# Execute Arbitrary Function With Revenge

Want to execute just a function inside a binary? Look no further

## Execute an arbitrary function in any binary

This tutorial shows you how to execute functions in any binary. So you don’t have to use ctypes.CDLL

Recently I get into a problem that I want to execute a function in a binary. The binary I’m analyzing is a fucked up binary, the header and the main function is corrupted, but some functions are executable. So there is a little chance for me to debug the binary or run it from the entry point.

So I have a few options:

• Copy instructions, emulate them

• Copy instruction by rewriting it. I can easily fail in 2 language problems. For example, I prefer to write code in Python, but how can I translate SAR(Arithmetic Shift) in ASM to Python? No, I want it quick.

• Try to execute it by manipulating Instruction Pointer (EIP/RIP)

Time after time I keep running into this problem, once, I use ctypes.DLL to get rand number, but that binary I was working with have custom rand so I can’t call the alternative rand in libc.

Life is so boring to repeatedly do the same thing over and over again.

Recently I found a framework write on top of Frida, I can run on any environment as long as you can execute the binary, (ARM, Linux, Windows), so that a good thing. Imagine you’re trying to reverse a check_password in ARM router, so you can just use a Raspberry Pi as environment and brute the password. No re-write needed.

Wanna see how effective this method is ?

First, I give you a binary.

This binary is taken from PicoCTF training platform.

Second, try to think how you can solve the binary as fast as possible. . . . Here is my solution:

You can see how it does by running this command

asciinema play execute_any_function/need-for-speed_asciinema.cast

I actually solve this challenge in 2 minutes, the binary I haven’t seen before.

So how the script works, you may wonder why.

Here is the recap how binary works, in main() function

It will have to run get_key, then print_flag

• In get_key, the calculate key and store result to global memory obj.key

• In print_flag, it decrypt_flag by using the global obj.key

• In decrypt_flag, it’s a bunch of instructions that I don’t want to touch it, anyway it writes result to obj.flag. So I need that address, s obj.flag give me the address 0x00201020.

So the flow to solve this challenge is to make call :

• decrypt_flag parameter must be obj.key, obj.key value is 0xed0cc64a from return value calculate_key

• Then the flag is automatically decrypted. Need to grab it out.

Firstly, Init the binary Secondly, grab the address of decrypt_flag and execute it Thirdly, grab the flag from memory and print it out.

See something weird, the framework handle PIE nicely. Lovely. The address I input is taken directly from Radare2.

## Create a mapping of a function

Have you ever get so tired that you have to check all input and output spaces of a function by rewriting the function. While I was poking Pico Mini challenge, I see otp challenge that kinda annoying, since Python can’t map SAR instruction. (Please let me know if you know how). So I use Revenge, create a mapping of jumble function is so easy.

The binary can be found here

So I just don’t care what valid_char I need to feed into the binary, I just brute all of them. Thus, same for jumble function. From the mapping, you can easily create a mapping (or dict-like in Python) to solve the challenge So you can easily map form a→b. Like Python Dict, with the help of Z3, I can easily create an equation and solve the entire program. This can be applied to other binary as well.

## Conclusion

TL;DR

Pros:

• Execute an arbitrary function in any binary

• Easy interface, handle PIE, easy to grab memory.

Cons:

• Can’t execute individual instruction

• I’m kinda happy that I don’t have to run into the same problem over and over again.