: "%1" %* (That is, type the following characters: quote-percent-one-quote-space-percent-asterisk.)
   
   NOTE: Registry Editor will automatically enclose the value in quotation marks. When you click OK, the (Default) value should look exactly like this:
   ""%1" %*"
   
   Make sure that you completely delete all the value data in the command key before typing the correct data. If you leave a space at the beginning of the entry, any attempt to run the program files will result in the error message, "Windows cannot find .exe." If this happens to you, start over from the beginning of this document, making sure to completely remove the current value data.
   
   
7. Restart the computer.

4. Deleting the files detected as W32.Yaha.Z@mm

1. Obtain the most recent virus definitions. There are two ways to do this:
   • Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
   • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).
     
     The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.
     
     
2. Start Norton AntiVirus and make sure that it is configured to scan all the 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 the files that are detected as W32.Yaha.Z@mm.