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CVE-2026-9595 webpack-dev-server CVE debrief

CVE-2026-9595 is a security vulnerability in webpack-dev-server that allows an attacker to bypass security features and leak sensitive information. 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.

Vendor
webpack-dev-server
Product
Unknown
CVSS
MEDIUM 5.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-15
Original CVE updated
2026-06-15
Advisory published
2026-06-15
Advisory updated
2026-06-15

Who should care

Developers and administrators using webpack-dev-server with a user-configured proxy and WebSocket forwarding enabled should be aware of this vulnerability and take steps to mitigate it.

Technical summary

The vulnerability has a CVSS score of 5.3 and is classified as MEDIUM severity. It was published on 2026-06-15T16:16:35.227Z and has not been modified since then. The vulnerability is related to CWE-346 and CWE-441.

Defensive priority

MEDIUM

Recommended defensive actions

  • Scope user-defined proxy context to specific paths instead of /
  • Omit ws: true from the proxy entry when WebSocket forwarding is not required
  • Update to [email protected] or later

Evidence notes

The vulnerability was reported by an unknown vendor and has a low confidence level. The evidence includes references to Openjsf and several GitHub commits and pull requests.

Official resources

CVE-2026-9595 was published on 2026-06-15T16:16:35.227Z and has not been modified since then.