1. Symantec/
  2. Security Response/
  3. W32.HLLW.Bymer


Risk Level 2: Low

October 9, 2000
February 13, 2007 11:50:29 AM
Also Known As:
Dnet.Dropper, W32/MsInit.worm.a [McAfee], Worm.Bymer.a [Kaspersky], TROJ_MSINIT.A [Trend], WORM_BYMER.A [Trend], W32/Bymer-A [Sophos], Win32.Bymer.A [Computer Associ
Systems Affected:

W32.HLLW.Bymer is a high-level language worm (HLLW). The Symantec AntiVirus Research Center (SARC) is currently aware of two different variants of this worm.

The first variation arrives as a file named Wininit.exe. The second variation is named Msinit.exe. Both variations have the same functionality, but their payloads vary slightly. Wininit.exe carries the Dnetc client with it, whereas Msinit.exe only copies it.

Because one variation carries the Dnetc client and the other does not, the size can be either approximately 22 KB or 220 KB. Because all received samples have been packed using different versions of UPX (a runtime compressor for Windows executable files), the file size may vary slightly.

Since the functionality of both versions is almost the same, the following information applies to both variations:

When first executed, the worm modifies one of the following registry keys:



This ensures execution upon restart. It then immediately attempts to spread by checking IP addresses for shared drives. It tries one IP address, sleeps for two seconds, and then tries the next address.

It uses some randomization when searching for IP addresses. If a shared drive is found, the worm checks to see whether the Windows folder is available. If so, it inserts itself into the \Windows\System folder and modifies the Load= line in the Win.ini file. This ensures that the worm will execute when the computer restarts. It also inserts or copies the Dnetc client, depending on the version.

The Dnetc client is not viral. Additional information can be found at distributed.net.


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: Neal Hindocha
Summary| Technical Details| Removal

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