---
id: "CVE-2026-9675"
kind: "vulnerability"
aliases: ["CVE-2026-9675", "GHSA-38rv-x7px-6hhq"]
title: "undici WebSocket client vulnerable to denial of service via cumulative fragment bypass"
severity: "high"
cvss: 7.5
epss: 0.004879999905824661
exploited: false
status: "open"
category: "package"
products: [{"name": "undici", "fixed": ["8.5.0"], "affected": ">= 8.0.0, < 8.5.0", "ecosystem": "npm"}, {"name": "nodejs undici", "fixed": ["8.5.0"], "affected": ">= 8.0.0, < 8.5.0", "ecosystem": "vendor"}]
ecosystems: ["npm", "vendor"]
tags: ["cwe-400", "cwe-770", "node.js", "package", "self-hosted", "undici", "websocket-client"]
not_affected_if: ["no-untrusted-websocket-endpoints", "undici-6.25.0-line", "undici-7.x", "undici-below-8.0.0"]
published_at: "2026-06-17T17:17:28+00:00"
first_seen_at: "2026-10-23T19:11:32+00:00"
updated_at: "2026-10-23T23:44:52+00:00"
sources: [{"source": "ghsa", "url": "https://github.com/advisories/GHSA-38rv-x7px-6hhq", "first_seen_at": "2026-10-23T19:11:32+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-9675", "first_seen_at": "2026-10-23T23:44:52+00:00"}]
---

# undici WebSocket client vulnerable to denial of service via cumulative fragment bypass

The undici WebSocket client does not cap the total size of fragmented messages, only each fragment, so a malicious WebSocket server can stream many small fragments until the client process runs out of memory. The result is a denial of service against the client process, not code execution.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- npm:undici affected >= 8.0.0, < 8.5.0 fixed 8.5.0
- vendor:nodejs undici affected >= 8.0.0, < 8.5.0 fixed 8.5.0

## How to check

Run `npm ls undici` in the project and check whether any resolved version is >= 8.0.0 and < 8.5.0.
Look for direct or transitive use of `new WebSocket(...)` from undici that connects to endpoints you do not control.

## Fix

Upgrade undici to 8.5.0 or later; no workaround is available per the advisory.

## Timeline

- 2026-10-23T19:11:32+00:00 ghsa https://github.com/advisories/GHSA-38rv-x7px-6hhq
- 2026-10-23T23:44:52+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-9675

## From the sources

## 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.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_