GHSA-2j22-pr5w-6gq8: 
Ruby vulnerability analysis and mitigation

Summary

Loofah::HTML5::Scrub.allowed_uri? does not correctly reject javascript: URIs when the scheme is split by HTML entity-encoded control characters such as 
 (carriage return), 
 (line feed), or 	 (tab).

Details

The allowed_uri? method strips literal control characters before decoding HTML entities. Payloads like java
script:alert(1) survive the control character strip, then 
 is decoded to a carriage return, producing java\rscript:alert(1). Note that the Loofah sanitizer's default sanitize() path is not affected because Nokogiri decodes HTML entities during parsing before Loofah evaluates the URI protocol. This issue only affects direct callers of the allowed_uri? string-level helper when passing HTML-encoded strings.

Impact

Applications that call Loofah::HTML5::Scrub.allowed_uri? to validate user-controlled URLs and then render approved URLs into href or other browser-interpreted URI attributes may be vulnerable to cross-site scripting (XSS). This only affects Loofah 2.25.0.

Mitigation

Upgrade to Loofah >= 2.25.1.

Credit

Responsibly reported by HackOne user @smlee.