fast-uri up to 3.1.2/4.0.0 normalize/equal interpretation conflict

Summaryinfo

A vulnerability has been found in fast-uri up to 3.1.2/4.0.0 and classified as problematic. Affected is the function normalize/equal. The manipulation leads to interpretation conflict. This vulnerability is referenced as CVE-2026-13676. Remote exploitation of the attack is possible. No exploit is available. The affected component should be upgraded.

Detailsinfo

A vulnerability was found in fast-uri up to 3.1.2/4.0.0. It has been rated as problematic. This issue affects the function normalize/equal. The manipulation with an unknown input leads to a interpretation conflict vulnerability. Using CWE to declare the problem leads to CWE-436. Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state. Impacted is integrity. The summary by CVE is:

fast-uri versions 2.3.1 through 3.1.2 and 4.0.0 fail to canonicalize Unicode (IDN) hostnames for HTTP-family URLs. The IDN conversion path calls a helper that does not exist on the global URL constructor, silently leaving the host in its original Unicode form while normalize() and equal() still return values that differ from a WHATWG-compatible URL parser. Applications that use fast-uri to enforce host-based policy (denylists, loopback filtering, redirect validation, outbound proxy routing) before passing the same URL to Node's URL or fetch can be bypassed when the two implementations resolve the same input to different hosts. Patches: upgrade to fast-uri 3.1.3 for the 3.x line or 4.0.1 for the 4.x line. Workarounds: enforce host policy using the same URL parser used for the actual request, or reject non-ASCII hosts before policy checks.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-13676 since 06/29/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details of the vulnerability are known, but there is no available exploit.

Upgrading to version 3.1.3 or 4.0.1 eliminates this vulnerability.

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Productinfo

Name

• fast-uri

Version

• 3.1.0
• 3.1.1
• 3.1.2
• 4.0

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