Friday, July 21, 2006

Extracting gzipped or Unix script files from pcap data

During an incident response, it's often handy to be able to examine the actual attack traffic, and if you're using Sguil, you probably have it handy. One common situation is that an intruder has transferred files to or from your network, and you'd really like to see what's in them.

There's a great tool for extracting arbitrary files from pcap dumps, tcpxtract. Similar to the way hard drive forensic tools look through bytes on the disk to find the "magic headers" at the beginning of various types of files, tcpxtract combs through pcaps looking for file types it knows, regardless of the transport protocol used to ship them over the network. When it finds them, it writes them out to individual files for manual analysis.

Tcpxtract is a great tool for NSM practicioners, and should be in everyone's standard kit. There are a few common types of files that it doesn't support, but you can easily fix this by simply editing the tcpxtract.conf file to add support for new types if you know their magic numbers.

My friend geek00L has already blogged about adding Windows PE executable support. Now I'm here to tell you how to add support for gzipped files and Unix script files like "#!/some/file" ("#!/bin/sh" or "#!/usr/bin/perl" for example).

Just add the following two lines to the end of tcpxtract.conf:

gzip(1000000, \x1f\x8b\x08);
script(1000000, \x23\x21\x2f);

A little anti-climactic after all that buildup, wasn't it? I've had some advice that the script detection is likely to throw lots of false positives in an SSL session, so maybe you should keep it commented out until you know there are script files in the session that you need to find.

1 comment:

C.S.Lee said...

I think we can add those signatures to improve tcpxtract signatures database. I have actually added the other two at the link here -

By the way, I think we can just avoid the false positive by filtering out ssl traffic with bpf since normally it is just encrypted and meaningless to run tcpxtract kind of tools on it, it is more suitable to use tcpxtract on generic network traffics indeed.

Cheers :]