CVE-2026-109855
HIGHDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Exploit Intelligence
Low risk: more likely to be exploited than 20% of all known CVEs.
References
Related Vulnerabilities
Other CWE-248 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2025-53366 | Unscored | - | 9.8% | - | - |
| CVE-2025-59538 | High | 7.5 | 9.4% | - | Fix |
| CVE-2021-37714 | High | 7.5 | 6.7% | - | Fix |
| CVE-2018-11466 | Critical | 9.8 | 4.1% | - | - |
| CVE-2025-59465 | High | 7.5 | 4.0% | - | Fix |
| CVE-2018-7852 | High | 7.5 | 3.6% | - | - |
Common questions
How do I fix CVE-2026-109855?
Published advisories record a fix for 2 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.
Is CVE-2026-109855 being actively exploited?
Not that we know of. CVE-2026-109855 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.28% is the estimated probability that it will be exploited in the next 30 days. That is higher than 20% of all scored CVEs.
How severe is CVE-2026-109855?
CVE-2026-109855 has a CVSS v3 base score of 7.5, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-109855 affect?
Published advisories record a fix for node-undici (Debian), undici (npm). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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Markdown
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-10-19.