PatchSiren cyber security CVE debrief

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: 2026-06-16
Advisory published: 2026-06-15
Advisory updated: 2026-06-16

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.

Sources and references

Verified primary and authoritative sources

CVE-2026-9595 CVE Program record
Publisher, destination, and source semantics verified

URL: https://www.cve.org/CVERecord?id=CVE-2026-9595

CVE Program - Official CVE Program record with source-provided CVE metadata.
CVE-2026-9595 NVD vulnerability detail
Publisher, destination, and source semantics verified

URL: https://nvd.nist.gov/vuln/detail/CVE-2026-9595

NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Source reference
Unverified legacy reference

URL: https://cna.openjsf.org/security-advisories.html

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Source reference
Unverified legacy reference

URL: https://github.com/facebook/create-react-app/pull/7444

ce714d77-add3-4f53-aff5-83d477b104bb
Source reference
Unverified legacy reference

URL: https://github.com/vuejs/vue-cli/commit/72ba7505aff2a8314e82aa5082379a77504a1fcb

ce714d77-add3-4f53-aff5-83d477b104bb
Source reference
Unverified legacy reference

URL: https://github.com/webpack/webpack-dev-server/pull/4316

ce714d77-add3-4f53-aff5-83d477b104bb
Source reference
Unverified legacy reference

URL: https://github.com/webpack/webpack-dev-server/security/advisories/GHSA-mx8g-39q3-5c79

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Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.