GHSA-v2hh-gcrm-f6hx — fast-uri vulnerable to host confusion via literal backslash authority delimiter

Affected: npm: fast-uri

### Impact

`fast-uri` v4.1.0 and earlier do not treat a literal backslash (U+005C) as an authority delimiter. Node's native WHATWG `URL` (used by `fetch()`, `undici`, and Node's `http`/`https` clients) normalizes `\` to `/` for special schemes (`http`, `https`, `ws`, `wss`, `ftp`, `file`), so the two parsers extract different hosts from the same input string.

For example, `http://evil.com\@allowed.com` is treated by `fast-uri` as host `allowed.com` with userinfo `evil.com\`, while Node's WHATWG URL parser and `fetch()` see host `evil.com` with path `/@allowed.com`.

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, including cloud metadata endpoints, loopback, or internal hosts.

### Patches

Upgrade to `fast-uri` v4.1.1, v3.1.4, or v2.4.3.

### Workarounds

None. Upgrade to the patched version.

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GHSA-v2hh-gcrm-f6hx — fast-uri vulnerable to host confusion via literal backslash authority delimiter