CVE-2026-19534

undici vulnerable to Denial of Service via unrequested WebSocket subprotocol

Workarounds

No workaround is available. The fix must be applied through an upgrade.

The undici WebSocket client throws an uncaught TypeError during the opening handshake when a server's 101 response includes a Sec-WebSocket-Protocol header that the client never requested. The throw occurs in a queueMicrotask callback with no surrounding try/catch, so it propagates as an uncaught exception and terminates the Node.js process. This is a remote, unauthenticated denial of service against any application that opens a WebSocket to an attacker controlled or compromised server, or over a plaintext ws:// connection subject to a machine-in-the-middle. It affects the default new WebSocket(url) usage, where no subprotocol is requested. Per RFC 6455 section 4.1, an unrequested subprotocol must fail the connection, not crash it.

All releases starting at undici 6.7.0 are affected.

CVE-2026-19534 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.28.1, 7.29.1, 8.10.2); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Upgrade to undici 6.28.1, 7.29.1, or 8.10.2.

1. What is CVE-2026-19534? CVE-2026-19534 is a high-severity security vulnerability in undici (npm), affecting versions >= 6.7.0, < 6.28.1. It is fixed in 6.28.1, 7.29.1, 8.10.2.
2. How severe is CVE-2026-19534? CVE-2026-19534 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of undici are affected by CVE-2026-19534? undici (npm) versions >= 6.7.0, < 6.28.1 is affected.
4. Is there a fix for CVE-2026-19534? Yes. CVE-2026-19534 is fixed in 6.28.1, 7.29.1, 8.10.2. Upgrade to this version or later.
5. Is CVE-2026-19534 exploitable, and should I be worried? Whether CVE-2026-19534 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-19534 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-19534?
   • Upgrade undici to 6.28.1 or later
   • Upgrade undici to 7.29.1 or later
   • Upgrade undici to 8.10.2 or later

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