CVE-2026-27962
CriticalCVSS 9.1Exploitation Probability (EPSS)
Low risk44th percentile - higher than 44% of all known CVEs
Summary
A vulnerability in the Authlib library (versions prior to 1.6.9) allows an attacker to inject a JWK key into the JWT header. This enables forging arbitrary JWT tokens that pass signature verification, as the library uses the attacker-supplied key instead of a trusted server key.
Risk Assessment
An attacker can completely bypass authentication and authorization mechanisms, impersonating any user or service. This can lead to unauthorized access to protected resources and data.
Recommendation
Immediately update the Authlib library to version 1.6.9 or later. If an update is not possible, ensure that JWS deserialization functions are not called with key=None.
Other vulnerabilities in Authlib
See all- CVE-2026-104056Critical
Authlib version 1.7.2 and below contains a vulnerability where discovery JSON metadata is cached without validation or issuer-origin binding. This allows a poisoned discovery response to replace all endpoint values with attacker-controlled values rather than endpoint URLs that share the origin of the configured server metadata URL.
- CVE-2026-96760Critical
Authlib (v1.7.2 and below) contains a signature verification bypass vulnerability. The JsonWebSignature.deserialize_json() method accepts a JSON Serialization JWS object and returns the payload as successfully verified without checking for a signature and without requiring a cryptographic key.
- CVE-2026-28802Critical
Authlib versions 1.6.5 to 1.6.6 incorrectly passed malicious JWT tokens with 'alg: none' and an empty signature, bypassing signature verification. The issue is fixed in version 1.6.7.
- CVE-2026-28498High
A vulnerability was found in the Authlib library (versions before 1.6.9) in the validation of OpenID Connect ID Tokens. The _verify_hash function incorrectly accepts unsupported cryptographic algorithms by returning True instead of rejecting the token. This allows an attacker to bypass token integrity by providing a forged token with an unknown algorithm.
Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS