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CVE-2026-28498 — Authlib: Fail-Open Cryptographic Verification in OIDC Hash Binding

Affected: authlib, Red Hat Ansible Automation Platform 2.6, Red Hat Quay 3.10, Red Hat Quay 3.12, Red Hat Quay 3.15, Red Hat Quay 3.16, Red Hat Lightspeed Core, Red Hat Ansible Automation Platform 2, Red Hat OpenShift AI (RHOAI), Red Hat Satellite 6

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, a 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. This issue has been patched in version 1.6.9.

GHSA-m344-f55w-2m6j — Authlib: Fail-Open Cryptographic Verification in OIDC Hash Binding

Affected: PyPI: authlib

## 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.

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## 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 fai…

PYSEC-2026-2117

Affected: PyPI: authlib