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CVE-2026-28498 high CVSS 7.5

Vulnerability in authlib (CVE-2026-28498)

Summary

Vulnerability in authlib (CVE-2026-28498). Data can be tampered with by attackers. Exploitable via _verify_hash. Mitigation: upgrade to 1.6.9 or later.

Original description

The description as published by the source.

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.

2. Technical Details & Root Cause

The vulnerability resides within the _verify_hash(signature, s, alg) function in authlib/oidc/core/claims.py:

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 fail…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-28498 has been found in authlib.
Attackers can target a specific entry point like _verify_hash over the network to misuse the product.
Data can be tampered with by attackers. CVSS score: 7.5/10.
What to do: upgrade authlib to 1.6.9 or later.
If unsure, ask your IT team or search "authlib CVE-2026-28498" on the vendor's site.

CVE-2026-28498 (authlib) — CWE-354 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: _verify_hash / at_hash / c_hash / alg
Patched: 1.6.9 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-354) exists in authlib. Attackers reach the vulnerable code path via _verify_hash without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of authlib. Attack surface: _verify_hash / at_hash / c_hash / alg.

🔥 Severity

Severity: High (CVSS 7.5/10). Data can be tampered with by attackers.

🔧 How to fix

Update to 1.6.9.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'authlib' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'authlib' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で authlib を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade authlib to 1.6.9

    ステージング環境で 1.6.9 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

PyPI authlib
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.6.9"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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