CVE-2026-12151

undici WebSocket client vulnerable to denial of service via fragment count bypass

Workarounds

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

The undici WebSocket client enforces maxPayloadSize on the cumulative byte count of fragments in a message but does not enforce a limit on the number of fragments. A malicious WebSocket server can stream many small or empty continuation frames that each pass per-frame and cumulative-size validation, collectively causing unbounded memory growth in the client process. The result is memory exhaustion and a denial of service.

Affected applications are those using the undici WebSocket client (new WebSocket(...)) or the WebSocketStream API that can be induced to connect to an attacker-controlled or compromised WebSocket endpoint.

All releases starting at undici 6.17.0 are affected.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-12151 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 (6.27.0, 7.28.0, 8.5.0); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Upgrade to undici v6.27.0, v7.28.0 or v8.5.0.

1. What is CVE-2026-12151? CVE-2026-12151 is a high-severity uncontrolled resource consumption vulnerability in undici (npm), affecting versions < 6.27.0. It is fixed in 6.27.0, 7.28.0, 8.5.0. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-12151? CVE-2026-12151 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-2026-12151? undici (npm) versions < 6.27.0 is affected.
4. Is there a fix for CVE-2026-12151? Yes. CVE-2026-12151 is fixed in 6.27.0, 7.28.0, 8.5.0. Upgrade to this version or later.
5. Is CVE-2026-12151 exploitable, and should I be worried? Whether CVE-2026-12151 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-2026-12151 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-2026-12151?
   • Upgrade undici to 6.27.0 or later
   • Upgrade undici to 7.28.0 or later
   • Upgrade undici to 8.5.0 or later

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