GHSA-5qhf-9phg-95m2 Loofah `allowed_uri?` does not detect `javascript:` URIs split by numeric character references without semicolons

• RubyGems loofah: >= 2.25.0, < 2.25.2 → fixed in 2.25.2

Summary `Loofah::HTML5::Scrub.allowed_uri?` does not correctly reject `javascript:` or `vbscript:` URIs when the scheme is split by a numeric character reference that has no trailing semicolon. A browser decodes such references and resolves the URL to an executable `javascript:` scheme, while `allowed_uri?` reports it safe. This is a bypass of the fix for GHSA-46fp-8f5p-pf2m, which handled numeric character references with a trailing `;` (`&#9;`, `&#10;`, `&#13;`) but did not cover the forms without semicolons. Details `allowed_uri?` decodes HTML entities with `CGI.unescapeHTML`, which decodes numeric character references only when they carry a trailing `;`. A reference without a semicolon such as `&#58` (colon) or `&#9` (tab) is left literal, so the scheme-detection check finds no scheme, and the method falls through to its scheme-less path and returns `true`. A browser, however, decodes numeric character references even without a trailing semicolon. An encoded colon such as `&#58` becomes the `:` scheme separator, so `javascript&#58alert(1)` resolves to `javascript:alert(1)`. Encoded whitespace such as `&#9` (tab) is decoded and then stripped from the URL, rejoining the surrounding text, so `java&#9script:alert(1)` also resolves to `javascript:alert(1)`. In both cases the URL executes while `allowed_uri?` approved it as safe. Note that Loofah's default `sanitize()` path is not affected, because Nokogiri decodes or entity-escapes HTML entities during parsing before Loofah evaluates the URI protocol. This issue only affects callers of the public `allowed_uri?` string-level helper that pass it HTML-encoded strings. Impact Callers that validate a user-controlled URL with `Loofah::HTML5::Scrub.allowed_uri?` and then render the approved value into an `href` or other browser-interpreted URI attribute may be vulnerable to cross-site scripting (XSS). This includes applications that call `allowed_uri?` directly, as well as higher-level features built on top of it, such as Action Text 8.2's markdown link validation. Mitigation Upgrade to Loofah >= 2.25.2. Credit Responsibly reported by GitHub user @MoonFuji.