Understand Game Hacking in One Post

Posted on Tue, 5 Sep 2017 at 15:06:36 EST

Tagged as tutorial, reverse-engineering, and game-hacking.

At a first glance, it might seem that game cheats like AimTux are something that could only be conjured by the most talented of reverse engineers. That was at least my initial view on it, especially since I always saw these game hackers using outlandish terms that I hadn't heard in over a year of playing in CTF's. Don't be fooled; game hacking isn't nearly as complex as its community makes it seem. In this post, I will explain the concepts in a way that is familiar to people with experience in binary exploitation and reverse engineering, but it shouldn't be too hard to understand if you lack that background.

You want to know the secret of game hacking? Editing memory. Much can be accomplished with nothing more than a few writes to process memory. This should be unsurprising if you've used Cheat Engine, scanmem, or even the Game Genie. Memory editing, despite the fact that much is nowadays validated on the server, remains king in the cheat market. Reading and writing memory be your primitives, and I'll show you just how effective they can be by walking you through a basic wallhack for CS:GO. I choose Counter-Strike as an example, because there is a wealth of information out there, and it has an active commmunity constantly hacking on it. In case you want to go forth and do more on your own, y'know?

First, I should explain the two methods of editing process memory. Developers of game hacks refer to the methods as "internal" and "external", where internal means a dynamic library that gets injected into the game's address space, and external means a separate process that manipulates memory by means of the operating system. AimTux is an example of an internal hack, and vcaim is an example of an external hack. We'll be writing an external cheat in this blog post. Although, if you want to learn more about writing internal cheats on Linux, this blog post by Aixxe is excellent.

Next, there's some terminology that people use when talking about memory-manipulating cheats. "Offsets," and "signatures." If you've ever performed a ret2libc attack on a system with ASLR, you already know about offsets. It's just a number you add to the address at which a library was loaded to get the position of something in memory. In the case of ret2libc, you're trying to get to a function like system(3), but in the case of CS:GO hacks, you're trying to get to get something like a list of entities currently in the game. You can try to find functions, too, which we'll be doing in this post to write wallhacks, but most legit CS:GO hacks go after entity data.

Games get updated and therefore recompiled quite often, so offsets are constantly changing. To combat this, cheat developers developed ways to scan for "signatures" in memory. That is, patterns of bytes that will reveal the offset - either by being around the desired offset, or being code that references it. If you get signatures from someone, it will probably look like "B9 ? ? ? ? 6A 00 FF 50 08 C3". Those are hexpairs, and the question marks are bytes that get ignored because they're an address or something that will likely end up changing in a future update.

Oh yeah, probably should've mentioned why we're using offsets instead of fixed addresses. It is because of ASLR - a lot of CS:GO's code is stored in shared libraries. Specifically, client_client.so and engine_client.so. Where these are depends on whether you're using an amd64 or an x86 processor. Just use find(1) in the Steam directory, man.

As a heads up, this cheat is mostly a paste I stole from Emma. I didn't come up with it myself, but I thought that it was simple enough to be an example for this post.

The way we're going to go about writing our wallhack is pretty primitive, patching the .text segment. Although we're going to do this by editing memory, not the binary on disk. In CS:GO, there's a "glow" effect that spectators have - allowing them to see the outlines of other players in gamemodes like Casual. If we can find the offset to the code that checks if we're a spectator or not and patch it, we can enable the glowing effect and see through walls.

The glow effect is also controlled by a "cvar," which is just a client-side configuration variable. Specifically, it checks "spec_show_xray". If we open up client_client.so in radare2, we can see that that's a plain ASCII string and that there are two references to it in the .text segment.

[0x005eef60]> iz~spec_show_xray
vaddr=0x0135c245 paddr=0x0135c245 ordinal=3016 sz=15 len=14 section=.rodata type=ascii string=spec_show_xray
[0x005eef60]> iS
[Sections]
...
idx=11 vaddr=0x005eef60 paddr=0x005eef60 sz=13998500 vsz=13998500 perm=--r-x name=.text
...

40 sections

[0x005eef60]> e search.from=0x005eef60
[0x005eef60]> e search.to=0x005eef60+13998500
[0x005eef60]> /r 0x0135c245
[0x01348878-0x01348904] data 0x6236aa leaq str.spec_show_xray, %rsi in unknown function
data 0x71817c leaq str.spec_show_xray, %rsi in unknown function

If we seek to the first one, we'll see a dissasembly listing like this

0x00623690      4c8d0de9c664.  leaq 0x00c6fd80, %r9
0x00623697      b980000800     movl $0x80080, %ecx
0x0062369c      4c8d05c5fdd9.  leaq 0x013c3468, %r8        ; "If set to 1, you can see player outlines and name IDs through walls - who you can see depends on your team and mode"
0x006236a3      488d159af1d3.  leaq 0x01362844, %rdx       ; "0"
0x006236aa      488d35948bd3.  leaq 0x0135c245, %rsi       ; "spec_show_xray"
0x006236b1      488d3d080df6.  leaq 0x065843c0, %rdi
0x006236b8      e8c3278e00     callq 0xf05e80

This is how cvars are "constructed" in the source engine. %rdi contains the address of the actual variable, which is at 0x065843c0. This is done so that the variable can be changed from the in-game console, if the player so desires. But what this means for us is that we can easily find the address of a cvar in memory. If we look for references to that address, we'll find a handful.

[0x006236aa]> /r 0x065843c0
[0x01348782-0x01348904] data 0x6236b1 leaq 0x065843c0, %rdi in unknown function
data 0x6236c3 leaq 0x065843c0, %rsi in unknown function
data 0x7b7f57 movq 0x01bd5180, %rdi in unknown function
data 0x7b901b movq 0x01bd5180, %rbx in unknown function
data 0xc5ac60 movq 0x01bd5180, %rax in unknown function
data 0xc664d4 leaq 0x065843c0, %rax in unknown function
data 0xc7e86c leaq 0x065843c0, %rax in unknown function
data 0xc8bc34 movq 0x01bd5180, %rax in unknown function
data 0xd78699 movq 0x01bd5180, %rax in unknown function
data 0xda8601 movq 0x01bd5180, %rax in unknown function
data 0xda9d0f movq 0x01bd5180, %rax in unknown function
data 0xe3db40 movq 0x01bd5180, %rax in unknown function

A little of trial and error, combined with looking at the OSX binaries with symbols, yields that 0xc664d4 is the address that we're looking for - the function responsible for glowing.

0x00c664c0      e80be7b3ff     callq 0x7a4bd0
0x00c664c5      84c0           testb %al, %al
0x00c664c7      0f84c3010000   je 0xc66690
0x00c664cd      488b3d24df91.  movq 0x065843f8, %rdi       ; [0x65843f8:8]=0
0x00c664d4      488d05e5de91.  leaq 0x065843c0, %rax
0x00c664db      4839c7         cmpq %rax, %rdi
...

That first call is the actual check, the symbol for it in the OSX binaries is "CanSeeSpectatorOnlyTools". So if we patch the jump at 0x00c664c7, we should be able to see the glow effect as long as "spec_show_xray" is set to 1.

This is pretty easy, since we just need to change 6 bytes. I initially considered using dd(1) for this, but it doesn't seem to like touching procfs mem files, so instead we'll edit it from a python REPL.

[jakob@Epsilon ~]$ sudo grep -i client_client.so /proc/$(pidof csgo_linux64)/maps
7f5029915000-7f502b0e4000 r-xp 00000000 08:12 41426690                   csgo/bin/linux64/client_client.so
7f502b0e4000-7f502b2e4000 ---p 017cf000 08:12 41426690                   csgo/bin/linux64/client_client.so
7f502b2e4000-7f502b571000 rw-p 017cf000 08:12 41426690                   csgo/bin/linux64/client_client.so
[jakob@Epsilon ~]$ sudo python
Python 3.6.2 (default, Jul 20 2017, 03:52:27)
[GCC 7.1.1 20170630] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> OFF = 0x7f5029915000 + 0x00c664c7
>>> with open("/proc/9052/mem", "wb") as mem:
...     mem.seek(OFF)
...     mem.write(b"\x90" * 6)
...
139982284502215
6
>>>

And it seems to work pretty well.

Wallhax in Action

I know I didn't go into a whole lot of depth about how you would actually come up with a cheat like this, but the reality is that a lot can be figured out using some basic reverse engineering skills. You already saw how much information leakage there is from a simple string reference. There's a lot of information out there already, including the source code for the Source 2013 Base. I'd also recommend taking a look at the UnknownCheats community if you're interested in learning more, they're (generally) helpful and quite friendly.

Further Reading:

https://www.unknowncheats.me/forum/counterstrike-global-offensive/169923-cs-cheat-rookie-rookies.html

https://www.unknowncheats.me/forum/general-programming-and-reversing/133228-implement-pattern-scanning-obtain-offsets-dynamically.html

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