fastify session up to 11.1.5 authentication replay

Summaryinfo

A vulnerability, which was classified as critical, has been found in fastify session up to 11.1.5. The affected element is an unknown function. The manipulation leads to authentication replay. This vulnerability is traded as CVE-2026-117499. It is possible to initiate the attack remotely. There is no exploit available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as critical, has been found in fastify session up to 11.1.5. This issue affects an unknown functionality. The manipulation with an unknown input leads to a authentication replay vulnerability. Using CWE to declare the problem leads to CWE-294. A capture-replay flaw exists when the design of the product makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes). Impacted is confidentiality, integrity, and availability. The summary by CVE is:

@fastify/session is a session plugin for Fastify. In all releases before 11.1.6, with the default rolling session behavior, a request that restored a session before a logout could save its own copy after another request had completed the session destroy call, recreating the destroyed session identifier with its authenticated data. An attacker who already holds a victim's valid signed session cookie and keeps a request in flight across the logout can replay that cookie to restore the session after it was meant to be revoked, without any cookie theft or signature forgery. The issue is fixed in @fastify/session 11.1.6, which adds a per session generation marker and a conditional store write so a session restored before a logout or regeneration cannot recreate or overwrite the current entry, and users should upgrade to 11.1.6 or later. There is no workaround, and custom session stores must implement the new conditional write.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-117499 since 10/27/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Neither technical details nor an exploit are publicly available. MITRE ATT&CK project uses the attack technique T1040 for this issue.

Upgrading to version 11.1.6 eliminates this vulnerability.

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Productinfo

Vendor

• fastify

Name

• session

Version

• 11.1.0
• 11.1.1
• 11.1.2
• 11.1.3
• 11.1.4
• 11.1.5

Website

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

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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