The WebDav vulnerability (described in Microsoft Security Bulletin MS03-007) using TCP port 80. The worm specifically targets machines running Microsoft IIS 5.0 using this exploit. The worm's use of this exploit will impact Windows 2000 systems and may impact Windows NT/XP systems.
The Locator service vulnerability using TCP port 445 (described in Microsoft Security Bulletin MS03-001). The worm specifically targets Windows 2000 machines using this exploit.
The presence of the file, %Windir%\system32\drivers\svchost.exe, is an indication of a possible infection.
This threat is compressed with UPX.
Note: Virus definitions dated February 11, 2004 revision 23 (20040211.023 or Defs Version 60211w) or later will detect this threat.
When W32.Welchia.B.Worm runs, it does the following:
Creates a mutex named "WksPatch_Mutex." This mutex allows only one instance of the worm to execute in memory.
Copies itself as %System%\drivers\svchost.exe.
Notes:
%System% is a variable. The worm 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).
There is a legitimate system file, %System%\svchost.exe, which has the same file size as the worm on Windows XP system.
Creates the following service:
Service name: WksPatch Service binary: %System%\drivers\svchost.exe Service display name: Constructed in the form of %string1% %string2% %string3%, where:
%string1% is one of the following:
System
Security
Remote
Routing
Performance
Network
License
Internet
%string2% is one of the following:
Logging
Manager
Procedure
Accounts
Event
and %string3% is one of the following:
Provider
Sharing
Messaging
Client
For example, the service display name can be "Security Logging Sharing."
Deletes the service named "RpcPatch," if it exists.
Overwrites the HOSTS file with the following text:
#
#
127.0.0.1 localhost
Generates random IP addresses, and sends exploit data to the IP addresses, in an attempt to infect the systems:
Sends data to TCP port 135 to exploit the DCOM RPC vulnerability.
Sends data to TCP port 80 to exploit the WebDav vulnerability.
Sends data to TCP port 445 to exploit the Workstation Service vulnerability.
Sends data to TCP port 445 to exploit the Locator service vulnerability.
Runs an HTTP server on a random TCP port, so that the vulnerable computers can reconnect to an infected computer, then locally download and execute the worm as WksPatch.exe.
Searches the files in the IIS Virtual Roots and %Windir%\Help\\IISHelp\common folders with the following extensions, if the version of the operating system of the infected machine is Japanese:
.shtml
.shtm
.stm
.cgi
.php
.html
.htm
.asp
Note: The Virtual Roots and IIS Help folders are installed as part of Microsoft's Internet Information Services server.
Overwrites the files it finds with the following .htm file:
Downloads one of the following patches from Microsoft's Windows Update Web site, if the version of the operating system of the infected machine is Chinese (Simplified), Chinese (Traditional), Korean, or English:
Note: Before downloading the patch, the worm checks whether the patch is already applied, and if so, does not attempt to download it.
If not already applied, the worm will try to download the patch first as a normal process and then as a service. If it fails to download, the worm will not try to download it again. If the download is successful, the worm will delete the patch file after it runs.
Installs the patch, and then restarts the computer.
The worm will self-terminate on June 1, 2004, or after running 120 days, whichever comes first.
Symantec Client Security
Antivirus component: An update for the Symantec Client Security AntiVirus engine to protect against W32.Welchia.B.Worm has been available since February 11, 2004 via LiveUpdate (see above).
Symantec Client Firewall:Symantec Client Firewall ships with the default ruleset as "High: Block everything until you allow it." It will notify the user of the backdoor connection attempt and prompt the user to Permit, Block, or Customize a rule for that connection attempt, which the worm opened.
Symantec Gateway Security 5400 Series and Symantec Gateway Security v1.0
Antivirus component: An update for the Symantec Gateway Security AntiVirus engine to protect against W32.Welchia.B.Worm is now available. Symantec Gateway Security users are advised to run LiveUpdate.
IDS/IPS component: An update for the Symantec Gateway Security IDS/IPS engine to protect against W32.Welchia.B.Worm was posted on February 18, 2004. Symantec Gateway Security administrators are advised to run LiveUpdate to ensure protection against this threat.
Full application inspection firewall component: By default, Symantec's full application inspection firewall technology protects against the propagation of the W32.Welchia.B.Worm, by blocking attackers from using the WebDAV, DCOM over HTTP, and RPC attack vectors.
In addition, the default security policy blocks attackers from using backdoor ports on infected machines.
Symantec Enterprise Firewall 7.0.x and Symantec VelociRaptor 1.5
By default, Symantec's full application inspection firewall technology protects against the propagation of W32.Welchia.B.Worm by blocking attackers from using the WebDAV, DCOM over HTTP, and RPC attack vectors.
In addition, the default security policy blocks attackers from using backdoor ports on infected machines.
Symantec HIDS 4.1.1
Symantec released a LiveUpdate package on February 17, 2004 for users of Symantec HIDS 4.1.1. See the Symantec Host IDS 4.1.1 Security Update 3 for additional information.
Symantec ManHunt
RPC DCOM
This vector is detected by the custom signature, MS RPC DCOM HEAP Overflow, which was released in Security Update 11.
SMB Workstation
This vector is detected by the custom signature, SMB Workstation Service Overflow, which was released in Security Update 12.
HTTP WebDAV
Symantec ManHunt Protocol Anomaly Detection technology detects the activity associated with this exploit as "HTTP Malformed URL (HTTP_BAD_REQURL5)." An event refinement rule has been released in Security Update 20 to specifically detect this as "HTTP IIS Welchia WebDAV SEARCH BO."
Locator Overflow
This vector is detected by the custom signature, MS NETBIOS Locator Service Buffer Overflow, which was released in Security Update 20.
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 using the W32.Welchia.Worm Removal Tool
Symantec Security Response has developed a removal tool to clean the infections of W32.Welchia.B.Worm. This is the easiest way to remove this threat and should be tried first. To obtain the W32.Welchia.Worm removal tool, read the document, "W32.Welchia.Worm Removal Tool."
Manual Removal
As an alternative to using the removal tool, you can manually remove this threat. The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.
Disable System Restore (Windows XP).
Update the virus definitions.
Restart the computer in Safe mode or VGA mode.
Run a full system scan and delete all the files detected as W32.Welchia.B.Worm.
For details on each of these steps, read the following instructions.
1. Disabling System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.
Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.
Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.
For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:
2. Updating the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:
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).
3. Restarting the computer in Safe mode or VGA mode
Shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in Safe mode or VGA mode.
For Windows 95, 98, Me, 2000, or XP users, restart the computer in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."
For Windows NT 4 users, restart the computer in VGA mode.
4. Scanning for and deleting the infected files
Start your Symantec antivirus program and make sure that it is configured to scan all the files.
If any files are detected as infected with W32.Welchia.B.Worm, click Delete.
Additional information:
Translations of this Document:
Given the time needed to translate documents into other languages, the translated versions of this document may vary in content if the English document was updated with new information during the translation process. The English document always contains the most up-to-date information.
