# CVE-2026-120216: Fastify denial of service via uncaught exception on handlerTimeout with streaming responses

Source: https://junglewise.ai/threats/cve-2026-120216-fastify-is-a-web-framework-for-node-js-in-versions-5-8-0-through  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-02. Updated: 2026-11-02  
License: CC BY 4.0 with a link back

CVE-2026-120216 · Severity: medium · CVSS 5.9 · Published 2026-11-02

Fastify is a web application framework for Node.js used to build web services. When configured with a nonzero handlerTimeout setting and a route streams data, an attacker can send a single request that keeps the connection open past the timeout, causing the server to crash and interrupt all other active requests. This results in complete unavailability of the service.

Vendors: [OpenJS Foundation](https://junglewise.ai/threats/vendors/openjs-foundation).

## Quick answers

**Is CVE-2026-120216 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-120216 being exploited in the wild.

**How severe is CVE-2026-120216?** Medium severity, with a CVSS score of 5.9, published 2 November 2026.

**Which products and versions are affected?** OpenJS Foundation Fastify 5.8.0 through 5.12.7.

**Is there a fix for CVE-2026-120216?** Yes: fixed in 5.12.8. Until then: Set handlerTimeout to 0, which disables the affected timeout path and restores the default non-vulnerable behavior. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-120216 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-120216?** Check whether you run OpenJS Foundation Fastify at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 5.12.8

Set handlerTimeout to 0, which disables the affected timeout path and restores the default non-vulnerable behavior.

## Technical details

The vulnerability is an uncaught exception (CWE-248) triggered when Fastify attempts to send an error response on a streaming response that has already transmitted headers. When handlerTimeout fires on a streaming endpoint, Fastify tries to send headers a second time, raising ERR_HTTP_HEADERS_SENT and terminating the Node.js process. An unauthenticated remote attacker can exploit this over the network with no user interaction required, provided handlerTimeout is set to a nonzero value (the default value of 0 is not vulnerable).

## Affected products

- OpenJS Foundation Fastify 5.8.0 through 5.12.7

## Timeline

- 2026-11-02: disclosed

## References

- [https://cna.openjsf.org/security-advisories.html](https://cna.openjsf.org/security-advisories.html)
- [https://github.com/fastify/fastify/security/advisories/GHSA-3qgw-9275-xf8w](https://github.com/fastify/fastify/security/advisories/GHSA-3qgw-9275-xf8w)