Writeups – So_Stealthy, Rescue Shell (Nuit Du Hack Quals)

Yesterday, I participated in the Nuit Du Hack Quals. In hindsight, I should’ve played this all day – this was a good CTF quality-wise, with a good variety of challenges and infrastructure which seemed to remain online.

I will present two writeups below, for posterity.

So_Stealthy

This challenge was presented as a PCAP file, which you can download here. The objective was to reverse engineer some malware included in the pcap. At a glance in wireshark, most data is HTTP or HTTPS traffic: so we start by extracting all the HTTP/HTTPS objects:

I don’t have an consistent way to attempt these “needle-in-a-haystack” style problems (is there one?), so I went with a manual approach, running “strings” across each of the extracted objects.

Eventually, I identified an odd-looking favicon(1).ico, comprised of obfuscated JS:

I quickly extracted the base64, which turned out to be a binary file, containing a .NET executable at a non-zero offset. We then extract the file and pop it into ILSpy to determine what it does:

It looks like this is a reasonably simple XOR cipher, but I couldn’t find the key initialization anywhere. Going back to the JS, we find our culprit:

We can then match this up with our disassembly, to note that the “Aa6b…” function takes the last 22 characters of it’s argument as the xor key:

Knowing this, a quick python script gives us the flag:

Rescue Shell

This challenge was presented as a Linux binary, which you can download here. This was ostensibly a shell in itself, but was disabled (and infact, did not import any shell-executing functions).

Popping this into IDA pro, I quickly got the sense that this was a classic stack overflow vulnerability, conveniently using read (so null bytes were safe).

A quick test later, and this is confirmed, with the return address from check_passphrase being controlled:

Controlling the stack to this extent makes the rest of the challenge trivial: we then wrangle a rop chain to leak the address of one of the elements in the GOT (using 0x40099a as a “leak” gadget, taking an argument from the stack) and then from there, bounce to a libc magic gadget to give us shell:

You can download the python script I used here.

As always, thanks to the NDH organisers for putting together another quality CTF event. There’s quite a number of CTF events next weeked to participate in, so I’ll likely see you all in UIUCTF next weekend.

About Norman

Sometimes, I write code. Occasionally, it even works.
This entry was posted in Bards, Computers, Jesting. Bookmark the permalink.

7 Responses to Writeups – So_Stealthy, Rescue Shell (Nuit Du Hack Quals)

  1. anon says:

    Nice writeup, i have a noob question, why do they provide the libc.so.6 ? Its for local testing purpose? How can i set the ELF to run with that libc? Thanks!!

  2. squalltan2000 says:

    Hi, i am quite confuse with the exploit here. So basically you are leaking the GOT of fread 0x601210 and calculate the base address with GOTfread – 0x6A460. After that may i know which libc function you are returning to? Because i dont see any function with offset 0x41320. I am guessing you might return to system but if it is the case, we still need the arguement of /bin/sh

    • Norman says:

      This is a magic libc gadget. In libc there’s a few places where if you return, AND some conditions are met (in this case – from memory, RAX had to be zero), it’s the equivalent of a complete execve /bin/sh.

      Take a look at one_gadget for more information.

  3. squalltan2000 says:

    Thanks. It is very clear for me. However i still have one more question that might seems dump. May i know why you choose fread GOT to leak? I try other GOT for example strncmp and i realize the base address is not correct after minus the offset. Do we have a clue on which GOT to choose?

    • Norman says:

      There isn’t a specific reason beyond strncmp (the first logical choice) didn’t seem to work when I initially tried it, getting directed to some other variant strncmp when I tested locally.

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