Tag Archives: video

SpyderSec: Challenge

I can’t believe it’s been nine months since my last post. I must have been busy. Or lazy.

Just as well I recently completed SpyderSec‘s nice little X-Files themed encryption challenge to get me out of my blog funk.

This was a really fun one, and totally worth a look if you’re interested in video steganography. This is a walkthrough to how I completed the challenge, so if you fancy trying yourself look away now.

Flag #1

The challenge spec already makes clear that this is a web application challenge and a full nmap TCP scan confirmed that only port 80 was open. The web page served doesn’t reveal too much:

Website front page

Just in case an administrator left some content or administration pages open, I scanned the host with dirb.

Search turns up a /v/ directory, but this is Forbidden.

Looking at the html source of the page reveals some packed JavaScript:

You can dump this packed program into one of a few public JavaScript unpackers, but I wanted to see if it would evaluate fine in nodejs, which it did:

The hex values appeared to be ASCII characters, so I tried converting to a string and got some interesting output:

I wasn’t sure what to make of “muder.fbi” at this point. Plugging it into the dirb search didn’t turn up any hidden content.

Plugging away at the page for a while, I turned to look at the logs of the requests I had made in BurpSuite. I noticed that the page was including a cookie and that it mentioned a URI:

I already knew of the /v/ directory, so it stood that there was a good chance I would find something interesting if I accessed this location directly. However, I found disappointment in the fact that the URI was also a forbidden location:

The challenge was beginning to feel very much like a hidden data exercise. I tried plugging various strings I had captured into the web server to see if I could find something. I can’t remember how long it took, but eventually putting the URI together with ‘mulder.fbi’ caused the web server to offer me a 13MB file:

A cursory examination of the file showed it as a video file.

Flag #2: Kill Switch

The video turned out to be a song with lyrics slides. (The song incidentally is a reference to an episode of The X-Files – I had to look that up.)

At this point I figured that there was something hidden in the video file. Hidden data – particularly where video is concerned – is really not my strong suit. I searched the file for strings (which didn’t help) and for container metadata (didn’t find any). On a hunch I decided to run the file through a video processor. I asked the program to make a direct stream copy of the audio and video, meaning that the whole content would be copied without recompression into a new file. If there was any hidden data in the file, it might reveal a difference in file size.

Several megabytes of data were missing from the stream copy, but the file played exactly the same; a strong indication that there was data to be found in the file. Since the data was not part of the video and audio streams, I felt I had a chance with this. (If the data had been hidden as data in the streams themselves then my chances of success would have been much lower.)

Still, I don’t really know much about video container formats, so I took the least resistance path and did some web searches first of all. Actually, doing a search for “hide data in mp4 video” turns up some pretty interesting stuff immediately. (That link is seriously worth a read if you’re interested.)

Working on the hunch that the video file contains a TrueCrypt encrypted volume, the challenge essentially becomes “find the correct password”. And the thing about TrueCrypt volumes is that they’re practically indiscernible from random data. All you can do is keep trying passwords until one works and even if you never find one that works, you will never know whether the data is actually random or you’ve just not found the correct password.

Anyway, you probably know where this is going. Eventually (too long) I found the password. I can’t remember exactly when I turned my attention to the website images (I had already gone over the BurpSuite logs several times to no avail, and was considering writing or searching for a tool to automate TrueCrypt password attempts) when I dumped the webpage content and started looking for strings.

The password was hidden in the metadata in the following file:

SpyderSec Challenge.png

And there we have it. The TrueCrypt password is A!Vu~jtH#729sLA;h4%. After that, getting to the final flag is a breeze…

SpyderSec TyueCrypt password

SpyderSec TrueCrypt volume mounted successfully

SpyderSec challenge completed