CVE-2026-9595 webpack-dev-server vulnerable to HMR WebSocket interception via permissive user proxies
Misconfigured proxies in webpack-dev-server leak cookies and bypass origin checks by intercepting HMR WebSockets, requiring scoped proxy contexts or disabling WebSocket forwarding.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-9595?
CVE-2026-9595 is a medium-severity vulnerability in webpack-dev-server, published on 15 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Impact: When a user-configured proxy on webpack-dev-server has a broad context (e.g. /) and ws: true, it also intercepts the dev server's own HMR WebSocket and forwards it to the proxy target. This leaks the browser's cookies and Origin header to the backend, bypasses the dev server's Host/Origin validation, and corrupts the HMR socket (both HMR and the proxy end up writing to the same socket).
Patches: Fixed in [email protected].
Workarounds: Scope user-defined proxy context to specific paths instead of /, or omit ws: true from the proxy entry when WebSocket forwarding is not required.
Which versions are affected by CVE-2026-9595?
2 open source packages ship an affected version, including webpack-dev-server. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
webpack-dev-server npm | All versions | 5.2.5 |
webpack-dev-server npm | All versions | 5.2.5 |
Products include Webpack.js Webpack-Dev-Server.
How do I fix CVE-2026-9595?
Upgrade webpack-dev-server to 5.2.5 or later.
Is CVE-2026-9595 being exploited?
There is no known exploitation of CVE-2026-9595 in the wild and no public exploit code.
Other vulnerabilities in webpack-dev-server
- CVE-2026-112085 webpack-dev-server vulnerable to cross-origin WebSocket access via proxied upgrades
- CVE-2018-14732 Missing Origin Validation in webpack-dev-server
- CVE-2025-30360 webpack-dev-server users' source code may be stolen when they access a malicious web site with non-Chromium based browser
- CVE-2026-114348 webpack-dev-server vulnerable to information disclosure via Host validation bypass in the open-editor endpoint
- CVE-2026-14631 webpack-dev-server vulnerable to denial of service via a malformed Host or Origin header
Sources
Also known as CLEANSTART-2026-UI84742, CLSA-2026-1787837620, GHSA-mx8g-39q3-5c79, MINI-22f4-5pgw-hff5, MINI-g5x9-2hrg-8v6r, MINI-j9cf-gq4v-9jfc and 5 more.