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CVE-2026-27962 critical CVSS 9.1

Vulnerability in authlib (CVE-2026-27962)

Summary

Vulnerability in authlib (CVE-2026-27962). Confidential information can be exposed externally. Exploitable via authlib. Mitigation: upgrade to 1.6.9 or later.

Original description

The description as published by the source.

Description

Summary

A JWK Header Injection vulnerability in authlib's JWS implementation allows an unauthenticated
attacker to forge arbitrary JWT tokens that pass signature verification. When key=None is passed
to any JWS deserialization function, the library extracts and uses the cryptographic key embedded
in the attacker-controlled JWT jwk header field. An attacker can sign a token with their own
private key, embed the matching public key in the header, and have the server accept the forged
token as cryptographically valid — bypassing authentication and authorization entirely.

This behavior violates RFC 7515 §4.1.3 and the validation algorithm defined in RFC 7515 §5.2.

Details

Vulnerable file: authlib/jose/rfc7515/jws.py
Vulnerable method: JsonWebSignature._prepare_algorithm_key()
Lines: 272–273

elif key is None and "jwk" in header:
    key = header["jwk"]   # ← attacker-controlled key used for verification

When key=None is passed to jws.deserialize_compact(), jws.deserialize_json(), or
jws.deserialize(), the library checks the JWT header for a jwk field. If present, it extracts
that value — which is fully attacker-controlled — and uses it as the verification key.

RFC 7515 violations:

  • §4.1.3 explicitly states the jwk header parameter is "NOT RECOMMENDED" because keys
    embedded by the token submitter cannot be trusted as a verification anchor.
  • §5.2 (Validation Algorithm) spe…

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-2026-27962 has been found in authlib.
Attackers can target a specific entry point like authlib over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 9.1/10.
What to do: upgrade authlib to 1.6.9 or later.
If unsure, ask your IT team or search "authlib CVE-2026-27962" on the vendor's site.

CVE-2026-27962 (authlib) — CWE-347 / CVSS v3 9.1
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: authlib / jwk / kid / None
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-347) exists in authlib. Attackers reach the vulnerable code path via authlib without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of authlib. Attack surface: authlib / jwk / kid / None.

🔥 Severity

Severity: Critical (CVSS 9.1/10). Confidential information can be exposed externally.

🔧 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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