Jail break - UTCTF 2026 write up

No Description

Challenge Description

We’ve built the world’s most secure Python sandbox. Nothing can escape. Probably. Hopefully. Run it locally:

python3 jail.py 

Analyzing Path

The challenge gives us a Python source file; first, try to open it:

import sys

_ENC = [0x37, 0x36, 0x24, 0x2e, 0x23, 0x25, 0x39, 0x32, 0x3b, 0x1d, 0x28, 0x23, 0x73, 0x2e, 0x1d, 0x71, 0x31, 0x21, 0x76, 0x32, 0x71, 0x1d, 0x2f, 0x76, 0x31, 0x36, 0x71, 0x30, 0x3f]
_KEY = 0x42

def _secret():
    return ''.join(chr(b ^ _KEY) for b in _ENC)

BANNED = [
    "import", "os", "sys", "system", "eval",
    "open", "read", "write", "subprocess", "pty",
    "popen", "secret", "_enc", "_key"
]

SAFE_BUILTINS = {
    "print": print,
    "input": input,
    "len": len,
    "str": str,
    "int": int,
    "chr": chr,
    "ord": ord,
    "range": range,
    "type": type,
    "dir": dir,
    "vars": vars,
    "getattr": getattr,
    "setattr": setattr,
    "hasattr": hasattr,
    "isinstance": isinstance,
    "enumerate": enumerate,
    "zip": zip,
    "map": map,
    "filter": filter,
    "list": list,
    "dict": dict,
    "tuple": tuple,
    "set": set,
    "bool": bool,
    "bytes": bytes,
    "hex": hex,
    "oct": oct,
    "bin": bin,
    "abs": abs,
    "min": min,
    "max": max,
    "sum": sum,
    "sorted": sorted,
    "reversed": reversed,
    "repr": repr,
    "hash": hash,
    "id": id,
    "callable": callable,
    "iter": iter,
    "next": next,
    "object": object,
}

# _secret is in globals but not documented - players must find it
GLOBALS = {"__builtins__": SAFE_BUILTINS, "_secret": _secret}

print("=" * 50)
print("  Welcome to PyJail v1.0")
print("  Escape to get the flag!")
print("=" * 50)
print()

while True:
    try:
        code = input(">>> ")
    except EOFError:
        break

    blocked = False
    for word in BANNED:
        if word.lower() in code.lower():
            print(f"  [BLOCKED] Nice try!")
            blocked = True
            break

    if blocked:
        continue

    try:
        exec(compile(code, "<jail>", "exec"), GLOBALS)
    except Exception as e:
        print(f"  [ERROR] {e}")

Based on the Python script, we identified some suspicious points:

– First, there is a secret function in this source code: python def _secret(): return ''.join(chr(b ^ _KEY) for b in _ENC)

– Next, there is a blacklist containing some banned strings: python BANNED = [ "import", "os", "sys", "system", "eval", "open", "read", "write", "subprocess", "pty", "popen", "secret", "_enc", "_key" ]

– Finally, GLOBALS is passed into exec, but it still contains _secret: python GLOBALS = {"__builtins__": SAFE_BUILTINS, "_secret": _secret}

– So we can observe that this program only blocks static strings; runtime-generated strings remain usable.

Overall, this source code contains some important points:

• _ENC is the encoded bytes array.
• _KEY = 0x42 is the XOR key.
• _secret() decodes _ENC with _KEY.
• The blacklist contains secret, but does not ban vars, chr, map, or getattr.

There is no embedded binary data, so binwalk/exiftool are not needed.

Decoding

Based on the secret function, we can decode directly by XOR with the KEY = 0x42:

''.join(chr(b ^ 0x42) for b in _ENC)

We created a payload, ran jail.py with it, and obtained the flag:

• Payload:

    name="_"+"".join(map(chr,[115,101,99,114,101,116]))
    fn=vars()[name]
    print(fn())
  

• Run result:

    $ python3 jail.py
    ==================================================
    Welcome to PyJail v1.0
    Escape to get the flag!
    ==================================================
  
    >>> name="_"+"".join(map(chr,[115,101,99,114,101,116]))
    fn=vars()[name]
    print(fn())>>> >>> 
    utflag{py_ja1l_3sc4p3_m4st3r}