CVE-2026-120216 fastify vulnerable to Denial of Service via uncaught exception on handlerTimeout with streaming responses
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2026-120216?
CVE-2026-120216 is a medium-severity vulnerability in fastify, published on 2 Nov 2026.
Official description
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.
Weakness: CWE-248
Which versions are affected by CVE-2026-120216?
How do I fix CVE-2026-120216?
Upgrade to a fixed version.
Is CVE-2026-120216 being exploited?
There is no known exploitation of CVE-2026-120216 in the wild and no public exploit code.
Sources
Also known as CLSA-2026-1793801979, GHSA-3qgw-9275-xf8w.