On Sat, Aug 22, 2015 at 12:43 AM, Yawning Angel yawning@schwanenlied.me wrote:
On Fri, 21 Aug 2015 17:51:20 -0700 Kevin P Dyer kpdyer@gmail.com wrote:
On Wed, Aug 19, 2015 at 11:58 AM, Yawning Angel yawning@schwanenlied.me wrote:
[snip]
The FTE semantic attack they presented isn't the easiest one I know of (the GET request as defined by the regex is pathologically malformed).
Very interesting! This is news to me. I'm assuming I did something silly. (Even though I tested it against bro, wireshark, etc.)
Huh. I brought it up in conversation with a few people and was under the impression it was passed on. I probably should have e-mailed you about it or something.
How is it pathologically malformed?
"manual-http-request": { "regex": "^GET\ \/([a-zA-Z0-9\.\/]*) HTTP/1\.1\r\n\r\n$" },
No "Host" header. All complaint requests MUST include one per RFC 2616, and all compliant servers MUST respond with a 400 if it is missing.
Ah, gotcha. It's not RFC compliant. RFC2616 was created in 1999 and there are tons of HTTP-like implementations since then that, ostensibly, don't need to follow it. (e.g., an HTTP-like client/server that only talk to each other.) A network monitor must deal with these cases too, and they'll broadcast HTTP/1.1 in their headers.
This [1] paper is a bit dated (2007) but my intuition is that real-world implementations have drifted even further from the RFC over the last 8 years. I swear there's a more recent paper on this topic, but I couldn't find it...
Since requests of that sort should invoke the error path on RFC compliant servers it's a really good distinguisher since legitimate clients will not do such a thing. Existing realistic adversaries already have "identify 'suspicious behavior', call back to confirm" style filtering in production, so false positive rate can be reduce to 0 if needed.
Based on our exploration of data, we found there's a wide range of implementations and most of which have non-RFC-compliant behaviors. See Section 4 of our paper for more details. For that reason I'd be very surprised if a host-header-check could result in a 0 FP rate.
With that being said, I'll add the host-header-check to the list of experiments that we want to do for the full version of our paper. Would be interesting to learn what the data tells us.
-Kevin
[1] https://www.ideals.illinois.edu/bitstream/handle/2142/11424/Non-compliant%20...