JWS JSON integrity protected headers are owerwritten by unprotected headers

[krkd]

Describe the bug

Deserialize JWS JSON will overwrite protected headers (protected) with values from header

To Reproduce

from authlib.jose import JsonWebSignature


jws = JsonWebSignature()

header_obj = {
    'protected': {
        'alg': 'HS256',
        'crit': ['jti', 'exp'],
        'kid': 'key',
        'jti': '1',
        'exp': '11111111111',
    },
    'header': {},
}
secret = b'secret'
payload = b'payload'

jws_json = jws.serialize_json(header_obj, payload, secret)

# assume that attacker got access to jws
jws_json['header'] = {
    'kid': 'another_key',
    'jti': '9',
    'exp': '99999999999',
}

decoded = jws.deserialize_json(jws_json, secret)

assert decoded['header']['kid'] == 'key'
assert decoded['header']['jti'] == '1'
assert decoded['header']['exp'] == '11111111111'

Expected behavior

deserialize_json must not overwrite protected headers with unprotected headers.

{
    'header': {
        'alg': 'HS256',
        'crit': ['jti', 'exp'],
        'kid': 'key',
        'jti': '1',
        'exp': '11111111111'
    },
    'payload': b'payload',
}

Environment:

• OS: ox X 11.2.3
• Python Version: 3.9.1
• Authlib Version: 0.15.3

[krkd]

Changing dict update order in JWSHeader should do the trick, but maybe i am missing the big picture

[mtueng]

Can you please label this Bug as a security issue, because it is one that affects both methods serialize_json and serialize. Thanks!

[lepture]

I don't understand the issue.

assume that attacker got access to jws

If the attacker can alter your code, you are already in danger.