I’ve found lots of examples in C of programs illustrating buffer Overflows, including those of pointer rewrites which has been of great help in understanding how a buffer overflow works and memory safety etc. but I’ve yet to be able to find an example illustrating how such a buffer overflow can rewrite a pointer in such a way that it actually results in code execution?
Is this just not a thing, or is my google-fu rust y? Tried ChatGPT and my local Mistral and they both seem unable to spit out precisely what I’m asking, so maybe I’m wording this question wrong.
If anyone in here knows, could point me in the right direction? Thanks y’all btw love this community 🧡
I really like this video, in it he demonstrates how a char pointer can be exploited to alter the return value in the stack and walks through an example of how it’s done. https://www.youtube.com/watch?v=1S0aBV-Waeo
Love computerphile!
There are various approaches, but the most common one in an x86 environment is overwriting the return address that was pushed onto the stack.
When you call a function, the compiler generally maps it to a CALL instruction at the machine language level.
At the time that a CALL instruction is invoked, the current instruction pointer gets pushed onto the stack.
kagis
https://www.felixcloutier.com/x86/call
When executing a near call, the processor pushes the value of the EIP register (which contains the offset of the instruction following the CALL instruction) on the stack (for use later as a return-instruction pointer). The processor then branches to the address in the current code segment specified by the target operand.
A function-local, fixed-length array will also live on the stack. If it’s possible to induce the code in the called function to overflow such an array, it can overwrite that instruction pointer saved on the stack. When the function returns, it hits a RET instruction, which will pop that saved instruction pointer off the stack and jump to it:
https://www.felixcloutier.com/x86/ret
Transfers program control to a return address located on the top of the stack. The address is usually placed on the stack by a CALL instruction, and the return is made to the instruction that follows the CALL instruction.
If what was overwriting the saved instruction pointer on the stack was a function pointer to malicious code, it will now be executing.
If you’re wanting to poke at this, I’d suggest familiarizing yourself with a low-level debugger so that you can actually see what’s happening first, as doing this blindly from source without being able to see what’s happening at the instruction level and being able to inspect the stack is going to be a pain. On Linux, probably
gdb. On Windows, I’m long out of date, but SoftICE was a popular low-level debugger last time I was touching Windows.You’ll want to be able to at least set breakpoints, disassemble code around a given instruction to show the relevant machine language, display memory at a given address in various formats, and single step at the machine language level.
I’d also suggest familiarizing yourself with the calling convention for your particular environment, which is what happens at a machine language level surrounding the call and return from a subroutine. Such buffer overflow attacks involve also overwriting other data on the stack, and understanding what is being overwritten is going to be necessary to understand such an attack.
Thanks! This is very helpful
I think you might find some illustrations & examples of what you want by searching for return-oriented programming, rather than just buffer overflows.
@LainTrain There used to be approximately a million examples floating around in the web. You could just write a simple program with a fixed-size stack buffer at a repeatable address, overflow a return address with a crafted string, return to the overwritten stack buffer full of shellcode. All of the mitigations (stack canaries, W^X, ASLR, CFI, canonical addresses, …) mean that you have to either use much more elaborate techniques (ROP/return to libc, address leaks, …) or you have to disable the mitigations to see a working exploit example, which is pretty unimpressive.
Thanks! The reason I was looking for an example is because I understand:
overflow a return address with a crafted string, return to the overwritten stack buffer full of shellcode
In principle, but not in practice. Especially the last part.
I have my char buf[16] and some char * ptr = buf; and then a gets() gets a 20 char string, causing a buffer overflow either then or when the buffer is read where it reads out of bounds.
I’ve done this many times, sometimes intentionally, and if I visualize the memory as one continuous line where the ptr is stored at the precise address buf[20] is at, allowing me to write into that memory location a new address for the pointer by having part of the string given to gets() be a new memory address at the address of ptr, so that next time that pointer is accessed in a program, it leads to an arbitrary memory read, and the arbitrary pointer address can be to still further down in the initial string we gave to gets(), e.g. buf[40] where our shellcode is stored, but how to do this - implement it in practice (so - in code), I don’t really know.
Specifically I don’t know how to make a pointer at a predictable constant address so it’s stored address can be overwritten, and how to make the reading of the resulting maliciously modified pointer also somehow execute code. I’m guessing it can’t just be a char pointer reading in data, right?
Specifically I don’t know how to make a pointer at a predictable constant address so it’s stored address can be overwritten,
This is one of the things that I mentioned in my above comment on mitigating buffer overflow attacks, that address randomization is one of the mitigations. Are you trying to create an exploit that will function in such an environment?
If so, I’d still start out understanding how the attack works in a non-mitigated environment – it’s simpler – and then learn about the mitigations and efforts to counter them.
I agree, I think for now I’d like to try to create a demo exploit and exploitable program like this without considering ASLR et al. and then at some point in the future perhaps look at a return to libc type deal to understand that as well.
@LainTrain The simplest case is overwriting the return address on the stack. If your stack layout looks like this (B for buffer, R for return address, A for function arguments):
BBBBBBBBRRRRAAAA
and you give a pointer to the first B byte to gets(), the input can overwrite the bytes of R.
You can try this with a 32-bit program complied with disabled mitigations. Run the program in a debugger, break in the function, inspect the stack pointer value. With ASLR disabled the addresses will remain the same for every program execution assuming the call graph at this point doesn’t change. You can then overwrite the bytes of R with the buffer address (assuming no stack canary), and overwrite the buffer bytes with machine code instructions. When the function attempts to return, it instead jumps to the instructions you left in the buffer, and executes them (assuming no W^X).Thank you! This is incredibly helpful and insightful.
I now understand how one would do this with manually writing in a debugger, am I correct in thinking that if I constructed the input to gets() in such a manner that BBBBBBB contains shellcode, and RRRR is a return address pointing to the beginning of BBBBB then that is how arbitrary code execution can be achieved with this in practice?
@LainTrain Yes, but “in practice” this simple approach worked 20 years ago. Modern processors, compilers and operating systems make exploitation of stack buffer overflows a lot more difficult.
That’s fine, I think for my purposes it’s better to start simple with the basic concept of it first, then add complexity by learning about the protections and how they have/could be circumvented.
IIRC, the Nintendo Game & Watch hack on YouTube covers this with a STM32H7 on the little Mario handheld game from a few years ago.