I attended BSides London almost a month ago now, and of course took a look at the CTF. There were a handful of reversing challenges, but multiple of them were MacOS (Mach-O) binaries. As I looked down at my Windows laptop and my Kali VM, I felt at a bit of a disadvantage. While I was able to solve one of the challenges just with IDA, I went looking for a way to run Mac binaries on a Linux OS. And I found Darwin. It took basically the rest of the day to install, so I didn’t get to any of the additional challenges, but I am happy to be semi-equiped the next time the need comes up.

## Darling

### Background

Darling is a translation layer allowing you to run macOS software on Linux. It is just like how Wine lets you run Windows exes on Linux.

### Installation

Installation isn’t hard, but it take a long time. The install instructions were straight forward, but I did have to adjust one thing for Kali. I used the Ubuntu apt install line for dependencies, but linux-headers-generic didn’t exist. I just removed that item, and the installed worked. I already had some headers installed, so if just removing it doesn’t work for you, try to install these, likely the common one:

root@kali# apt list --installed linux-headers-*
Listing... Done
linux-headers-4.18.0-kali1-amd64/now 4.18.6-1kali1 amd64 [installed,local]
linux-headers-4.18.0-kali1-common/now 4.18.6-1kali1 all [installed,local]
linux-headers-4.19.0-kali1-amd64/now 4.19.13-1kali1 amd64 [installed,local]
linux-headers-4.19.0-kali1-common/now 4.19.13-1kali1 all [installed,local]
linux-headers-4.19.0-kali4-amd64/now 4.19.28-2kali1 amd64 [installed,local]
linux-headers-4.19.0-kali4-common/now 4.19.28-2kali1 all [installed,local]
linux-headers-amd64/now 4.19+104+kali1 amd64 [installed,upgradable to: 4.19+105+kali1]


Then it was just a matter of git clone, followed by build/cmake/make. The build process took about 6 hours on my machine.

### Testing

Now I can run the test that is the top of the Darling website:

root@kali# uname
Linux
root@kali# darling shell
Darling [/root/bsides-london-2019]# uname
Darwin


### Next Steps

I won’t get to it today, but there’s a project on GitHub that provides gdb for Darling. I’d like to install this and play around with it. That could be a future post. If it is, I’ll join it to this one.

## BSides London CTF: coffee

### Challenge

I originally solved this with just ida, but it’s nice to see the program run. I’m given a file, coffee:

root@kali# file coffee
coffee: Mach-O 64-bit x86_64 executable, flags:<NOUNDEFS|DYLDLINK|TWOLEVEL|PIE>


When I run it, I get a sad face with a usage telling me I need to give it a key:

### Testing

If I run it with some incorrect input, it puts out a message, but that message is garbled:

Darling [~/bsides-london-2019]# ./coffee 0xdf
a$$a a$$a
a$$a a$$$$a a$$   $a a$          $a a$$a a$$$ a
aa
aa
a    a
aa
a$$a a$$a
I <3 Java

Mr Sadface is sad, can you unlock his happiness by giving him the right key?

If you know, you know -> |9<doyccqEkYkPAfFcyP^gOPzCkAo[UXc[bPBfMgULzZUzYnmJw


Putting in numbers seems to get the output changing:

Darling [~/bsides-london-2019]# for i in {1..5}; do ./coffee \$i | grep 'If you know'; done
If you know, you know -> }8=eanxbbpDjXjQ@gGbxQ_fNQ{Bj@nZTYbZcQCgLfTM{[T{XolKv
If you know, you know -> ~;>fbm{aasGi[iRCdDa{R\eMRxAiCmYWZaYR@dOeWNxXWx[loHu
If you know, you know -> :?gclzrFhZhSBeEzS]dLSy@hBlXV[XaSAeNdVOyYVyZmnIt
If you know, you know -> x=8dk}gguAo]oTEbBg}TZcKT~GoEk_Q\g_fTFbIcQH~^Q~]jiNs
If you know, you know -> y<9aej|fft@n\nUDcCf|U[bJUFnDj^P]f^gUGcHbPI_P\khOr


### Reversing

I’ll open this in IDA and take a look. There’s only two real functions, _main and _coffee_sadface. The latter just prints the sadface message I’ve seen a couple times now.

_main is just a couple of branches, with one loop:

Looking at the bottom right area where the loop is, I can see a couple things:

1. A string that’s very similar to what I saw in the output is loaded and passed to memcpy, putting it into what I’ve named buffer.
2. A variable I’ve named counter will start at 0, and then the loop will run until the counter hits 53, then it will jump out.
3. When it jumps out, it will call printf with the “If you know” string and buffer.
4. In the loop, it reads the next byte, xors it with the input number, and writes it back to the buffer.

At this point, I can jump over to python. I know the flags take the format S33n0eViL{} (or it might have been S3en0eViL…I don’t quite remember, and you’ll see why I can’t prove it in a minute). So I can just xor the first byte in the seed with S and get the key:

root@kali# python3
Python 3.7.3rc1 (default, Mar 13 2019, 11:01:15)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> seed = '|9<doyccqEkYkPAfFcyP^gOPzCkAo[UXc[bPBfMgULzZUzYnmJw'
>>> ord(seed[0])^ord('S')
47
>>> ''.join([chr(47^ord(x)) for x in seed])
'S\x16\x13KO@VLL^jDvD\x7fnIiLV\x7fqH\x7fUlDn@tzwLtM\x7fmIbHzcUuzUvABeX'


Well… that’s not exactly what I was hoping for. Could it be a multi-byte xor? What if I look for the second byte:

>>> ord(seed[1])^ord('3')
10
>>> ''.join([chr(10^ord(x)) for x in seed])
'v36njesii{OaSaZKlLisZTmEZpIaKeQ_RiQhZHlGm_FpP_pSdg@}'


That actually looks really good, including the {} in the right spots. I can try this to get the full flag:

>>> ''.join([chr(10^ord(x)) if i % 2 else chr(47^ord(x)) for i,x in enumerate(seed)])
'S3\x13nOeViL{java\x7fKILLs\x7fTHE\x7fplanet_with\x7fHIGH_cpu_USAge}'


I can’t quite explain what the 5 non-ascii bytes are. But with a couple guesses, I can fill them in by hand, and get a flag that is accepted:

S33nOeViL{java_KILLs_THE_planet_with_HIGH_cpu_USAge}