After some deliberation, we feel that the Snort Back Orifice pre-processor vulnerability could become a big problem very fast. As a result, we turned the Infocon status to 'yellow'.
A number of exploits for this vulnerability has now been published ranging from denial of service to remote code execution exploits.
You have a problem if you run Snort Version 2.4 (other then 2.4.3), and if you have the 'bo' preprocessor enabled.
Why do we think this is a big deal:
* The exploit is rather easy to write. Yes, its specific to a particular binary, but there are a number of common binaries deployed in large numbers.
* It uses a single UDP packet, which can lead to very fast spreading worms.
* The UDP packet can be spoofed, and can use any port combination.
* Snort is very popular. A fast spreading (noisy) UDP worm could lead to local slowdowns/outages.
The quick fix is to disable the BO preprocessor. Please do so NOW (if you haven't already). Worry about upgrading snort later, after you have done your testing. But going through this myself, its not that hard.
Snort before version 2.4 is not vulnerable. Neither is any Snort install that does not have the bo preprocessor enabled.
Please let us know if you see exploits posted, or have other details to share. We expect to stay on 'yellow' for about 12-24 hrs unless there are any new developments.
Handler's Diary October 20th 2005
Snort BO status update (NEW)
Last Updated: 2005-10-20 05:27:33 UTC by Kyle Haugsness (Version: 1)
Here is an update regarding the Snort Back Orifice pre-processor vulnerability...(Kyle Haugsness Oct. 20 05:30 UTC)
When this vulnerability was announced yesterday, I was curious to see how difficult this would be to exploit due to the widespread nature of Snort. After doing a little research on the encryption method in Back Orifice, I was able to develop working exploit code in 2 hours. Bad news!! Of course, we aren't in the business of releasing exploits, so this code is staying private. Now, it appears that HD Moore is very close to having exploit code working as a plugin to metasploit. If we haven't said it loudly enough already, PLEASE UPGRADE your Snort sensors or disable the BO pre-processor if running the vulnerable versions of Snort 2.4 series. I checked the 2.3.2 source tree today and it is not vulnerable.
How about defensive measures? If you are running Snort and are able to upgrade, then the new version should detect the exploit attempt. But I am working on two additional defensive tools. The first is a Snort signature that should catch the exploit attempt. This should be available real soon now (tm).
The second tool may prove to be much more valuable. This tool is necessary because of the fact that the exploit can be triggered on any UDP port (except 31337) and that all Back Orifice traffic is encrypted. I don't want to give away more information at this point, since it will help the exploit writers. The tool is a standalone program that utilizes libpcap to sniff traffic and decode UDP traffic looking for the exploit. It will be useful to folks that can't upgrade their Snort daemon to get the new detection it provides, but still want to see if they are being attacked. Secondly, this will be useful to people running a different IDS system that can't decode the Back Orifice encryption. Third, it will probably be very useful in identifying a global worm outbreak.
Since time is of the essence here, I am hoping to have this tool available very shortly. It will require libpcap and is being developed on Debian Linux. It will not require Snort to be running. Since code portability isn't my strong suit, we may be looking for people to test and port the code to FreeBSD, Solaris, etc. Please drop us an e-mail if you would be willing to help in this area. The source code is currently about 800 lines.