Fuzz Test Random Bytes Input Crash in Python

import random


def parse_header(data: bytes) -> dict:
    """Parse a fake binary header format."""
    if len(data) < 8:
        raise ValueError("header too short")

    magic = data[:4]
    if magic != b'PARS':
        raise ValueError("bad magic")

    version = data[4]
    if version != 1:
        raise ValueError("unsupported version")

    flags = data[5]
    if flags & 0x80:
        raise ValueError("invalid flag bit")

    payload_len = int.from_bytes(data[6:8], "big")
    if payload_len > 1000:
        raise ValueError("payload too large")

    return {"version": version, "flags": flags, "payload_len": payload_len}


def fuzz(num_trials: int = 10000) -> None:
    """Randomly generate inputs and look for crashes."""
    for _ in range(num_trials):
        length = random.randint(0, 20)
        data = bytes(random.getrandbits(8) for _ in range(length))
        try:
            parse_header(data)
        except ValueError:
            pass  # expected for malformed input
        except Exception as e:
            print(f"CRASH: input={data!r} -> {type(e).__name__}: {e}")
            return

    print(f"All {num_trials} fuzz trials passed without crashes.")


if __name__ == "__main__":
    fuzz(5000)