Wednesday, May 28, 2008

Snort Evasion Vulnerability in Frag3

I saw this Snort news item reporting a "potential evasion in Snort." This should have been listed in the release notes for 2.8.1, which is said to fix the problem. I found the original iDefense Labs advisory which credits Silvio Cesare, who probably sold the vulnerability to iDefense Labs. From the advisory:

Snort does not properly reassemble fragmented IP packets. When receiving incoming fragments, Snort checks the Time To Live (TTL) value of the fragment, and compares it to the TTL of the initial fragment. If the difference between the initial fragment and the following fragments is more than a configured amount [5], the fragments will be silently discard[ed]. This results in valid traffic not being examined and/or filtered by Snort...

Exploitation of this vulnerability allows an attacker to bypass all Snort rules. In order to exploit this vulnerability, an attacker would have to fragment IP packets destined for a targeted host, ensuring that the TTL difference is greater than the configured maximum.

This is a problem in Frag3, as you can see in the spp_frag3.c diff. The change log mentions it too. You can see a change to README.frag3.

The SecurityFocus BID shows 2.8.0 and 2.6.x as being vulnerable.

iDefense is the only place where I've seen a workaround posted:

In the snort.conf file, set the ttl_limit configuration value to 255 as shown below.

preprocessor frag3_engine: ttl_limit 255

This will set the allowable difference to the maximum possible value, and prevent fragments from being dropped.

This will probably affect performance, but it's worth it until you can update to 2.8.1.

The National Vulnerability Database points to other relevant advisories.

The good aspect of this issue is the amount of transparency offered by Snort being an open source product. We can directly inspect the code to see what has changed and how it was documented.

This bad aspect of this issue is it reminds us not to rely on a single tool for network security monitoring purposes. If we only rely on Snort to "tell us what is bad," we could miss a large amount of activity initiated by a smart intruder who understands this vulnerability.

This problem is an example of the most subtle sort of network security evasion. Now that the vulnerability is public, how do we tell if we have been evaded in the past? I imagine it could only be detected by reviewing full content data already captured to disk. TTL values are not captured by default in session data, and statistical data is usually not granular enough either.

However, if a site is capturing full content data, two options arise. The first would be to re-process stored full content data through Snort 2.8.1 to see if any alerts appear that were not triggered in Snort 2.8.0 and earlier. The second is to write a custom analyzer to check TTL values for unusual properties.

In addition to reviewing network data, this issue demonstrates the value of non-network data. Host-based logs, application logs, DNS records, and other sources could all have pointed to problems that might have been ignored by Snort. Also remember that a decrease in the number of security events reported per unit time can be just as significant as an increase in the number of events. For example, people always worry when they see a surge in alerts, or bandwidth usage, and so on. It could be just as important when a large decrease in alerts, or bandwidth, or whatever is observed. In this Snort vulnerability, it would have indicated a potential evasion condition being exploited.


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Anonymous said...

Interesting. I wonder how many other IDSes are susceptible to similar issues. Might be interesting to watch for any advisories from other IPS vendors in the coming weeks. I'm guessing most of them cut some corners when handling fragmented packets, so it should be interesting.

Anonymous said...

Richard -- talk a little about how hard it would be to get your ttl to vary by 5 in a reassembled session.

Richard Bejtlich said...

With the right tool, trivial. There's nothing "out of the box," however. You need to write something yourself or make good use of something like Scapy.