# CVE-2026-120215: @fastify/cors vulnerable to Denial of Service on a CORS preflight with the onSend hook

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

@fastify/cors is the CORS plugin for the Fastify web framework. In versions 8.2.0 through 11.3.x, an application that registers the plugin with the non-default onSend hook option crashes on a CORS preflight. On an OPTIONS request that carries the Origin and Access-Control-Request-Method headers, the plugin calls reply.send from inside the onSend hook and re-enters the send pipeline, so a second write of the response headers throws an uncaught ERR_HTTP_HEADERS_SENT that can terminate the Node.js process when no process-level handler is installed. An unauthenticated remote client can trigger this with a single preflight request, so the impact is availability only, and exploitation requires the non-default onSend configuration. The issue is fixed in @fastify/cors 11.4.0, where onSend is no longer an accepted hook value and registration fails with a clear error instead of crashing at request time, and users should upgrade to 11.4.0 or later. As a workaround, use any other hook such as the default onRequest, or set preflight to false.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- @fastify/cors / @fastify/cors (@fastify/cors): ≥ 8.2.0; < 11.4.0 (affected, semver); 11.4.0 (unaffected, semver). Source: CNA: openjs

## Severity and prioritization

- CVSS 3.1: 3.7 (low); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L; source: CNA: openjs

- CVSS 3.1: 3.7 (low); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L; source: NVD: ce714d77-add3-4f53-aff5-83d477b104bb

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-248

## Exploit evidence

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

## References

- https://github.com/fastify/fastify-cors/security/advisories/GHSA-8c3j-mm64-257c (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-cors/security/advisories/GHSA-8c3j-mm64-257c (ce714d77-add3-4f53-aff5-83d477b104bb)

## Provenance

- CNA: openjs: https://www.cve.org/CVERecord?id=CVE-2026-120215; retrieved 2026-11-02; source updated 2026-11-02

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-120215; retrieved 2026-11-02; source updated 2026-11-02

Revision: c6de5776abb5c849128204e8b69f82b1f697ceef3e845d67657c34ae8b7c28e8

Canonical: https://y2.dev/cve/CVE-2026-120215