Bug 2565121 (CVE-2026-120216)

Summary: CVE-2026-120216 fastify: Fastify: Denial of Service via streaming response timeout handling
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: bbrownin, sdawley
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Fastify. An unauthenticated remote attacker can cause a Denial of Service (DoS) by sending a request to a streaming endpoint when a custom handler timeout is configured. If a streaming response remains open past the configured timeout after response headers have already been sent, the server attempts to issue an error on an active stream. This failure raises an unhandled exception that terminates the Node.js process and interrupts all in-flight requests.
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Description OSIDB Bzimport 2026-11-02 11:23:46 UTC 
fastify is a web framework for Node.js. In versions 5.8.0 through 5.12.7, when the server is configured with a nonzero handlerTimeout and a route returns a streaming response, a request whose stream has already sent its headers but stays open past the timeout crashes the server. When the timeout fires, Fastify tries to send a new error response on a response that has already started, raising an uncaught ERR_HTTP_HEADERS_SENT that terminates the Node.js process and interrupts any other in-flight requests. An unauthenticated remote client can trigger this with a single request to an affected streaming endpoint, so the impact is availability only, and exploitation requires the non-default handlerTimeout to be set. The default handlerTimeout of 0 is not affected. The issue is fixed in fastify 5.12.8, and users should upgrade to 5.12.8 or later. As a workaround, set handlerTimeout to 0, which disables the affected timeout path.