CVE-2026-117499: @fastify/session vulnerable to session revocation bypass via a logout race condition

Severity: low (CVSS 3.7 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-27 · updated 2026-10-27
Weakness: CWE-362

Description

@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.

Affected products

@fastify/session: before 11.1.6 — fixed in 11.1.6

How to fix

Upgrade to a fixed version: @fastify/session 11.1.6. Check the vendor advisory for mitigations.

References

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