webpack-dev-middleware vulnerable to Path Traversal via non-slash-terminated publicPath

Why it is Amber

• CVSS 7.4 high
• Remotely exploitable without logging in
• On your watchlist: npm

Affected packages

References

• https://github.com/advisories/GHSA-g84c-rxfj-3j2c
• https://github.com/webpack/webpack-dev-middleware/security/advisories/GHSA-g84c-rxfj-3j2c
• https://nvd.nist.gov/vuln/detail/CVE-2026-76844
• https://github.com/webpack/webpack-dev-middleware/pull/2404
• https://github.com/webpack/webpack-dev-middleware/commit/13344e78b26d88fe9b7c8381a3860e248a52359b
• https://github.com/webpack/webpack-dev-middleware/commit/cefccbac330c29052fc6dffe85fd292ea2ac782b
• https://github.com/advisories/GHSA-wr3j-pwj9-hqq6
• https://github.com/webpack/webpack-dev-middleware/blob/v8.1.1/src/middleware.js
• https://github.com/webpack/webpack-dev-middleware/releases/tag/v7.4.6
• https://github.com/webpack/webpack-dev-middleware/releases/tag/v8.3.0

Original description

> [!IMPORTANT] > CVE-2026-76844 was assigned and published for this issue by VulnCheck on 2026-08-24 without prior coordination with the webpack maintainers or the OpenJS Foundation, which holds the CVE Numbering Authority scope for webpack projects. Neither the maintainers nor the OpenJS CNA were notified before publication, and no fix was available at that time. This advisory is the coordinated record produced by the webpack maintainers and the OpenJS Foundation CNA. ### Impact `webpack-dev-middleware` resolves a request to a local file in `getFilenameFromUrl`. When the configured `publicPath` has no trailing slash (for example `/assets`), a request that shares that prefix without being under it (for example `/assets../secret`) still passes the containment check, and the `..` guard does not catch it because it runs on the path before the `publicPath` prefix is stripped. The stripped remainder then escapes the configured output directory once joined, so an unauthenticated request can read files outside the output root. Applications serving `webpack-dev-middleware` (directly or via `webpack-dev-server`) with a `publicPath` that does not end in a slash are affected. A development server reachable on the network can be attacked with a single request and no victim interaction. ### Patches Patched in `webpack-dev-middleware` 8.3.0. There is no backport to the 5.x, 6.x, or 7.x lines; users on those lines should upgrade to 8.3.0 or later. ### Workarounds Configure `publicPath` with a trailing slash (for example `/assets/` instead of `/assets`), or upgrade to a patched version. ### References - [CVE-2024-29180](https://www.cve.org/CVERecord?id=CVE-2024-29180) ([GHSA-wr3j-pwj9-hqq6](https://github.com/webpack/webpack-dev-middleware/security/advisories/GHSA-wr3j-pwj9-hqq6)), the earlier path traversal in the same component that this issue bypasses.