In short
- CVE-2026-9595 is a MEDIUM (CVSS 4.3) vulnerability affecting webpack-dev-server (webpack.js), webpack-dev-server.
- Chance of exploitation in the next 30 days (EPSS): 0.16%, higher than 5.5% of all vulnerabilities.
- Fix: update to 5.2.5 or later.
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 webpack-dev-server 5.2.5.
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.
NVD 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.
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