# CVE-2026-117518: @fastify/reply-from vulnerable to path traversal via backslash dot-segment prefix escape

Status: published. Published: 2026-11-02. Updated: 2026-11-02.

@fastify/reply-from is a Fastify plugin that forwards requests to an upstream HTTP server. In versions up to and including 12.8.0, the plugin validated proxy paths against forward-slash dot-segments only and then built the target with the WHATWG URL parser, which normalizes a backslash and its percent-encoded form to a forward slash on the http, https, ws, and wss schemes. A path that uses backslash dot-segments therefore bypasses the traversal check and resolves to a different upstream path, so an application that scopes proxying to an upstream prefix, or relies on route guards in front of the forwarding call, can have an unauthenticated request escape the intended prefix and reach other upstream endpoints. The resulting access to unintended upstream resources is primarily a confidentiality impact with limited integrity impact. The issue is fixed in @fastify/reply-from 12.8.1, where the path is normalized and any segment resolving to a single or double dot, along with control characters and ambiguous whitespace, is rejected before the URL is constructed, and users should upgrade to 12.8.1 or later. There is no workaround other than upgrading.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- @fastify/reply-from / @fastify/reply-from (@fastify/reply-from): ≥ 0; < 12.8.1 (affected, semver); 12.8.1 (unaffected, semver). Source: CNA: openjs

## Severity and prioritization

- CVSS 3.1: 8.2 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N; source: CNA: openjs

- CVSS 3.1: 8.2 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N; source: NVD: ce714d77-add3-4f53-aff5-83d477b104bb

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-22

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-vp2r-3vgv-32g9 (CNA: openjs)

- https://cna.openjsf.org/security-advisories.html (CNA: openjs)

- https://cna.openjsf.org/security-advisories.html (ce714d77-add3-4f53-aff5-83d477b104bb)

- https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-vp2r-3vgv-32g9 (ce714d77-add3-4f53-aff5-83d477b104bb)

## Provenance

- CNA: openjs: https://www.cve.org/CVERecord?id=CVE-2026-117518; retrieved 2026-11-02; source updated 2026-11-02

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-117518; retrieved 2026-11-02; source updated 2026-11-02

Revision: 3b4e3b285cb65f0ca0b9f0d1095aa71bca21615adad359f4f428892df45a462b

Canonical: https://y2.dev/cve/CVE-2026-117518