VRT Advisories

VRT Rules 2006-01-25

Sourcefire VRT Update

Date: 2006-01-25


The Sourcefire VRT has been investigating the Black/Nyxem worm and its variants that are known to affect hosts using the Microsoft Windows operating system.


The VRT has done extensive analysis of the worm. The VRT has posted detailed pcaps from our lab here. You can also read a VRT engineer's candid review here.

The VRT has posted detailed pcaps of this worm from our lab. Users can download and view these pcaps here.

Initial reports from other sources make the following assertions about the worm behavior:

  1. Spreads mostly by email, and relies on people actually clicking on an attachment to infect themselves.
  2. Attempts to spread by SMB shares C$ and ADMIN$
  3. Hits a counter page that reports number of infected hosts. http://webstats.web.rcn.net/cgi-bin/Count.cgi?df=765247
  4. Attempts to check connectivity by hitting microsoft.com

From the initial binary analysis the VRT agrees with 1, 2, and 3. The worm is currently running in the lab to confirm all of these behaviors, and to confirm that the current rules in the VRT rule set will detect infected hosts.

The web stats counter for this worm is showing a large number of infections, however the accuracy of this counter can not be relied on since the counter does not verify the authenticity of the request. The VRT has not encountered any instances of infection whatsoever.

Infected hosts only make one request to the counter page, so our current opinion is that the best detection method for finding infected hosts is looking for attempts to spread via SMB shares, as it looks like it generates a large number of requests on this protocol.

In order to determine if an infection is in progress on a network, check Snort logs for the following SID's (these SID's detect attempts to connect to C$ and ADMIN$).

2177 NETBIOS SMB startup folder unicode access
2466 NETBIOS SMB-DS IPC$ unicode share access
2470 NETBIOS SMB C$ unicode share access
2472 NETBIOS SMB-DS C$ unicode share access
2473 NETBIOS SMB ADMIN$ unicode share access
2475 NETBIOS SMB-DS ADMIN$ unicode share access
1042 WEB-IIS view source via translate header
1149 WEB-CGI count.cgi access
1917 SCAN UPnP service discover attempt
466 ICMP L3retriever Ping
538 NETBIOS SMB IPC$ unicode share access

Additionally, the IP address for webstats.web.rcn.net has not changed in days so an alternate detection method may be to try the following rule to find hosts as they get infected.

alert tcp $HOME_NET any -> 80 (msg:"COMMUNITY VIRUS Possible BlackWorm or Nymex infected host"; uricontent:"/cgi-bin/Count.cgi?df=765247"; sid:100000226; rev:1;)

It should be noted however, that subsequent variants of the worm may display different behavior and this rule could be subject to generating false negative events.

This rule is in the latest Community rule pack and is identified as sid 100000197. If this rule proves correct in the lab and can be improved upon, the VRT will update the VRT Certified Ruleset accordingly.

Additional References:





About the VRT:

The Sourcefire VRT is a group of leading edge intrusion detection and prevention experts working to proactively discover, assess and respond to the latest trends in hacking activity, intrusion attempts and vulnerabilities. This team is also supported by the vast resources of the open source Snort community, making it the largest group dedicated to advances in network security industry.