Backdoor.Coreflood

Backdoor.Coreflood is a Backdoor Trojan horse that is primarily designed to conduct Denial of Service (DoS) attacks. The Trojan connects to an IRC server and gives control of the infected computer to an attacker.

The detection for Backdoor.Coreflood was modified on October 16, 2003 to account for a minor variant of this Trojan.

Also Known As:	Backdoor.Apdoor [KAV], CoreFlood [McAfee], CoreFlood.dll [McAfee]. Autoproxy Trojan [LURHQ]
Type:	Trojan Horse
Infection Length:	43,008 bytes, 24,576 bytes, 28,160 bytes, 69,632 bytes
Systems Affected:	Windows 95, Windows 98, Windows NT, Windows 2000, Windows XP, Windows Me
Systems Not Affected:	Windows 3.x, Macintosh, OS/2, UNIX, Linux

Virus Definitions (LiveUpdate™ Weekly) December 04, 2002 Virus Definitions (Intelligent Updater) December 02, 2002

Wild Number of infections: More than 1000 Number of sites: More than 10 Geographical distribution: High Threat containment: Easy Removal: Moderate

Threat Metrics Wild: Medium Damage: Medium Distribution: Low

Damage

• Payload:
  • Compromises security settings: Allows a hacker to gain unauthorized access to an infected computer.

The Trojan consists of two parts:

• A .exe file, which is the loader.
• A .dll file, which contains the primary code.

1. Extracts the .dll file from itself, and then copies itself and the .dll file to the %System% folder. The file names are arbitrary.
   
   Note: %System% is a variable. The Trojan locates the System folder and copies itself to that location. By default this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).
   
   
2. Next, the .exe file calls a function within the .dll file to begin executing the main code. The main code hooks the Explorer.exe process in a way so that all of its actions run under the process context of Explorer.exe.
   
   
3. Sets itself to run when Windows is started. It does this in various ways:
   • Older variants add the value:
     
     "<file name>"="%system%\<file name.ext>"
     
     to the registry key:
     
     HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Windows/CurrentVersion/Run
     
     The Trojan monitors this registry key and may re-add it if it is modified or removed.
     
     
   • Newer variants discovered in July 2004 create the following registry keys instead:
     
     HKEY_CURRENT_USER/CLSID/{<random clsid 1>}
     
     HKEY_CURRENT_USER/CLSID/{<random clsid 2>}
     
     HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/
     Explorer/ShellIconOverlayIdentifiers/<random name>/(Default) = {<random clsid 1>}
     
     HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/
     Explorer/Browser Helper Objects/{<random clsid 2>}
     
     They also add the value:
     
     (Default) = %System%\<random name>.dll
     
     to the registry keys:
     
     HKEY_CURRENT_USER/CLSID/{<random clsid 1>}/InprocServer32
     HKEY_CURRENT_USER/CLSID/{<random clsid 2>}/InprocServer32
     
     Note: {<random clsid X>} is a random value of the form, {########-####-####-####-############}, for example, {01234567-89ab-cdef-0123-456789abcdef}.
     
     
4. Connects to an IRC server and joins a predefined chat channel. It listens for commands to execute. These commands allow an attacker to gain access to the computer and potentially conduct a DoS attack against other computer systems.

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.

The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

1. Disable System Restore (Windows Me/XP).
2. Update the virus definitions.
3. Restart the computer in Safe mode or VGA mode.
4. Run a full system scan and delete all the files detected as Backdoor.Coreflood.
5. Reverse the changes that were made to the registry.

• "How to disable or enable Windows Me System Restore"
• "How to turn off or turn on Windows XP System Restore"

• 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.

• For Windows 95, 98, Me, 2000, or XP users, shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in Safe mode. For instructions, refer to the document, "How to start the computer in Safe Mode."
• For Windows NT 4 users, shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in VGA mode.

1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
   • For Norton AntiVirus consumer products: Read the document, "How to configure Norton AntiVirus to scan all files."
   • For Symantec AntiVirus Enterprise products: Read the document, "How to verify that a Symantec Corporate antivirus product is set to scan all files."
2. Run a full system scan.
3. If any files are detected as infected with Backdoor.Coreflood, write down the full path to and the file name of the infected file or files.
   
   Important: Do not skip this step. You will need the file names later in the removal.
   
   
4. Click Delete. If your Symantec antivirus product reports that it cannot delete an infected file, proceed to section 5 (modifying the registry), then restart the computer and manually delete the file.

1. Click Start > Run.
2. Type regedit
   
   Then click OK.
   
   
3. Navigate to the key:
   
   HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Run
   
   
4. In the right pane, delete any value that refers to any files that were detected as Backdoor.Coreflood.
   
   Note: All the variants do not add an entry to this key.
   
   
5. Navigate to and select the key:
   
   HKEY_LOCAL_MACHINE/Software/Classes/CLSID
   
   
6. Click Edit > Find.
   
   
7. In the "Find what" box, type the file name of the .dll file that was detected as Backdoor.Coreflood in section 4.
   
   
8. If you find an entry of the form:
   
   "(Default)"="%System%\<detected file name>.dll
   
   in the registry key:
   
   HKEY_LOCAL_MACHINE\Software\Classes\CLSID\{<random clsid>}\InProcServer32
   
   then write down the <random clsid> value>
   
   Then, in the left pane, delete the subkey:
   
   HKEY_LOCAL_MACHINE\Software\Classes\CLSID\{<random clsid>}
   
   
9. Next, click Edit > Find to repeat the search, as there may be more than one such key. Delete any that are found.
   
   
10. Navigate to and delete the key:
    
    HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion
    \Explorer/Browser Helper Objects/{<random clsid>}
    
    where {<random clsid>} matches one of the values found and deleted in the previous searches.
    
    
11. Navigate to and delete the key:
    
    HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/
    Explorer/ShellIconOverlayIdentifiers/<detected file name>
    
    Note: <detected file name> should match the name of the infected dll file. For example, if abcdwxyz.dll was detected as Backdoor.Coreflood, then delete the registry key:
    
    HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/
    Explorer/ShellIconOverlayIdentifiers/abcdwxyz
    
    
12. Exit the Registry Editor.
    
    
13. Restart the computer. If you could not delete any files in section 4, use Windows Explorer to locate and delete them.

Revision History:

• July 8, 2004: Updated Technical Description and removal instructions to account for new variants.
• July 31, 2003: Updated alias information, as well as removal instructions to include Safe Mode.
  

Write-up by: Eric Chien