CVE-2026-120215

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CVE-2026-120215

Summary: @fastify/cors vulnerable to Denial of Service on a CORS preflight with the onSend hook

Details: @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.

References: https://cna.openjsf.org/security-advisories.html, https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/120xxx/CVE-2026-120215.json, https://github.com/fastify/fastify-cors/security/advisories/GHSA-8c3j-mm64-257c, https://nvd.nist.gov/vuln/detail/CVE-2026-120215