9.1

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-27962

CVE-2026-27962: Authlib JWS JWK Header Injection: Signature Verification Bypass

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, 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 issue has been patched in version 1.6.9.

(GitHub Advisory)

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CVE-2026-27962

CVE-2026-27962:

9.1

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
authlib	pip	<= 1.6.8	1.6.9

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability lies in the authlib library's handling of JSON Web Signatures (JWS). Specifically, the JsonWebSignature._prepare_algorithm_key function in authlib/jose/rfc7515/jws.py would fall back to using a key provided in the jwk header of the token if the application did not supply a key for verification. This allows an attacker to craft a token, sign it with their own private key, and embed their public key in the jwk header. When the server processes this token and the key lookup returns None (a common case, for example, when a key ID kid is not found), authlib would use the attacker's public key from the header to validate the signature. The signature would pass verification, and the application would trust the claims in the forged token, leading to a complete authentication and authorization bypass. The fix, as seen in commit a5d4b2d4c9e46bfa11c82f85fdc2bcc0b50ae681, removes this dangerous fallback logic. A similar vulnerability was also patched in the prepare_key function within authlib/jose/rfc7516/jwe.py.

Vulnerable functions

JsonWebSignature._prepare_algorithm_key

authlib/jose/rfc7515/jws.py

The function insecurely uses a JSON Web Key (JWK) from the header of a JSON Web Signature (JWS) as the verification key when no key is provided by the application. This allows an attacker to forge a JWS by signing it with their own private key and embedding the corresponding public key in the 'jwk' header. The server will then use the attacker's key to validate the signature, leading to a signature validation bypass.

prepare_key

authlib/jose/rfc7516/jwe.py

The function insecurely uses a JSON Web Key (JWK) from the header of a JSON Web Encryption (JWE) as the key when no key is provided by the application. This could lead to scenarios where an attacker-controlled key is used in cryptographic operations.

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WAF Protection Rules

WAF Rule

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Reasoning

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