CVE-2026-13676: fast-uri vulnerable to host confusion via failed IDN canonicalization

fast-uri versions >= 2.3.1, <= 4.0.0 fail to canonicalize Unicode/IDN hostnames for HTTP-family URLs. The IDN conversion path calls URL.domainToASCII(...) on the global WHATWG URL constructor, where that helper does not exist. The resulting TypeError is silently routed into parsed.error, but parse(), normalize(), and equal() all return with the host left in its original Unicode form.

For example, http://127。0。0。1/ is treated by fast-uri as host 127。0。0。1, while Node’s WHATWG URL parser and fetch() canonicalize the same input to 127.0.0.1.

Applications that use fast-uri to enforce host-based policy (denylists, loopback filtering, redirect validation, outbound proxy routing) before passing the same URL into Node’s URL or fetch() consumers see a policy/use desync and can be steered to an unintended destination.