[SECURITY] Algorithm Confusion Through kid Header

[paragonie-security]

Starting here:

python-jose/jose/jws.py

Lines 250 to 258 in be8e914

	def _verify_signature(signing_input, header, signature, key="", algorithms=None):
	alg = header.get("alg")
	if not alg:
	raise JWSError("No algorithm was specified in the JWS header.")
	if algorithms is not None and alg not in algorithms:
	raise JWSError("The specified alg value is not allowed")

This correctly rejects invalid alg headers, as JWT implementations MUST to be secure.

python-jose/jose/jws.py

Lines 259 to 262 in be8e914

	keys = _get_keys(key)
	try:
	if not _sig_matches_keys(keys, signing_input, signature, alg):
	raise JWSSignatureError()

However, the algorithm associated with the key returned from _get_keys() is not validated.

python-jose/jose/jws.py

Lines 217 to 247 in be8e914

	def _get_keys(key):
	if isinstance(key, Key):
	return (key,)
	try:
	key = json.loads(key, parse_int=str, parse_float=str)
	except Exception:
	pass
	if isinstance(key, Mapping):
	if "keys" in key:
	# JWK Set per RFC 7517
	return key["keys"]
	elif "kty" in key:
	# Individual JWK per RFC 7517
	return (key,)
	else:
	# Some other mapping. Firebase uses just dict of kid, cert pairs
	values = key.values()
	if values:
	return values
	return (key,)
	# Iterable but not text or mapping => list- or tuple-like
	elif isinstance(key, Iterable) and not (isinstance(key, str) or isinstance(key, bytes)):
	return key
	# Scalar value, wrap in tuple.
	else:
	return (key,)

Which is unfortunate, since the underlying verify method expects a Key object with the alg specified:

python-jose/jose/jws.py

Lines 207 to 208 in be8e914

	if not isinstance(key, Key):
	key = jwk.construct(key, alg)

Consequently, it's possible to use a set of keys with mismatching algorithms (i.e. in frameworks that consume this library), which would in turn make those libraries susceptible to algorithm confusion (see also: the HS256/RS256 attack from a few years ago).

This is identical to the problem in firebase/php-jwt#351 https://seclists.org/fulldisclosure/2021/Aug/14

Note: This particular sharp edge isn't covered by the JWT Best Practices RFC.