Python sandbox / eval escape

When it applies

User input reaches eval(), exec(), a "safe expression" evaluator, a template engine that runs Python, or a feature that runs user formulas — and a blocklist tries to make it safe by banning words like import, os, or system. If you can evaluate Python at all, the blocklist is almost always bypassable.

Why it works

String-based keyword filters remove names, but the objects they'd name are still reachable through Python's object graph. From any object you can walk __class__ → __base__ → __subclasses__() to find a class whose module gives you code execution (subprocess.Popen, os.system, builtins), without ever typing a banned identifier literally.