CVE-2026-28498
OpenID Connect Token Validation Bypass in Authlib Python Library
Publication date: 2026-03-16
Last updated on: 2026-03-17
Assigner: GitHub, Inc.
Description
Description
CVSS Scores
EPSS Scores
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| authlib | authlib | to 1.6.9 (exc) |
Helpful Resources
Exploitability
| CWE ID | Description |
|---|---|
| CWE-354 | The product 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. |
| CWE-573 | The product does not follow or incorrectly follows the specifications as required by the implementation language, environment, framework, protocol, or platform. |
Attack-Flow Graph
AI Powered Q&A
Can you explain this vulnerability to me?
This vulnerability exists in the Authlib Python library, which is used to build OAuth and OpenID Connect servers. Before version 1.6.9, the library's internal hash verification logic (_verify_hash) for validating the at_hash (Access Token Hash) and c_hash (Authorization Code Hash) claims had a flaw. When the library encountered an unsupported or unknown cryptographic algorithm in the ID Token's alg header parameter, it would incorrectly treat the validation as successful (fail-open behavior). This means an attacker could supply a forged ID Token with a deliberately unrecognized algorithm, bypassing mandatory integrity protections and violating cryptographic and OpenID Connect specifications. The issue was fixed in version 1.6.9.
How can this vulnerability impact me? :
This vulnerability can allow an attacker to bypass the integrity checks of OpenID Connect ID Tokens by using a forged token with an unsupported cryptographic algorithm. As a result, the attacker could gain unauthorized access or impersonate users by exploiting the fail-open validation logic, potentially compromising authentication and authorization processes in applications relying on the vulnerable Authlib versions.
How does this vulnerability affect compliance with common standards and regulations (like GDPR, HIPAA)?:
I don't know
How can this vulnerability be detected on my network or system? Can you suggest some commands?
I don't know
What immediate steps should I take to mitigate this vulnerability?
To mitigate this vulnerability, upgrade the Authlib Python library to version 1.6.9 or later, where the issue with the _verify_hash function has been patched.