CVE-2026-9595 – webpack-dev-server vulnerable to HMR WebSocket interception via permissive user proxies

CVE ID :CVE-2026-9595

Published : June 15, 2026, 4:16 p.m. | 1 hour, 25 minutes ago

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.

Severity: 5.3 | MEDIUM

Visit the link for more details, such as CVSS details, affected products, timeline, and more…