CVE-2026-9595

webpack-dev-server vulnerable to HMR WebSocket interception via permissive user proxies

CVSS 3.1 5.3 MEDIUMEUVD CVSS 3.1 5.3 CVSS 3.1 · ce714d77-add3-4f53-aff5-83d477b104bb · Secondary 5.3 MEDIUMCVSS 3.1 · [email protected] · Primary 4.3 MEDIUMCVSS_V3 Publisher rating MODERATEFixed by webpack-dev-server 5.2.5KEV status unknown

Original advisory

cve-program · CVE-2026-9595

Published

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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euvd · EUVD-2026-36729

Published

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nvd · CVE-2026-9595

Published

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osv · GHSA-mx8g-39q3-5c79

Published

### 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.

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Affected versions, risk and source history

Exploitation and risk

Source status

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Risk history

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Advisory history

12 saved versions

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13 Nov 2026 18:06:53 UTC