Bug 2565024 (CVE-2026-120215)

Summary: CVE-2026-120215 @fastify/cors: @fastify/cors: Denial of Service via CORS preflight requests
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: low    
Version: unspecifiedCC: sdawley
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in @fastify/cors. An unauthenticated remote attacker can cause a Denial of Service (DoS) by sending a Cross-Origin Resource Sharing (CORS) preflight request to an application configured with a non-default hook. Under these conditions, the plugin improperly attempts to write response headers multiple times, triggering an unhandled error that can crash the server process.
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Description OSIDB Bzimport 2026-11-02 10:31:58 UTC 
@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.