CVE-2026-28498
Description
A flaw was found in Authlib, a Python library used for building OAuth and OpenID Connect (OIDC) servers. This vulnerability allows a remote attacker to bypass critical integrity checks in OIDC ID Tokens. Specifically, the library's internal hash verification logic fails open when encountering an unsupported cryptographic algorithm, accepting a forged ID Token as valid. This can lead to an authentication bypass, granting unauthorized access to systems relying on Authlib for OIDC authentication.
Statement
This IMPORTANT vulnerability in Authlib allows attackers to bypass OIDC ID Token integrity verification. The at_hash and c_hash validation fails open for unknown algorithms, accepting forged tokens as valid. Exploitation requires no authentication or user interaction. Impact is high to confidentiality and integrity. Red Hat products using Authlib for OIDC validation are affected. Fixed in version 1.6.9.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 9.1 | 7.5 | 9.1 |
| Attack Vector | Network | Network | Network |
| Attack Complexity | Low | Low | Low |
| Privileges Required | None | None | None |
| User Interaction | None | None | None |
| Scope | Unchanged | Unchanged | Unchanged |
| Confidentiality | High | None | High |
| Integrity Impact | High | High | High |
| Availability Impact | None | None | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
cve.org: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Understanding the Weakness (CWE)
Access Control
Technical Impact: Bypass Protection Mechanism
Confidentiality,Integrity
Technical Impact: Read Application Data; Modify Application Data
Accountability,Non-Repudiation
Technical Impact: Hide Activities
Frequently Asked Questions
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