---
id: CVE-2026-9675
title: >-
  undici WebSocket client vulnerable to denial of service via cumulative
  fragment bypass
summary: >-
  undici WebSocket client vulnerable to denial of service via cumulative
  fragment bypass
severity: high
cvss: 7.5
cwe:
  - CWE-400
  - CWE-770
vendor: undici
product: undici
affected:
  - 'undici >= 8.0.0, < 8.5.0'
patched:
  - undici 8.5.0
published: '2026-06-18'
updated: '2026-06-18'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-38rv-x7px-6hhq'
references:
  - url: 'https://github.com/nodejs/undici/security/advisories/GHSA-38rv-x7px-6hhq'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-9675'
  - url: 'https://cna.openjsf.org/security-advisories.html'
  - url: 'https://github.com/advisories/GHSA-38rv-x7px-6hhq'
tags:
  - ghsa
  - npm
epss: 0.00426
epssPercentile: 0.36514
ingestedAt: '2026-06-19T03:39:00.824Z'
ecosystem: npm
---

## Overview

## 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.

## Affected packages

- `undici >= 8.0.0, < 8.5.0`

## Remediation

Upgrade to a patched release:

- `undici 8.5.0`