undici WebSocket client vulnerable to Denial of Service via failed HTTP/2 handshake

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-109855: 0.28% estimated exploitation activity in the next 30 days; percentile 20.41 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact undici's experimental WebSocket client (enabled by default via `allowH2: true`) crashes the Node.js process when it connects to a malicious server over HTTP/2. If the server answers the WebSocket upgrade with a response that fails handshake validation, undici aborts the request, but the abort is a no-op because the request is already marked completed, so the underlying `ClientHttp2Stream` is left open with no error listener. When the server then destroys the connection, the orphaned stream emits an unhandled `error` and the process exits. A remote, unauthenticated server can therefore deny service to any client that opens a WebSocket to it over HTTP/2. undici 8.11.0 and 8.11.1 are affected; the regression was introduced in 8.11.0.

Patches Upgrade to `8.11.2`.

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

Published by the source

27 Oct 2026, 16:09 UTC

Last modified by the source

27 Oct 2026, 16:15 UTC

Source last checked

28 Oct 2026, 06:00 UTC

1. Revision 2 · amended · 28 Oct 2026

   Amended from osv record GHSA-qq8j-7v2v-3rwp

2. Revision 1 · created · 27 Oct 2026

   Published from ghsa record GHSA-qq8j-7v2v-3rwp

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# Patch Next advisory brief: CVE-2026-109855

Advisory: undici WebSocket client vulnerable to Denial of Service via failed HTTP/2 handshake
IDs: CVE-2026-109855, GHSA-qq8j-7v2v-3rwp
Record: https://patchnext.com/advisories/cve-2026-109855 (revision 2, source checked 28 Oct 2026, 06:00 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: undici 8.11.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm undici: affected >= 8.11.0, < 8.11.2; fixed in 8.11.2

## Suggested inspection (read-only)
- `npm ls undici --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on undici.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-qq8j-7v2v-3rwp (GitHub-reviewed advisory GHSA-qq8j-7v2v-3rwp lists affected ranges: npm undici >=8.11.0 <8.11.2)
- Verified: https://osv.dev/vulnerability/GHSA-qq8j-7v2v-3rwp (OSV record GHSA-qq8j-7v2v-3rwp lists affected ranges: npm undici >=8.11.0 <8.11.2 (fixed))

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