### Impact
`fast-uri` v4.1.1 and earlier require a literal `//` to recognize a URI authority, so a reference that uses `\\`, `/\`, or `\/` as the authority introducer (in place of `//`, after an optional scheme) is parsed with no authority: the sequence and everything after it fold into the path. Node's native WHATWG `URL` (used by `fetch()`, `undici`, and Node's `http`/`https` clients) instead treats `\` as interchangeable with `/` for special schemes (`http`, `https`, `ws`, `wss`, `ftp`, `file`), so the two parsers extract different hosts from the same input.
For example, `fast-uri` resolves `\\evil.com/path` against base `https://allowed.com/` to `https://allowed.com/%5C%5Cevil.com/path` (confined to the trusted host), while Node's WHATWG URL resolves the same reference to `https://evil.com/path`.
Applications that use `fast-uri` to enforce host-based policy (allowlists, denylists, loopback/SSRF 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.
### Patches
Upgrade to `fast-uri` v4.1.2, v3.1.5, v2.4.4.
### Workarounds
None. Upgrade to the patched version.