Fastify fast-uri up to 2.4.2/3.1.3/4.1.0 URL Parser server-side request forgery

Summaryinfo

A vulnerability was found in Fastify fast-uri up to 2.4.2/3.1.3/4.1.0. It has been declared as critical. This affects an unknown part of the component URL Parser. Such manipulation leads to server-side request forgery. This vulnerability is uniquely identified as CVE-2026-16221. The attack can be launched remotely. No exploit exists.

Detailsinfo

A vulnerability, which was classified as critical, has been found in Fastify fast-uri up to 2.4.2/3.1.3/4.1.0. Affected by this issue is some unknown processing of the component URL Parser. The manipulation with an unknown input leads to a server-side request forgery vulnerability. Using CWE to declare the problem leads to 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. Impacted is confidentiality, integrity, and availability. CVE summarizes:

Impact: fast-uri versions from 2.3.1 through 4.1.0 (including the 3.x line up to 3.1.3 and the 2.x line up to 2.4.2) do not treat a literal backslash character (U+005C) as an authority delimiter. Node's native WHATWG URL parser, used by fetch, undici, and Node's http and https clients, normalizes the backslash to a forward slash for special schemes such as http, https, ws, wss, ftp, and file. As a result, the two parsers extract different hosts from the same input string. Applications that use fast-uri to enforce host-based policy such as allowlists, denylists, loopback or SSRF filtering, redirect validation, or outbound proxy routing before passing the same URL into Node's URL or fetch consumers can be steered to an unintended destination, including cloud metadata endpoints, loopback, or internal hosts. Patches: upgrade to fast-uri 4.1.1, 3.1.4, or 2.4.3. Workarounds: none.

The advisory is available at github.com. This vulnerability is handled as CVE-2026-16221 since 07/18/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. The technical details are unknown and an exploit is not available.

Upgrading to version 2.4.3, 3.1.4 or 4.1.1 eliminates this vulnerability.

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Productinfo

Vendor

• Fastify

Name

• fast-uri

Version

• 2.4.0
• 2.4.1
• 2.4.2
• 3.1.0
• 3.1.1
• 3.1.2
• 3.1.3
• 4.0
• 4.1.0

Website

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

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