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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-9595: 0.16% estimated exploitation activity in the next 30 days; percentile 5.51 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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.

Published by the source

17 Jun 2026, 18:13 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 12:55 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-mx8g-39q3-5c79

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-mx8g-39q3-5c79

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# Patch Next advisory brief: CVE-2026-9595

Advisory: webpack-dev-server vulnerable to HMR WebSocket interception via permissive user proxies
IDs: CVE-2026-9595, GHSA-mx8g-39q3-5c79
Record: https://patchnext.com/advisories/cve-2026-9595 (revision 2, source checked 26 Oct 2026, 12:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: webpack-dev-server 5.2.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm webpack-dev-server: affected < 5.2.5; fixed in 5.2.5

## Suggested inspection (read-only)
- `npm ls webpack-dev-server --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on webpack-dev-server.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-mx8g-39q3-5c79 (OSV record GHSA-mx8g-39q3-5c79 lists affected ranges: npm webpack-dev-server >=0 <5.2.5 (fixed))
- Verified: https://github.com/advisories/GHSA-mx8g-39q3-5c79 (GitHub-reviewed advisory GHSA-mx8g-39q3-5c79 lists affected ranges: npm webpack-dev-server >=0 <5.2.5)

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