---
id: CVE-2026-120215
title: >-
  @fastify/cors vulnerable to Denial of Service on a CORS preflight with the
  onSend hook
summary: >-
  @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 ca…
severity: low
cvss: 3.7
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L'
cvssSource: cna
cwe:
  - CWE-248
vendor: '@fastify/cors'
product: '@fastify/cors'
affected:
  - '@fastify/cors >= 8.2.0 < 11.4.0'
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-11-02T17:03:55.457723Z'
published: '2026-11-02'
updated: '2026-11-02'
sourceUpdated: '2026-11-02T17:04:11.417Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-120215'
references:
  - url: >-
      https://github.com/fastify/fastify-cors/security/advisories/GHSA-8c3j-mm64-257c
  - url: 'https://cna.openjsf.org/security-advisories.html'
tags:
  - cve.org
epss: 0.00291
epssPercentile: 0.21837
ingestedAt: '2026-11-05T19:30:42.717Z'
---

## Overview

@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.

## Affected

- `@fastify/cors >= 8.2.0 < 11.4.0`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.