Loofah allowed_uri? treats data:text/html as safe text/plain

Summary

Loofah::HTML5::Scrub.allowed_uri? returns true for a data: URI whose mediatype is written with the HTML5 named character reference / in place of the solidus. Loofah reads that mediatype as malformed and falls back to the allowed text/plain, while a browser decodes the reference and reads the same URI as text/html.

Details

allowed_uri? decodes HTML entities with CGI.unescapeHTML, which handles numeric character references but not HTML5 named character references. / therefore survives normalization, so the mediatype carries no /, fails the MIME grammar, and falls back to the allowed text/plain under the WHATWG rule for a malformed mediatype.

:, /, 	 and 
 are the only named character references in the HTML5 table that decode to characters that change how a URI parses (U+003A, U+002F, U+0009 and U+000A; there is no named reference for U+000D), so this closes the set.

Loofah's default sanitize() path is not affected, because Nokogiri decodes the entity during parsing before Loofah evaluates the URI. This issue affects only callers of the public allowed_uri? string-level helper that pass it HTML-encoded strings.

Impact

Applications that validate a user-controlled URL with Loofah::HTML5::Scrub.allowed_uri? and then render the approved value into an href or another browser-interpreted URI attribute may be vulnerable to cross-site scripting. Rails::HTML::Sanitizer.allowed_uri? re-exports the same helper.

Mitigation

Upgrade to Loofah >= 2.25.3.

Credit

Responsibly reported by GitHub user @ngpestelos.