GreyCTF'23: CrashPython

This was a pretty simple python challenge that makes use of a segmentation fault (segfault). A segfault occurs when a program tries to access a memory location that it is not allowed to access.

The Challenge

Explanation

The main gist of this challenge was that there was a placeholder to input python code to run to cause a crash (through segmentation fault), but the code had to fulfil the requirements of the blacklist. The blacklist included keywords that couldn’t be used in the code.

Solution

With the restrictions, how can we still cause a segfault? I read up more about segfaults in python and found this.

In Python, segmentation faults can occur when using C extensions, which allow Python to interact with native libraries and code. These extensions can be written in C or C++ and can provide significant performance benefits to Python programs. However, if these extensions are poorly written or contain bugs, they can cause segmentation faults.

One of the libraries that provide C compatible data types in Python is ctypes, hence I tried to utilise this library.

import ctypes
def segfault():
    # Access an invalid memory location
    ctypes.string_at(0)

segfault()

This code attempts to read a string from memory address 0, which is typically off-limits for user-level programs for security reasons. Thus, the operating system intervenes to prevent any unintended consequences, leading to a segfault.

And that worked! Giving us the flag: