Summary
Loofah allowed_uri? does not detect javascript: URIs split by numeric character references without semicolons
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 ; (	, , ) 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 : (colon) or 	 (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 : becomes the : scheme separator, so javascript:alert(1) resolves to javascript:alert(1). Encoded whitespace such as 	 (tab) is decoded and then stripped from the URL, rejoining the surrounding text, so java	script: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.
Mitigation
Upgrade to Loofah >= 2.25.2.
Credit
Responsibly reported by GitHub user @MoonFuji.
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.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
Affected versions
Security releases
Kodem intelligence
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Frequently Asked Questions
- What is GHSA-5QHF-9PHG-95M2? GHSA-5QHF-9PHG-95M2 is a low-severity cross-site scripting (XSS) vulnerability in loofah (rubygems), affecting versions >= 2.25.0, < 2.25.2. It is fixed in 2.25.2. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- Which versions of loofah are affected by GHSA-5QHF-9PHG-95M2? loofah (rubygems) versions >= 2.25.0, < 2.25.2 is affected.
- Is there a fix for GHSA-5QHF-9PHG-95M2? Yes. GHSA-5QHF-9PHG-95M2 is fixed in 2.25.2. Upgrade to this version or later.
- Is GHSA-5QHF-9PHG-95M2 exploitable, and should I be worried? Whether GHSA-5QHF-9PHG-95M2 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-5QHF-9PHG-95M2 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-5QHF-9PHG-95M2? Upgrade
loofahto 2.25.2 or later.