Uncaught Exception Affecting fastify package, versions >=5.8.0 <5.12.8>=6.0.0-alpha.0 <6.0.0-alpha.6
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-JS-FASTIFY-22148885
- published3 Nov 2026
- disclosed2 Nov 2026
- creditSLRHR
Introduced: 2 Nov 2026
NewCVE-2026-120216 (opens in a new tab)How to fix?
Upgrade fastify to version 5.12.8, 6.0.0-alpha.6 or higher.
Overview
fastify is an overhead web framework, for Node.js.
Affected versions of this package are vulnerable to Uncaught Exception via the handlerTimeout path in lib/route.js when a route returns a streaming response. When the server is configured with a nonzero handlerTimeout and a streaming response has already transmitted its headers but remains open past the timeout, Fastify attempts to send a new error response on the already-started response, raising an uncaught ERR_HTTP_HEADERS_SENT that terminates the Node.js process and interrupts all in-flight requests. An unauthenticated remote client can trigger this crash with a single request to an affected streaming endpoint.
Note: This is only exploitable when handlerTimeout is explicitly set to a nonzero value; the default handlerTimeout: 0 is not affected.
Workaround
Set handlerTimeout to 0 (the default), which disables the affected timeout path.