CVE-2026-114616 in Authlib
Summary
by MITRE • 10/22/2026
Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to 1.9.1, Authlib's OctKey.import_key() and HMAC signing and verification path accept zero-length key material. If an application's secret lookup can return an empty string or empty bytes, a remote attacker can compute the deterministic HS256, HS384, or HS512 signature for arbitrary token claims and have Authlib treat the token as authentic. This issue is fixed in version 1.9.1.
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Analysis
by VulDB Data Team • 10/22/2026
The vulnerability identified within the Authlib Python library represents a critical failure in cryptographic key validation during the initialization of HMAC-based JSON Web Tokens, specifically affecting versions prior to 1.9.1. Authlib serves as a foundational component for implementing OAuth and OpenID Connect protocols in Python applications, handling the complex logic required for secure authentication flows. The core technical flaw resides within the OctKey.import_key() method and the subsequent signing and verification pathways for HMAC algorithms such as HS256, HS384, and HS512. These cryptographic functions are designed to use a shared secret key to generate and verify digital signatures that ensure data integrity and authenticity. However, the implementation fails to enforce minimum length requirements on the provided key material, allowing zero-length keys or empty byte strings to be accepted without raising an exception or triggering a validation error.
This technical deficiency creates a severe security gap when application developers implement secret lookup mechanisms that may return null values, missing configuration entries, or empty strings due to misconfiguration or runtime errors. In such scenarios, the library proceeds with cryptographic operations using an effectively non-existent key. Because HMAC algorithms are deterministic, meaning they produce the same output for the same input and key, a zero-length key results in a fixed, predictable signature value. A remote attacker can exploit this determinism by crafting arbitrary token claims and computing the corresponding valid signature offline without possessing any actual secret material. When these forged tokens are presented to the vulnerable Authlib instance during verification, the library computes the HMAC using its own empty or null key state, which matches the attacker's pre-computed signature, thereby treating the maliciously crafted token as authentic.
The operational impact of this vulnerability is profound, effectively bypassing authentication and authorization controls entirely. An adversary can forge access tokens with arbitrary claims, such as elevating privileges to administrative roles, impersonating other users, or accessing restricted resources that should be protected by OAuth2 standards. This undermines the fundamental trust model of OpenID Connect and OAuth implementations relying on Authlib. The flaw aligns with CWE-347 Improper Verification of Cryptographic Signature, as the system fails to ensure that a digital signature was generated by an authorized party using a valid secret key. Furthermore, from a threat modeling perspective consistent with MITRE ATT&CK techniques, this vulnerability facilitates Token Forgery and Impersonation, allowing attackers to bypass authentication mechanisms (T1078) and potentially escalate privileges within the application ecosystem.
Mitigation strategies must prioritize immediate software updates alongside defensive coding practices. The primary remediation is upgrading Authlib to version 1.9.1 or later, where the developers have implemented strict validation checks that reject empty or insufficiently long key materials during initialization. For applications unable to upgrade immediately due to dependency constraints, a temporary workaround involves implementing custom middleware or wrapper functions around the token verification process. These wrappers should explicitly validate that any retrieved secret keys are non-empty and meet minimum entropy requirements before being passed to Authlib's signing or verification methods. Additionally, developers should audit their configuration management systems to ensure that missing environment variables do not resolve to empty strings but instead trigger explicit errors during application startup, preventing the deployment of configurations with invalid cryptographic parameters.