Loofah has improper detection of disallowed URIs via `allowed_uri?`
Date: 2026-03
Affected: rubygems/loofah
Attack vector: xss
Category: vulnerability-disclosure
Severity: Low
CWE: CWE-116
CWE-79

## 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 `&#13;` (carriage return), `&#10;` (line feed), or `&#9;` (tab). ## Details The `allowed_uri?` method strips literal control characters before decoding HTML entities. Payloads like `java&#13;script:alert(1)` survive the control character strip, then `&#13;` 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.


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