fastify fastify-secure-session up to 8.4.1 Cookie Unsign Routine improper authentication

Summaryinfo

A vulnerability categorized as critical has been discovered in fastify fastify-secure-session up to 8.4.1. This affects an unknown part of the component Cookie Unsign Routine. Such manipulation leads to improper authentication. This vulnerability is traded as CVE-2026-117549. The attack may be launched remotely. There is no exploit available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in fastify fastify-secure-session up to 8.4.1. It has been rated as critical. Affected by this issue is an unknown functionality of the component Cookie Unsign Routine. The manipulation with an unknown input leads to a improper authentication vulnerability. Using CWE to declare the problem leads to CWE-287. When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct. Impacted is confidentiality, integrity, and availability. CVE summarizes:

@fastify/secure-session is a session plugin for Fastify that stores an encrypted session in a cookie. In versions 7.5.0 through 8.4.1, when the plugin is configured with the signed cookie option, it calls the cookie unsign routine but, when the signature check reports the value as invalid, still passes the original cookie value to the session decoder, so a session whose signature did not validate is decrypted anyway. The practical effect appears when an application supplies the cookie signing secret independently of the session encryption key and rotates only the signing secret to retire existing sessions while keeping the encryption key active, because a holder of an old session cookie can strip the now obsolete signature and keep using the session. The attacker must already possess an unexpired encrypted session cookie and needs neither secret, and cannot forge session data, extend expiry, or survive rotation of the encryption key. The issue is fixed in @fastify/secure-session 8.4.2, which no longer decodes the session when the signed cookie option is set and the signature check fails, and users should upgrade to 8.4.2 or later. As a workaround, rotate the session encryption key rather than relying on rotation of the cookie signing secret alone.

The advisory is available at github.com. This vulnerability is handled as CVE-2026-117549 since 10/27/2026. The exploitation is known to be easy. The attack may be launched remotely. The technical details are unknown and an exploit is not available.

Upgrading to version 8.4.2 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-115124). If you want to get best quality of vulnerability data, you may have to visit VulDB.

Productinfo

Vendor

• fastify

Name

• fastify-secure-session

Version

• 8.4.0
• 8.4.1

Website

• Product: https://github.com/fastify/fastify-secure-session/

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CPE 2.2info

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If you want to get best quality of vulnerability data, you may have to visit VulDB.

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