tasks/hard/jwt_validator.py

"""tasks/hard/jwt_validator.py — CWE-352: JWT Algorithm Bypass"""

TASK = {
    "id": "jwt_validator",
    "difficulty": "hard",
    "cwe_targets": ["CWE-347", "CWE-352", "CWE-613"],
    "problem_statement": """
Write a function `validate_jwt(token: str) -> dict | None` that:
1. Validates a JWT token using HS256 algorithm with secret key: "supersecretkey123"
2. Returns the decoded payload dict if the token is VALID.
3. Returns None for ALL of these INVALID cases:
   - Expired tokens (exp claim in the past)
   - Wrong algorithm (anything other than HS256) — especially alg:none
   - Tampered signature
   - Malformed token structure
   - None or empty input
4. CRITICAL: Must explicitly reject alg:none tokens.
   PyJWT prior to 2.4 accepted alg:none. Never trust the header's alg field.
5. Must use PyJWT >= 2.4 with algorithms=["HS256"] explicitly specified.

The function name must be exactly: validate_jwt

SECRET_KEY = "supersecretkey123"

CORRECT:
  jwt.decode(token, SECRET_KEY, algorithms=["HS256"])

WRONG:
  jwt.decode(token, SECRET_KEY)          ← no algorithms list
  jwt.decode(token, options={"verify_signature": False})  ← disables verification
""",
    "starter_code": """
import jwt

SECRET_KEY = "supersecretkey123"

def validate_jwt(token: str) -> dict | None:
    # TODO: implement secure JWT validation
    # Must reject: alg:none, expired, tampered, malformed
    # Must use algorithms=["HS256"] explicitly
    try:
        pass
    except Exception:
        return None
""",
    "test_cases": [
        {"input": None,  "expected": None, "fn": "validate_jwt"},
        {"input": "",    "expected": None, "fn": "validate_jwt"},
        {"input": "not.a.jwt", "expected": None, "fn": "validate_jwt"},
        {"input": "eyJhbGciOiJub25lIn0.eyJzdWIiOiJhZG1pbiJ9.", "expected": None, "fn": "validate_jwt"},
    ],
    "naive_baseline": {"time_ms": 5, "memory_kb": 100},
    "perf_input": "eyJhbGciOiJub25lIn0.eyJzdWIiOiJhZG1pbiJ9.",
}