CVE-2026-19534 is a high-severity (CVSS 7.5) CWE-248 vulnerability in undici. A fix is available for undici — see the affected versions and patch details below.
undici vulnerable to Denial of Service via unrequested WebSocket subprotocol
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-19534.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-19534 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,415 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
undicinpmDescription
undici's WebSocket client crashes the whole Node.js process during the opening handshake when a server responds with a subprotocol that the client never requested. A default WebSocket connection sends no subprotocol, but if the server's 101 response includes a Sec-WebSocket-Protocol header, undici dereferences a null value while checking it against the requested list and throws an uncaught TypeError. Because that code runs inside a microtask with no surrounding error handling, the exception propagates and terminates the process under Node's default behavior, instead of gracefully failing the connection as required by the WebSocket protocol. Any application that opens a WebSocket to an attacker-controlled or compromised server, or over a plaintext connection subject to a machine-in-the-middle, can be crashed remotely without authentication in the default configuration. This affects undici versions from 6.7.0 up to 6.28.1, from 7.0.0 up to 7.29.1, and from 8.0.0 up to 8.10.2. Users should upgrade to undici 6.28.1, 7.29.1, or 8.10.2.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | undici | ≥ 6.7.0&&< 6.28.1 | 6.28.1npm install undici@6.28.1 |
| 📦npm | undici | ≥ 7.0.0&&< 7.29.1 | 7.29.1npm install undici@7.29.1 |
| 📦npm | undici | ≥ 8.0.0&&< 8.10.2 | 8.10.2npm install undici@8.10.2 |
Affected Products
undicinodejsDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for undici, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update undici to 6.28.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-19534 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This is an Important denial of service flaw in undici's WebSocket client, which can lead to a Node.js process crash. The vulnerability is triggered when an application connects to a malicious or compromised server that responds with an unrequested WebSocket subprotocol, allowing a remote, unauthenticated attacker to…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-19534 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Enterprise Linux 10 | nodejs24-1:24.21.0-1.el10_2 | RHSA-2026:73428 |
| Red Hat Enterprise Linux 10 | nodejs22-1:22.23.2-3.el10_2 | RHSA-2026:75864 |
| Red Hat Enterprise Linux 10.0 Extended Update Support | nodejs22-1:22.23.2-2.el10_0 | RHSA-2026:84703 |
| Red Hat Enterprise Linux 8 | nodejs:24-8100020260921144227.6d880403 | RHSA-2026:74085 |
| Red Hat Enterprise Linux 8 | nodejs:22-8100020260928121333.6d880403 | RHSA-2026:75870 |
| Red Hat Enterprise Linux 9 | nodejs:24-9080020260921144317.rhel9 | RHSA-2026:73838 |
| Red Hat Enterprise Linux 9 | nodejs:22-9080020260928121441.rhel9 | RHSA-2026:76750 |
| Red Hat Enterprise Linux 9.6 Extended Update Support | nodejs:22-9060020261009101511.rhel9 | RHSA-2026:84589 |
Frequently Asked Questions
Is CVE-2026-19534 in your dependencies?
Find it across npm, including transitive dependencies.