GHSA-5QHF-9PHG-95M2
Vulnerability from github – Published: 2026-07-21 22:03 – Updated: 2026-07-21 22:03Summary
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.
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.
{
"affected": [
{
"package": {
"ecosystem": "RubyGems",
"name": "loofah"
},
"ranges": [
{
"events": [
{
"introduced": "2.25.0"
},
{
"fixed": "2.25.2"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [],
"database_specific": {
"cwe_ids": [
"CWE-184",
"CWE-79"
],
"github_reviewed": true,
"github_reviewed_at": "2026-07-21T22:03:11Z",
"nvd_published_at": null,
"severity": "LOW"
},
"details": "## Summary\n\n`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.\n\nThis is a bypass of the fix for [GHSA-46fp-8f5p-pf2m](https://github.com/flavorjones/loofah/security/advisories/GHSA-46fp-8f5p-pf2m), which handled numeric character references with a trailing `;` (`\u0026#9;`, `\u0026#10;`, `\u0026#13;`) but did not cover the forms without semicolons.\n\n## Details\n\n`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 `\u0026#58` (colon) or `\u0026#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`.\n\nA browser, however, decodes numeric character references even without a trailing semicolon. An encoded colon such as `\u0026#58` becomes the `:` scheme separator, so `javascript\u0026#58alert(1)` resolves to `javascript:alert(1)`. Encoded whitespace such as `\u0026#9` (tab) is decoded and then stripped from the URL, rejoining the surrounding text, so `java\u0026#9script:alert(1)` also resolves to `javascript:alert(1)`. In both cases the URL executes while `allowed_uri?` approved it as safe.\n\nNote that Loofah\u0027s 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.\n\n## Impact\n\nCallers 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\u0027s markdown link validation.\n\n## Mitigation\n\nUpgrade to Loofah \u003e= 2.25.2.\n\n## Credit\n\nResponsibly reported by GitHub user @MoonFuji.",
"id": "GHSA-5qhf-9phg-95m2",
"modified": "2026-07-21T22:03:11Z",
"published": "2026-07-21T22:03:11Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/flavorjones/loofah/security/advisories/GHSA-5qhf-9phg-95m2"
},
{
"type": "WEB",
"url": "https://github.com/flavorjones/loofah/commit/f1be9d893b5a8dd79240441a912d8897e74c38c0"
},
{
"type": "PACKAGE",
"url": "https://github.com/flavorjones/loofah"
},
{
"type": "WEB",
"url": "https://github.com/flavorjones/loofah/releases/tag/v2.25.2"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N",
"type": "CVSS_V4"
}
],
"summary": "Loofah `allowed_uri?` does not detect `javascript:` URIs split by numeric character references without semicolons"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
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