## 1. Executive Summary A critical library-level vulnerability was identified in the **Authlib** Python library concerning the validation of OpenID Connect (OIDC) ID Tokens. Specifically, the internal hash verification logic (`_verify_hash`) responsible for validating the `at_hash` (Access Token Hash) and `c_hash` (Authorization Code Hash) claims exhibits a **fail-open** behavior when encountering an unsupported or unknown cryptographic algorithm. This flaw allows an attacker to bypass mandatory integrity protections by supplying a forged ID Token with a deliberately unrecognized `alg` header parameter. The library intercepts the unsupported state and silently returns `True` (validation passed), inherently violating fundamental cryptographic design principles and direct OIDC specifications. --- ## 2. Technical Details & Root Cause The vulnerability resides within the `_verify_hash(signature, s, alg)` function in `authlib/oidc/core/claims.py`: ```python def _verify_hash(signature, s, alg): hash_value = create_half_hash(s, alg) if not hash_value: # ← VULNERABILITY: create_half_hash returns None for unknown algorithms return True # ← BYPASS: The verification silently passes return hmac.compare_digest(hash_value, to_bytes(signature)) ``` When an unsupported algorithm string (e.g., `"XX999"`) is processed by the helper function `create_half_hash` in `authlib/oidc/core/util.py`, the internal `getattr(hashlib, hash_type, None)` call fails, and the function correctly returns `None`. However, instead of triggering a `Fail-Closed` cryptographic state (raising an exception or returning `False`), the `_verify_hash` function misinterprets the `None` return value and explicitly returns `True`. Because developers rely on the standard `.validate()` method provided by Authlib's `IDToken` class—which internally calls this flawed function—there is **no mechanism for the implementing developer to prevent this bypass**. It is a strict library-level liability. --- ## 3. Attack Scenario This vulnerability exposes applications utilizing Hybrid or Implicit OIDC flows to **Token Substitution Attacks**. 1. An attacker initiates an OIDC flow and receives a legitimately signed ID Token, but wishes to substitute the bound Access Token (`access_token`) or Authorization Code (`code`) with a malicious or mismatched one. 2. The attacker re-crafts the JWT header of the ID Token, setting the `alg` parameter to an arbitrary, unsupported value (e.g., `{"alg": "CUSTOM_ALG"}`). 3. The server uses Authlib to validate the incoming token. The JWT signature validation might pass (or be previously cached/bypassed depending on state), progressing to the claims validation phase. 4. Authlib attempts to validate the `at_hash` or `c_hash` claims. 5. Because `"CUSTOM_ALG"` is unsupported by `hashlib`, `create_half_hash` returns `None`. 6. Authlib's `_verify_hash` receives `None` and silently returns `True`. 7. **Result:** The application accepts the substituted/malicious Access Token or Authorization Code without any cryptographic verification of the binding hash. --- ## 4. Specification & Standards Violations This explicit fail-open behavior violates multiple foundational RFCs and Core Specifications. A secure cryptographic library **MUST** fail and reject material when encountering unsupported cryptographic parameters. **OpenID Connect Core 1.0** * **§ 3.2.2.9 (Access Token Validation):** "If the ID Token contains an `at_hash` Claim, the Client MUST verify that the hash value of the Access Token matches the value of the `at_hash` Claim." Silencing the validation check natively contradicts this absolute requirement. * **§ 3.3.2.11 (Authorization Code Validation):** Identically mandates the verification of the `c_hash` Claim. **IETF JSON Web Token (JWT) Best Current Practices (BCP)** * **RFC 8725 § 3.1.1:** "Libraries MUST NOT trust the signature without verifying it according to the algorithm... if validation fails, the token MUST be rejected." Authlib's implementation effectively "trusts" the hash when it cannot verify the algorithm. **IETF JSON Web Signature (JWS)** * **RFC 7515 § 5.2 (JWS Validation):** Cryptographic validations must reject the payload if the specified parameters are unsupported. By returning `True` for an `UnsupportedAlgorithm` state, Authlib violates robust application security logic. --- ## 5. Remediation Recommendation The `_verify_hash` function must be patched to enforce a `Fail-Closed` posture. If an algorithm is unsupported and cannot produce a hash for comparison, the validation **must** fail immediately. **Suggested Patch (`authlib/oidc/core/claims.py`):** ```python def _verify_hash(signature, s, alg): hash_value = create_half_hash(s, alg) if hash_value is None: # FAIL-CLOSED: The algorithm is unsupported, reject the token. return False return hmac.compare_digest(hash_value, to_bytes(signature)) ``` --- ## 6. Proof of Concept (PoC)