Backdoor.Nerte

SUMMARY

Backdoor.Nerte is a backdoor Trojan that allows a hacker complete access your computer. It can be programmed by the hacker to run when you start Windows.

Protection

• Virus Definitions (Intelligent Updater) November 9, 2001

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
• Payload: Allows unauthorized access
• Releases Confidential Info: Allows a hacker full access to all data on the infected computer.
• Compromises Security Settings: Allows a hacker full access to all data on the infected computer.

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

Backdoor.Nerte acts as a typical backdoor Trojan, and it allows a hacker to take control of your computer.

When it is executed, Backdoor.Nerte does the following:

It add the value:

ScanRegistry  <Path> srvnt.exe

to the registry key

HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRun

NOTES:

• ScanRegistry is a legitimate Windows program that is included with Windows 98/Me. By default, if this program exists, it loads from the Run key when you start Windows. The normal value is ScanRegistry  "C:Windowsscanregw.exe /autorun"
• If the ScanRegistry value is present, the original value is overwritten by the Trojan to the value shown.
• Nsrvnt.exe is a Trojan file.

The Trojan also adds the following keys:

HKEY_LOCAL_MACHINESoftwareNerte
HKEY_LOCAL_MACHINESoftwareNerteTR

and then add the following values:

Visib   "3"
Auto   "Yes"
Accept   "Yes"
Port   "80"

to HKEY_LOCAL_MACHINESoftwareNerteTR

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

Delete files that are detected as Backdoor.Nerte, and reverse the changes that the Trojan made to the registry.

To remove the Trojan:

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 Backdoor.Nerte. Deleted files must be either replaced from a clean backup or reinstalled.

To edit the registry:

CAUTION: We strongly recommend that you back up the system registry before you make any changes. Incorrect changes to the registry could result in permanent data loss or corrupted files. Please make sure that you modify only the keys that are specified. Please see the document How to back up the Windows registry before you proceed.

1. Click Start, and click Run. The Run dialog box appears.
2. Type regedit and then click OK. The Registry Editor opens.
3. Navigate to and delete the following keys:
   
   HKEY_LOCAL_MACHINESoftwareNerte
   HKEY_LOCAL_MACHINESoftwareNerteTR
   
4. Navigate to the key
   
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRun
   
5. Follow the instructions for your operating system:
   • Windows 95/NT
     1. In the right pane, delete the value
        
        ScanRegistry  <Path> srvnt.exe
        
     2. Exit the Registry Editor.
        
        
   • Windows 98/Me
     1. In the right pane, double-click the value
        
        ScanRegistry  <Path> srvnt.exe
        
     2. In the Value Data box, change the text to
        
        C:Windowsscanregw.exe /autorun
        
        NOTE: If you installed Windows to a location other then C:Windows, make the appropriate path substitution when you enter the text.
        
     3. Click OK, and then exit the Registry Editor.