Fastify reply-from up to 12.8.0 URL Parser path traversal

Summaryinfo

A vulnerability marked as critical has been reported in Fastify reply-from up to 12.8.0. Impacted is an unknown function of the component URL Parser. The manipulation leads to path traversal. This vulnerability is uniquely identified as CVE-2026-117518. The attack is possible to be carried out remotely. No exploit exists. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as critical, has been found in Fastify reply-from up to 12.8.0. This issue affects an unknown code of the component URL Parser. The manipulation with an unknown input leads to a path traversal vulnerability. Using CWE to declare the problem leads to CWE-22. The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

@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.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-117518 since 10/27/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Neither technical details nor an exploit are publicly available. MITRE ATT&CK project uses the attack technique T1006 for this issue.

Upgrading to version 12.8.1 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-115127). If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Productinfo

Vendor

• Fastify

Name

• reply-from

Version

• 12.0
• 12.1
• 12.2
• 12.3
• 12.4
• 12.5
• 12.6
• 12.7
• 12.8.0

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• 🔒
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CPE 2.2info

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If you want to get the best quality for vulnerability data then you always have to consider VulDB.

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