Origin validation error in webpack-dev-server - CVE-2026-9595
Published: June 16, 2026
Vulnerability identifier: #VU134578
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-9595
CWE-ID: CWE-346
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: webpack
Affected software:
webpack-dev-server
webpack-dev-server
Detailed vulnerability description
The vulnerability allows a remote attacker to cause a denial of service.
The vulnerability exists due to an origin validation error in the HMR WebSocket handling when processing WebSocket connections through a user-configured proxy with a broad context and WebSocket forwarding enabled. A remote attacker can cause the dev server's HMR WebSocket to be intercepted and forwarded to the proxy target to cause a denial of service.
The issue can leak the browser's cookies and Origin header to the backend and bypass the dev server's Host and Origin validation.
How to mitigate CVE-2026-9595
Install security update from vendor's website.