CVE-2026-9675

Description

Impact: The undici WebSocket client enforces maxPayloadSize per-frame but does not enforce the cumulative size of fragmented uncompressed messages. A malicious WebSocket server can stream many small fragments that each pass per-frame validation but collectively exceed the configured limit, 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(...)) that can be induced to connect to an attacker-controlled or compromised WebSocket endpoint. This is a regression specific to undici 8.1.0. The 6.25.0 line shipped the equivalent cumulative check from the start and is unaffected. The 7.x line never had the maxPayloadSize feature and is also unaffected. Patches: Upgrade to undici >= 8.5.0. Workarounds: No workaround is available. The fix must be applied through an upgrade.

CVSS Metrics

• v3.1•HIGH•Score: 7.5CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Trends

Current EPSS score: 0.43%• Percentile: 34%

Techniques & Countermeasures

• CWE-400•Uncontrolled Resource Consumption

  The product does not properly control the allocation and maintenance of a limited resource.

• CWE-770•Allocation of Resources Without Limits or Throttling

  The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.

Affected Systems

• Npm•undici

  ≥ 8.0.0, < 8.5.0

• undici•undici

  ≥ 8.0.0, < 8.5.0

References (4)

• https://github.com/nodejs/undici/security/advisories/GHSA-38rv-x7px-6hhq
• https://cna.openjsf.org/security-advisories.html
• https://nvd.nist.gov/vuln/detail/CVE-2026-9675
• https://github.com/nodejs/undici