fastify fastify-jwt up to 10.2.1 Option Merge request.jwtVerify improper authorization

Summaryinfo

A vulnerability was found in fastify fastify-jwt up to 10.2.1. It has been declared as critical. The impacted element is the function request.jwtVerify of the component Option Merge. Such manipulation leads to improper authorization. This vulnerability is traded as CVE-2026-18500. The attack may be launched remotely. There is no exploit available.

Detailsinfo

A vulnerability was found in fastify fastify-jwt up to 10.2.1. It has been rated as critical. Affected by this issue is the function request.jwtVerify of the component Option Merge. The manipulation with an unknown input leads to a improper authorization vulnerability. Using CWE to declare the problem leads to CWE-285. The product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action. Impacted is confidentiality, integrity, and availability. CVE summarizes:

@fastify/jwt is a JSON Web Token plugin for Fastify. In versions before 10.2.2, a per-request verification key passed to request.jwtVerify({ key }) is silently overridden by the plugin's globally configured secret, because the option merge applies the global key last. Applications that use different keys for different authorization domains, for example separate user and admin keys, therefore accept a token signed with the global key on a route that explicitly requires another key. This lets an ordinary authenticated user cross a key-based trust boundary without knowing either secret. The issue is fixed in @fastify/jwt 10.2.2, where an explicit per-call key takes precedence over the global secret. Users should upgrade to 10.2.2.

The advisory is available at github.com. This vulnerability is handled as CVE-2026-18500 since 07/31/2026. The exploitation is known to be easy. The attack may be launched remotely. Technical details are known, but there is no available exploit. This vulnerability is assigned to T1548.002 by the MITRE ATT&CK project.

Upgrading to version 10.2.2 eliminates this vulnerability.

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Productinfo

Vendor

• fastify

Name

• fastify-jwt

Version

• 10.2.0
• 10.2.1

Website

• Product: https://github.com/fastify/fastify-jwt/

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