1. Symantec/
  2. Security Response/
  3. W32.Downadup.E


Risk Level 2: Low

April 9, 2009
April 13, 2009 9:03:35 PM
Also Known As:
Infection Length:
119,296 bytes
Systems Affected:
CVE References:
The worm may be downloaded or delivered silently through Web exploits and then executed.

Once executed, the worm copies itself as the following file and then deletes the file:

It then drops the following file and runs it as a randomly named service driver:
%System%\0[RANDOM FILE NAME].tmp

The driver modifies the following file in order to disable the half-open connections limit:

It also modifies the following registry entry:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\"TcpNumConnections" = "00FFFFFE"

The worm removes the driver service from the compromised computer.

The worm creates the following registry entries:
  • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\CurrentVersion\Applets\"ds" = [ENCRYPTED DLL]
  • HKEY_CURRENT_USER\Software\Microsoft\CurrentVersion\Applets\"ds" = [ENCRYPTED DLL]

The worm then loads the encrypted DLLs from the registry into memory and drops a copy of W32.Downadup.C as the following file:
%System%\000[RANDOM FILE NAME].tmp

It also checks for the existence of the following registry entries:
  • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\CurrentVersion\Applets\"xl"
  • HKEY_CURRENT_USER\Software\Microsoft\CurrentVersion\Applets\"xl"

The worm also connects to the following URL to get its IP address:
  • http://checkip.dyndns.com
  • http://checkip.dyndns.org
  • http://www.findmyip.com
  • http://www.findmyipaddress.com
  • http://www.ipaddressworld.com
  • http://www.ipdragon.com
  • http://www.myipaddress.com
  • http://www.whatsmyipaddress.com

The worm then creates an HTTP server on the compromised computer on a random TCP port. The worm then sends this URL to remote computers. If successful, the remote computer will then connect back to this URL and download the worm so that each exploited computer can spread the worm, instead of downloading the worm from a predetermined location.

Next, the worm connects to a UPnP router and opens the HTTP port.

It then attempts to locate the network device registered as the Internet gateway on the network and opens the previously mentioned random TCP port, allowing access to the compromised computer from external networks.

The worm spreads by exploiting the Microsoft Windows Server Service RPC Handling Remote Code Execution Vulnerability (BID 31874).

Next, the worm periodically contacts the following sites to check the speed of the current Internet connection:
  • http://myspace.com
  • http://msn.com
  • http://ebay.com
  • http://cnn.com
  • http://aol.com

On May 3, 2009, the worm sets itself to be removed when the computer restarts. This does not remove the dropped copy of W32.Downadup.C.


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: Kachun Leung, Yana Liu and Sean Kiernan
Summary| Technical Details| Removal

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