High [CVE-2026-13676] Security policy bypass due to improper Unicode hostname canonicalization

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. A remote attacker could exploit this to circumvent security controls and potentially access unauthorized resources or perform malicious redirects.

This Important flaw in `fast-uri` allows a remote attacker to bypass host-based security policies. Applications that rely on `fast-uri` for URL parsing and policy enforcement, such as denylists or redirect validations, can be circumvented due to inconsistent handling of Unicode hostnames.