# CVE-2026-120216: fastify vulnerable to Denial of Service via uncaught exception on handlerTimeout with streaming responses

Status: published. Published: 2026-11-02. Updated: 2026-11-02.

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.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- fastify / fastify (fastify): ≥ 5.8.0; < 5.12.8 (affected, semver); 5.12.8 (unaffected, semver). Source: CNA: openjs

## Severity and prioritization

- CVSS 3.1: 5.9 (medium); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H; source: CNA: openjs

- CVSS 3.1: 5.9 (medium); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H; source: NVD: ce714d77-add3-4f53-aff5-83d477b104bb

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-248

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/fastify/fastify/security/advisories/GHSA-3qgw-9275-xf8w (CNA: openjs)

- https://cna.openjsf.org/security-advisories.html (CNA: openjs)

- https://cna.openjsf.org/security-advisories.html (ce714d77-add3-4f53-aff5-83d477b104bb)

- https://github.com/fastify/fastify/security/advisories/GHSA-3qgw-9275-xf8w (ce714d77-add3-4f53-aff5-83d477b104bb)

## Provenance

- CNA: openjs: https://www.cve.org/CVERecord?id=CVE-2026-120216; retrieved 2026-11-02; source updated 2026-11-02

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-120216; retrieved 2026-11-02; source updated 2026-11-02

Revision: fbaf1f566026682c9925367dfc7c1644611507f3cb119be325b62d318a107147

Canonical: https://y2.dev/cve/CVE-2026-120216