What’s Vulnerable › Vulnerabilities

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

• Severity: medium (CVSS 4.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-06-15 · updated 2026-06-17
• Weakness: CWE-346

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.

Affected products

• webpack-dev-server: before 5.2.5 — fixed in 5.2.5
• Npm Webpack-Dev-Server: before 5.2.5 — fixed in 5.2.5

How to fix

Upgrade to a fixed version: webpack-dev-server 5.2.5; Npm Webpack-Dev-Server 5.2.5. Check the vendor advisory for mitigations.

References

• https://cna.openjsf.org/security-advisories.html
• https://github.com/facebook/create-react-app/pull/7444
• https://github.com/vuejs/vue-cli/commit/72ba7505aff2a8314e82aa5082379a77504a1fcb
• https://github.com/webpack/webpack-dev-server/pull/4316
• https://github.com/webpack/webpack-dev-server/security/advisories/GHSA-mx8g-39q3-5c79

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