# CVE-2026-109855: undici WebSocket client denial of service via failed HTTP/2 handshake

Source: https://junglewise.ai/threats/cve-2026-109855-undici-is-an-http-client-library-for-node-js-in-versions-8-11-0  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-15. Updated: 2026-10-19  
License: CC BY 4.0 with a link back

CVE-2026-109855 · Severity: high · CVSS 7.5 · Published 2026-10-15

undici is an HTTP client library for Node.js used to handle web requests in server applications. In versions 8.11.0 and 8.11.1, its WebSocket client can be remotely crashed by a malicious server during an HTTP/2 connection upgrade, causing the entire Node.js process to exit and resulting in a denial of service. This affects any application using undici to open WebSocket connections over HTTP/2.

Technologies: [undici (npm)](https://junglewise.ai/threats/technologies/undici). Vendors: [Nodejs](https://junglewise.ai/threats/vendors/nodejs), [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-109855 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-109855 being exploited in the wild.

**How severe is CVE-2026-109855?** High severity, with a CVSS score of 7.5, published 15 October 2026.

**Which products and versions are affected?** Node.js Undici 8.11.0, 8.11.1; undici (npm) >= 8.11.0, < 8.11.2, SEMVER:introduced=8.11.0,fixed=8.11.2.

**Is there a fix for CVE-2026-109855?** Yes. undici (npm): upgrade to 8.11.2 or later. Until then: Set allowH2 to false to disable HTTP/2 for WebSocket connections, or avoid opening WebSocket connections to untrusted servers until upgrading. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-109855 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-109855?** Check whether you run Node.js Undici, Nodejs Undici or undici (npm) 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

- undici (npm): upgrade to 8.11.2 or later

  ```sh
  npm install undici@8.11.2
  ```


Set allowH2 to false to disable HTTP/2 for WebSocket connections, or avoid opening WebSocket connections to untrusted servers until upgrading.

## Technical details

The vulnerability is an unhandled exception in the WebSocket client's HTTP/2 handshake logic: when a WebSocket upgrade fails validation, the abort is marked as a no-op because the request is already completed, leaving the underlying ClientHttp2Stream open without an error listener. When the malicious server closes the connection, the orphaned stream emits an unhandled error that crashes the process. The attack requires network access and an unauthenticated connection to a WebSocket endpoint.

## Affected products

- Node.js undici 8.11.0, 8.11.1

## Identifiers

- CVE-2026-109855
- GHSA-qq8j-7v2v-3rwp

## Timeline

- 2026-10-15: disclosed
- 2026-10-15: patched

## References

- [https://cna.openjsf.org/security-advisories.html](https://cna.openjsf.org/security-advisories.html)
- [https://github.com/nodejs/undici/security/advisories/GHSA-qq8j-7v2v-3rwp](https://github.com/nodejs/undici/security/advisories/GHSA-qq8j-7v2v-3rwp)

## Related threats

- [undici cache poisoning via cross-origin redirect revalidation](https://junglewise.ai/threats/cve-2026-116630-undici-is-an-http-client-for-node-js-in-versions-7-0-0-through-7)
- [undici cross-origin credential disclosure via array-of-tuples headers](https://junglewise.ai/threats/cve-2026-120415-undici-is-an-http-client-for-node-js-in-versions-7-0-0-through-7)
- [undici HTTP client denial of service in decompress interceptor](https://junglewise.ai/threats/cve-2026-87029-undici-is-an-http-client-library-for-node-js-in-versions-7-15-0)
- [undici denial of service via TLS 1.2 certificate renegotiation](https://junglewise.ai/threats/cve-2026-87018-undici-is-an-http-client-library-for-node-js-in-versions-4-2-1)
- [undici WebSocket client denial of service via unbounded PONG buffering](https://junglewise.ai/threats/cve-2026-109932-undici-is-an-http-client-library-for-node-js-in-versions-5-15-0)