Authlib Python OIDC ID Token Hash Fail-Open <1.6.9
CVE-2026-28498 Published on March 16, 2026

Authlib: Fail-Open Cryptographic Verification in OIDC Hash Binding
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.

Github Repository NVD

Vulnerability Analysis

CVE-2026-28498 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. Public availability of a proof of concept (POC) exploit exists for CVE-2026-28498. The potential impact of an exploit of this vulnerability is considered to have a high impact on confidentiality and integrity, and no impact on availability.

Attack Vector:
NETWORK
Attack Complexity:
LOW
Privileges Required:
NONE
User Interaction:
NONE
Scope:
UNCHANGED
Confidentiality Impact:
HIGH
Integrity Impact:
HIGH
Availability Impact:
NONE

Weakness Types

Improper Validation of Integrity Check Value

The software does not validate or incorrectly validates the integrity check values or "checksums" of a message. This may prevent it from detecting if the data has been modified or corrupted in transmission. Improper validation of checksums before use results in an unnecessary risk that can easily be mitigated. The protocol specification describes the algorithm used for calculating the checksum. It is then a simple matter of implementing the calculation and verifying that the calculated checksum and the received checksum match. Improper verification of the calculated checksum and the received checksum can lead to far greater consequences.

Improper Following of Specification by Caller

The software does not follow or incorrectly follows the specifications as required by the implementation language, environment, framework, protocol, or platform. When leveraging external functionality, such as an API, it is important that the caller does so in accordance with the requirements of the external functionality or else unintended behaviors may result, possibly leaving the system vulnerable to any number of exploits.

Missing Cryptographic Step

The product does not implement a required step in a cryptographic algorithm, resulting in weaker encryption than advertised by the algorithm.


Products Associated with CVE-2026-28498

Want to know whenever a new CVE is published for Red Hat products? stack.watch will email you.

Red Hat Ansible Automation Platform
 
Red Hat Quay
 
Red Hat Satellite
 
Red Hat Lightspeed Core
 
Red Hat Openshift Ai
 

Affected Versions

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 Ansible Automation Platform 2: Red Hat Satellite 6: Red Hat Lightspeed Core: Red Hat OpenShift AI (RHOAI):

Vulnerable Packages

The following package name and versions may be associated with CVE-2026-28498

Package Manager Vulnerable Package Versions Fixed In
pip authlib <= 1.6.8 1.6.9

Exploit Probability

EPSS
0.20%
Percentile
9.95%

EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.