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Discovered: September 09, 2002
Updated: February 13, 2007 11:40:22 AM
Also Known As: W32/NullBot [Security Focus]
Type: Trojan Horse
Systems Affected: Windows

Trojan.Nullbot is an IRC Trojan that allows a hacker to gain control of the infected computer. It is written in the Microsoft C programming language and may be compressed two times with UPX and ASPack.

NOTE: This threat includes several variants. Symantec antivirus products may detect some of the variants as IRC Trojan.

Antivirus Protection Dates

  • Initial Rapid Release version September 10, 2002
  • Latest Rapid Release version March 03, 2008 revision 035
  • Initial Daily Certified version September 10, 2002
  • Latest Daily Certified version March 03, 2008 revision 037
  • Initial Weekly Certified release date September 11, 2002

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Writeup By: Yana Liu

Discovered: September 09, 2002
Updated: February 13, 2007 11:40:22 AM
Also Known As: W32/NullBot [Security Focus]
Type: Trojan Horse
Systems Affected: Windows

When Trojan.Nullbot runs, it does the following:

It copies itself as C:\%system%\System.exe.

NOTE : %system% is a variable. The Trojan locates the \Windows\System folder (by default this is C:\Windows\System or C:\Winnt\System32) and copies itself to that location.

The Trojan adds the value

Microsoft IPC system.exe

to one of these registry keys:


so that it runs each time that you start Windows.

The Trojan contains its own Internet Relay Chat (IRC) client. This allows the Trojan to connect to an IRC channel that was coded into the Trojan. Using the IRC channel, the Trojan listens for commands from the hacker. The hacker accesses the Trojan by using a password-protected authorization. The commands allow the hacker to perform any of the following actions:

  • Manage the installation of the backdoor
  • Dynamically update the installed Trojan
  • Uninstall itself
  • Deliver system and network information to the hacker
  • Control the IRC client on the compromised computer
  • Send the Trojan to other IRC channels in an attempt to compromise more computers
  • Download and execute files
  • Perform Denial of Service (DoS) attacks against a target that is defined by the hacker


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • 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.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a 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.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. 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.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.

Writeup By: Yana Liu

Discovered: September 09, 2002
Updated: February 13, 2007 11:40:22 AM
Also Known As: W32/NullBot [Security Focus]
Type: Trojan Horse
Systems Affected: Windows

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

  1. Update the virus definitions.
  2. Restart the computer in Safe mode
  3. Run a full system scan, and delete all files that are detected as Trojan.Nullbot.
  4. Delete the value

    Microsoft IPC system.exe

    from the following registry keys:


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

To update the virus definitions:
All virus definitions receive full quality assurance testing by Symantec Security Response before being posted to our servers. There are two ways to obtain the most recent virus definitions:
  • Run LiveUpdate, which is the easiest way to obtain virus definitions. These virus definitions 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 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 restart in Safe mode:
All Windows 32-bit operating systems, except Windows NT, can be restarted in Safe mode. For instructions on how to do this, read the document How to start the computer in Safe Mode .

To scan for and delete the infected files:
  1. Start your Symantec antivirus program, and make sure that it is configured to scan all files.
  2. Run a full system scan.
  3. If any files are detected as infected with Trojan.Nullbot, click Delete.

To remove the values from the registry:

CAUTION : Symantec strongly recommends that you back up the registry before you make any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify only the keys that are specified. Read the document How to make a backup of the Windows registry for instructions.
  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 each of these keys:


    For each one, look in the right pane, and delete this value:

    Microsoft IPC system.exe
  4. Click Registry, and click Exit.

Writeup By: Yana Liu