High [CVE-2026-75931] Host confusion via skipped IDN canonicalization

fast-uri is a URI parser for Node.js. It canonicalizes a host to its ASCII form only when the input carries an explicit scheme, so a scheme-relative reference such as a host preceded by two slashes is returned with its host verbatim and no error set.

An application that extracts a host with fast-uri to check it against a policy list and then resolves the same reference can make its decision on one host while the destination is another, enabling host confusion and policy bypass.

The affected versions are 2.4.2 up to but not including 2.4.5, 3.1.3 up to but not including 3.1.6, and 4.0.1 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3, which canonicalize the host consistently across the resolve path.

Users should upgrade to a patched version. This vulnerability arises because the parser fails to consistently convert internationalized domain names (IDN) to their standard ASCII form when processing scheme-relative references.

This inconsistency can lead to host confusion, where an application might validate a URI against a security policy using one hostname, but then resolve to a different, potentially malicious, hostname. This discrepancy could allow an attacker to bypass security policies and access unauthorized resources.

Red Hat severity: Important — CVSS 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N). Weakness: CWE-444.