Wednesday, 13 January 2016

Writeup for n00b16CTF

Ah! Year 2 of n00b ctf @ backdoor. Quite a nice and fun contest. Though this contest was for n00bs, I decided to take part again (just coz I won last time - find the writeup for those questions here). This time, I came second, since I couldn't solve 2 of the questions, and one other team (dcua) solved all except 1. However, a short while after the contest was done, I solved another. Now only the question titled EULA remains. Hopefully, I'll update this post with its writeup soon too :)

Here's a bunch of writeups (more like small hints) for each of the questions I solved:


Take a look at the URL as you click on the items. Maybe higher numbered items are interesting (even if they cannot be directly accessed). The Hackbar add-on for Firefox makes this very easy to go through, btw.


Okay, I didn't even run the binary. The answer was just lying there in one of the strings (hint hint) inside it directly. Pre-execution-analysis itself showed up the flag.


We can disassemble this executable and change the path of execution to either circumvent the check (through gdb) or by flipping one bit in the binary. Each of these would print the flag. Another thing is to try to work out how the flag is being made in the memory, but that's just wasting your time. Take the easy path :P


Still unsolved by me. However the Accept-Language HTTP parameter seems very interesting, especially with things like en, it etc.


A very high stake challenge (it had 150 points) but it a very short while, it was obvious it is a format string bug. After that it is trivial to attack it (read up on FSBs if it isn't). I made a stupid mistake though which took a couple of minutes to realize. I was sending a single quote at the beginning and at the end. Doh!


A multi byte xor cipher. My favourite tool for this ( works directly.


The = at the end gives away that it is base64 encoded. Decoding it gives another cipher but the spacing makes it obvious that it must be some simple ancient cipher. Caesar would be proud.


I couldn't solve this challenge during the contest, but solved it just after it, and damn, was I over thinking it. During the contest, I realized that the gs were too regular (every 7th place) so I split it into 32 lines of 6 characters each (after removing the gs at the end of each line). All these were hex digits, and with the 6 character thing, I naturally thought of colours (for example, #00ff00 is green). Lot of time was wasted in this direction, but after the contest and a small hint of transposing the matrix (i.e. making it into 6 rows of 32 characters each), it was obvious that each of those could be hashes. Online MD5 crackers then gave away the answers.


Restricted bash - where cd wasn't allowed. Well, ls -l .. worked. And there was the flag right there to be cated to the screen. Later on, I found out that you could also execute shell inside this rbash and then you could do anything you wanted!


The name of this challenge gave away that we needed to look at robots.txt (that pesky little file that web crawlers are supposed to adhere to, but only lead to attackers finding things they shouldn't).


Whoa! That is a lot of data, but seems like the javascript console in the browser nicely handles it. Turns out this is the esoteric language JSFuck.

Did you find any other cool/new ways of solving any of these tasks? If so, leave a comment below.