Microcorruption

24/04/2019

Hey folks. After successfully completing the original Cryptopals exercises, I’ve decided to try the Microcorruption challenge Thomas Ptacek helped designing. They are about binary exploitation, the art of turning a buffer overflow into arbitrary code execution. There’s plenty of write-ups about solving the challenges, so mine will be about the low-level debugging involved. While the notes below are specific to the challenge, you may find them helpful for understanding debuggers in general. For more debugging insights, I can recommend reading “The Art of Debugging” by Norman Matloff and Peter Jay Salzman.

Challenge-specific points

Every program in the challenge follows the same pattern of munging some data, asking for user input and conditionally unlocking a door if all requirements have been met. Your task is figuring out user input that will unlock the door in a reproducible manner. The big revelation is that unlike in a reverse engineering challenge you don’t even have to provide a valid password, all that matters is that you trick the system into unlocking the door.

The challenge tries to make it as simple as possible to figure out what’s going on. The programs run on a MSP430 microcontroller, a RISC architecture so simple that the full list of mnemonics fits on two manual pages. Most of the reverse engineering work has been done for you by giving you a disassembled view of the program code, with function names for each section. There is a separate page for assembling and disassembling code. The debugger is graphical and features a dashboard of widgets to follow the program’s control flow. This makes it easy to spot patterns, as opposed to textual debuggers where you need to keep all relevant information in your head. It’s a bit like the difference between Nethack (top-down rogue-like) and Zork (classic text adventure).

Using the debugger

The most fundamental tool you have in your toolbox is the breakpoint. With it you put stop signs on your program and can skip the boring parts easily. Typically you put breakpoints on functions, but you can put them on absolute addresses as well. Note that if you reset the debugger (as opposed to rebooting it), breakpoints are preserved, this makes it easy to try a different way of executing the program. Another pattern that comes up is creating a breakpoint to skip ahead to some point of the program, then undoing it later to inspect that part of the program more closely.

Stepping through code is crucial to get right. You can continue execution to proceed to the next breakpoint or user input action. To proceed one instruction, use step which is also known as step in. This is because, when faced with a call to a function, step would step into it, with the next instruction being inside that function. To avoid this there’s next which is also known as step over, it behaves almost the same except that, if faced with a call, it will not enter the function, but step over it so that you stay at the same code snippet. If you find yourself in a function you’d want to get out quickly, use finish. This will execute instructions until the function’s end.

There are short forms of these commands, c (continue), s (step), n (next) and f (finish). To repeat the last command, use the enter key. It’s possible to repeat commands with a numerical argument and to define macros (mostly useful for more complex repetitive patterns), but I haven’t made use of these yet.

Poking the memory

To understand how memory can be corrupted in ways useful to an attacker, it’s important to know how a running program is represented in memory. For this, it’s useful to track the memory and register views as the program is running. You’ll find that in this challenge there are separate regions of memory dedicated to the stack, heap and program code. If the memory layout permits it, interesting things can happen when the attacker writes beyond the designated boundaries. For example, if the stack pointer points to attacker-controlled memory, anything reading memory from the stack will be influenced, such as the return address to the last function (which could be changed to jump to a different function). Another example is overwriting adjacent stack memory to change a local variable’s contents. It’s even possible to put machine code (also known as shellcode because it’s typically designed to spawn a shell) on the stack, then jump into it and continue execution from there on, provided that the stack is executable.

Endianness is a subtle point here. The MSP430 architecture is little-endian, so the least significant byte comes first. The number 256 for example is encoded as the bytes 0x00 and 0x01. This is important to know when trying to make sense of numbers in the disassembly and interpreting user input (like, when entering an address).