Loofah allowed_uri? treats javascript&#x3a alert(1) as safe

Summary

Loofah::HTML5::Scrub.allowed_uri? returns true for a javascript: or vbscript: URI whose scheme is split by a numeric character reference without a trailing semicolon, followed by a character that allowed_uri? strips, such as a space or tab. A browser ends the reference at that character and reads javascript&#x3a alert(1) as javascript: alert(1).

Details

allowed_uri? strips control characters and whitespace before it decodes numeric character references. Removing the character that ends a semicolon-less reference lets the reference run on into the following text, so &#x3a alert decodes as &#x3aa (U+03AA) instead of a colon, and no scheme is detected. The same applies to decimal references and to references that encode a scheme letter, as in &#x6a\tavascript:alert(1).

Loofah's default path is affected, because Nokogiri decodes the entity during parsing before Loofah evaluates the URI. This issue affects only callers of the public string-level helper that pass it HTML-encoded strings.