Description
@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.
Published: 2026-11-02
Score: 8.2 High
EPSS: n/a
KEV: No
Impact: n/a
Action: n/a
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Remediation

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History

Mon, 02 Nov 2026 17:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Mon, 02 Nov 2026 10:45:00 +0000

Type Values Removed Values Added
Description @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.
Title @fastify/reply-from vulnerable to path traversal via backslash dot-segment prefix escape
Weaknesses CWE-22
References
Metrics cvssV3_1

{'score': 8.2, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N'}


Subscriptions

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cve-icon MITRE

Status: PUBLISHED

Assigner: openjs

Published:

Updated: 2026-11-02T16:58:04.730Z

Reserved: 2026-10-27T09:25:28.170Z

Link: CVE-2026-117518

cve-icon Vulnrichment

Updated: 2026-11-02T16:57:59.686Z

cve-icon NVD

Status : Deferred

Published: 2026-11-02T11:17:30.660

Modified: 2026-11-02T17:17:38.393

Link: CVE-2026-117518

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-11-02T11:30:14Z

Weaknesses