fastify fast-uri up to 2.4.3/3.1.4/4.1.1 Authority Parsing server-side request forgery

Summaryinfo

A vulnerability labeled as critical has been found in fastify fast-uri up to 2.4.3/3.1.4/4.1.1. This vulnerability affects unknown code of the component Authority Parsing. The manipulation results in server-side request forgery. This vulnerability is known as CVE-2026-18446. It is possible to launch the attack remotely. No exploit is available.

Detailsinfo

A vulnerability was found in fastify fast-uri up to 2.4.3/3.1.4/4.1.1. It has been classified as critical. Affected is an unknown part of the component Authority Parsing. The manipulation with an unknown input leads to a server-side request forgery vulnerability. CWE is classifying the issue as CWE-918. The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

fast-uri before 4.1.2, 3.1.5, and 2.4.4 requires a literal double forward slash to recognize a URI authority, so a reference that uses a backslash based introducer in place of it (backslash backslash, forward slash backslash, or backslash forward slash) is parsed with no authority and folds into the path. Node's native WHATWG URL parser instead treats a backslash as interchangeable with a forward slash for special schemes, so the two parsers extract different hosts from the same input. Applications that use fast-uri to enforce host based policy such as allowlists, SSRF filtering, or redirect validation before passing the same URL into Node's URL or fetch consumers can be steered to an unintended host. Upgrade to fast-uri 4.1.2, 3.1.5, or 2.4.4.

The advisory is shared for download at github.com. This vulnerability is traded as CVE-2026-18446 since 07/31/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are neither technical details nor an exploit publicly available.

Upgrading to version 2.4.4, 3.1.5 or 4.1.2 eliminates this vulnerability.

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Productinfo

Vendor

• fastify

Name

• fast-uri

Version

• 2.4.0
• 2.4.1
• 2.4.2
• 2.4.3
• 3.1.0
• 3.1.1
• 3.1.2
• 3.1.3
• 3.1.4
• 4.1.0
• 4.1.1

Website

• Product: https://github.com/fastify/fast-uri/

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