High · CVSS 7.5CVE-2026-109855

undici WebSocket client vulnerable to Denial of Service via failed HTTP/2 handshakeCVE-2026-109855

Answer in brief

CVE-2026-109855 records a High severity (CVSS 7.5) vulnerability in undici WebSocket client vulnerable to Denial of Service via failed HTTP/2 handshake. The current sources do not mark it as known exploited. The current feed maps undici/undici (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Oct 15, 2026Updated Oct 15, 2026Source checked Oct 15, 2026First seen by HOL Oct 15, 2026Material review Oct 15, 2026

Upstream Advisory

Key facts

Risk: High · CVSS 7.5
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

CVSS is 7.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps undici/undici (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
undici/undicigeneric	>=8.11.0 <8.11.2	8.11.2

Recommended response

Check inventory. Check lockfiles and deployed manifests for undici/undici.
Review the reported fix. Update undici/undici to 8.11.2 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Oct 15, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Oct 15, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Oct 15, 2026

Sources and claim methodology

Upstream source description

undici is an HTTP client library for Node.js. In versions 8.11.0 and 8.11.1, its experimental WebSocket client, which is enabled by default when HTTP/2 is allowed, mishandles a failed WebSocket handshake over HTTP/2: the request is already marked completed, so the abort becomes a no-op and the underlying HTTP/2 stream is left open with no error listener. When a malicious server then destroys the connection, the orphaned stream emits an unhandled error and the Node.js process exits. A remote, unauthenticated server can therefore crash any client that opens a WebSocket to it over HTTP/2, resulting in a denial of service. The issue is fixed in undici 8.11.2, and users should upgrade to 8.11.2 or later. As a workaround, set allowH2 to false to disable HTTP/2 for WebSocket connections, or avoid opening WebSocket connections to untrusted servers.

Quoted source text, attributed separately from HOL analysis.