# CVE-2026-117549: @fastify/secure-session vulnerable to signature verification bypass via cookie signature removal

Status: published. Published: 2026-11-02. Updated: 2026-11-02.

@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.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- @fastify/secure-session / @fastify/secure-session (@fastify/secure-session): ≥ 7.5.0; < 8.4.2 (affected, semver); 8.4.2 (unaffected, semver). Source: CNA: openjs

## Severity and prioritization

- CVSS 3.1: 4.2 (medium); CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N; source: CNA: openjs

- CVSS 3.1: 4.2 (medium); CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N; source: NVD: ce714d77-add3-4f53-aff5-83d477b104bb

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-347

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/fastify/fastify-secure-session/security/advisories/GHSA-4cgq-x5f7-r8jv (CNA: openjs)

- https://cna.openjsf.org/security-advisories.html (CNA: openjs)

- https://cna.openjsf.org/security-advisories.html (ce714d77-add3-4f53-aff5-83d477b104bb)

- https://github.com/fastify/fastify-secure-session/security/advisories/GHSA-4cgq-x5f7-r8jv (ce714d77-add3-4f53-aff5-83d477b104bb)

## Provenance

- CNA: openjs: https://www.cve.org/CVERecord?id=CVE-2026-117549; retrieved 2026-11-02; source updated 2026-11-02

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-117549; retrieved 2026-11-02; source updated 2026-11-02

Revision: 0014813688c5aeb84c035d5ee728c6750f0e6b177714ca1bfbe8c848646ca050

Canonical: https://y2.dev/cve/CVE-2026-117549