Vulnerability details

### Impact `fast-uri` percent-decodes the scheme component with the legacy global `unescape()` before validating it, so an input whose decoded scheme is a valid but dangerous token is emitted as a live scheme. RFC 3986 section 3.1 does not allow percent-encoding in a scheme, so no RFC or WHATWG parser treats such input as having that scheme, but `fast-uri` does. For example, `normalize('%6Aavascript:alert(1)')` returns `javascript:alert(1)`, `resolve('https://a.example/', '%6Aavascript:alert(1)')` returns `javascript:alert(1)`, and `parse('%6Aavascript:alert(1)').scheme` returns `javascript`. The `%uXXXX` form (`%u006Aavascript:`) produces the same result, and `equal('%68ttp://a/', 'http://a/')` returns `true`. An application that screens dangerous schemes on the raw input and then stores or renders `normalize(input)` or `resolve(base, input)` turns an inert string into an executable `javascript:`, `data:`, or `vbscript:` URL, a stored or reflected cross-site scripting vector in the consuming application. `equal()` also treats an encoded and a plain scheme as the same, which can bypass equality-based allowlists. ### Patches Upgrade to `fast-uri` `2.4.8`, `3.1.9`, or `4.2.3`. ### Workarounds Reject input whose scheme part (before the first `:`, and before any `/`, `?`, or `#`) contains `%`. Alternatively, check `parse(x).scheme` against an allowlist after normalization and use that same parsed result rather than the raw string.