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Discovered: August 18, 2003
Updated: August 11, 2017 2:01:31 PM
Type: Worm
Infection Length: Varies
Systems Affected: Windows
CVE References: CVE-2003-0109

W32.Welchia.Worm is a worm that propagates by exploiting the Microsoft Windows DCOM RPC Interface Buffer Overrun Vulnerability (BID 8205) and the Microsoft Windows ntdll.dll Buffer Overflow Vulnerability (BID 7116).

This worm attempts to remove W32.Blaster.Worm (MCID 1761) and patch the system.

As of February 26, 2004, due to a decreased rate of submissions, Symantec Security Response has downgraded this threat to a Category 2 from a Category 3.

W32.Welchia.Worm is a worm that exploits multiple vulnerabilities, including:

  • The DCOM RPC vulnerability using TCP port 135. The worm specifically targets Windows XP machines using this exploit.
  • The WebDav vulnerability using TCP port 80. The worm specifically targets machines running Microsoft IIS 5.0 using this exploit. As coded in this worm, this exploit will impact Windows 2000 systems and may impact Windows NT/XP systems.
W32.Welchia.Worm does the following:
  • Attempts to download the DCOM RPC patch from Microsoft's Windows Update Web site, install it, and then restart the computer
  • Checks for active machines to infect by sending an ICMP echo request, or PING, which will result in increased ICMP traffic
  • Attempts to remove W32.Blaster.Worm

Antivirus Protection Dates

  • Initial Rapid Release version August 18, 2003
  • Latest Rapid Release version May 04, 2019 revision 019
  • Initial Daily Certified version August 18, 2003
  • Latest Daily Certified version May 05, 2019 revision 002
  • Initial Weekly Certified release date August 20, 2003

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

Technical Description

When W32.Welchia.Worm is executed, it performs the following actions:

Copies itself to:

  • %System%\Wins\Dllhost.exe

Note: %System% is a variable. The worm locates the System folder and copies itself to that location. By default, this is C:\Winnt\System32 (Windows 2000) or C:\Windows\System32 (Windows XP).

Makes a copy of %System%\Dllcache\Tftpd.exe as %System%\Wins\svchost.exe.

Note: Tftpd is a legitimate program, which is not malicious, and therefore Symantec antivirus products do not detect it.

Adds the subkeys:
  • RpcPatch

  • RpcTftpd

to the registry key:
  • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services

Creates the following services:
  • Service Name: RpcTftpd
  • Service Display Name: Network Connections Sharing
  • Service Binary: %System%\wins\svchost.exe

This service will be set to start manually.

  • Service Name: RpcPatch
  • Service Display Name: WINS Client
  • Service Binary: %System%\wins\dllhost.exe

This service will be set to start automatically.

Ends the process, Msblast, and deletes the %System%\msblast.exe file, which W32.Blaster.Worm drops.

Selects the victim IP address in two different ways: The worm uses either A.B.0.0 from the infected machine's IP of A.B.C.D and counts up, or it will construct a random IP address based on some hard-coded addresses.

After selecting the start address, the worm counts up through a range of Class B-sized networks; for example, if the worm starts at A.B.0.0, it will count up to at least A.B.255.255.

Sends an ICMP echo request, or PING, to check whether the constructed IP address is an active machine on the network.

Once the worm identifies a machine as being active on the network, it will either send data to TCP port 135, which exploits the DCOM RPC vulnerability, or it will send data to TCP port 80 to exploit the WebDav vulnerability.

Creates a remote shell on the vulnerable host, which reconnects to the attacking computer on a random TCP port, between 666 and 765, to receive instructions.

Note: In the vast majority of the cases, the port is 707, because of the way the worm-threading model interacts with the implementation of the Windows C runtime .dll.

Launches the TFTP server on the attacking machine and instructs the victim machine to connect and download Dllhost.exe and Svchost.exe from the attacking machine. If the %System%\dllcache\tftpd.exe file exists, the worm may not download svchost.exe.

Checks the computer's operating system version, Service Pack number, and System Locale. It also attempts to connect to Microsoft's Windows Update and download the appropriate DCOM RPC vulnerability patch.

Once the update has been downloaded and executed, the worm restarts the computer so that the patch is installed.

Checks the computer's system date. If the year is 2004, the worm will disable and remove itself as follows:

Deletes the file %System%\Wins\Dllhost.exe

Deletes the services, RpcPatch and RpcTftpd, and removes the associated registry keys:
  • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RpcPatch
  • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RpcTftpd

The worm does not delete the file, %System%\Wins\Svchost.exe, which is a nonmalicious tftp server.

  • The worm activates its removal routine only if the worm is started in the year 2004. If the worm has been running continuously since 2003, it will not remove itself after January 1, 2004 unless you manually restart the computer or worm.
  • The W32.Welchia.Worm removal tool will still function normally in 2004.

Intruder Alert
On August 19, 2003, Symantec released Intruder Alert 3.6 W32_Welchia_Worm Policy .

Norton Internet Security/Norton Internet Security Professional
On August 20, 2003, Symantec released IDS signatures via LiveUpdate to detect W32.Welchia.Worm activity.

Symantec Client Security
On August 20, 2003, Symantec released IDS signatures via LiveUpdate to detect W32.Welchia.Worm activity.

Symantec ManHunt
  • Symantec ManHunt Protocol Anomaly Detection technology detects the activity associated with this exploit as "Portsweep." Although ManHunt can detect activity associated with this exploit with the Protocol Anomaly Detection technology, you can use the "Microsoft DCOM RPC Buffer Overflow" custom signature, released in Security Update 4, to precisely identify the exploit being sent.
  • Security Update 7 has been released to provide signatures specific to W32.Welchia.Worm to include the detection of more W32.Welchia.Worm attributes.

Symantec Gateway Security
  • On August 18, 2003, Symantec released an update for Symantec Gateway Security 1.0.
  • Symantec's full application inspection firewall technology protects against this Microsoft vulnerability, blocking all the above listed TCP ports by default. For maximum security, third-generation, full application inspection technology intelligently blocks the tunneling of DCOM traffic over HTTP channels; thus, providing an extra layer of protection not readily available on most common network filtering firewalls.

Symantec Host IDS
On August 19, 2003, Symantec released an update for Symantec Host IDS 4.1.


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.


You may have arrived at this page either because you have been alerted by your Symantec product about this risk, or you are concerned that your computer has been affected by this risk.

Before proceeding further we recommend that you run a full system scan . If that does not resolve the problem you can try one of the options available below.

If you are a Norton product user, we recommend you try the following resources to remove this risk.

Removal Tool

If you have an infected Windows system file, you may need to replace it using the Windows installation CD .

How to reduce the risk of infection
The following resources provide further information and best practices to help reduce the risk of infection.

If you are a Symantec business product user, we recommend you try the following resources to remove this risk.

Identifying and submitting suspect files
Submitting suspicious files to Symantec allows us to ensure that our protection capabilities keep up with the ever-changing threat landscape. Submitted files are analyzed by Symantec Security Response and, where necessary, updated definitions are immediately distributed through LiveUpdate™ to all Symantec end points. This ensures that other computers nearby are protected from attack. The following resources may help in identifying suspicious files for submission to Symantec.

Removal Tool

If you have an infected Windows system file, you may need to replace it using the Windows installation CD .

How to reduce the risk of infection
The following resource provides further information and best practices to help reduce the risk of infection.
Protecting your business network

The following instructions pertain to all current Symantec antivirus products.

1. Performing a full system scan
How to run a full system scan using your Symantec product

2. Restoring settings in the registry
Many risks make modifications to the registry, which could impact the functionality or performance of the compromised computer. While many of these modifications can be restored through various Windows components, it may be necessary to edit the registry. See in the Technical Details of this writeup for information about which registry keys were created or modified. Delete registry subkeys and entries created by the risk and return all modified registry entries to their previous values.

Writeup By: Frederic Perriot