CVE-2026-73492 in Loofah
Summary
by MITRE • 08/12/2026
Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. From 2.25.0 until 2.25.2, Loofah::HTML5::Scrub.allowed_uri? does not reject javascript: or vbscript: URIs whose scheme is split by semicolon-less numeric character references such as :, 	, 
, or 
. CGI.unescapeHTML leaves these references encoded, so allowed_uri? reports the URL safe even though a browser decodes an encoded colon or strips encoded whitespace and executes the resulting URI scheme. This issue affects only callers that pass HTML-encoded strings directly to allowed_uri?; Loofah's default sanitize() path is not affected. This issue is fixed in version 2.25.2.
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Analysis
by VulDB Data Team • 08/13/2026
The vulnerability resides in the Loofah library's HTML5 scrubbing functionality, specifically within the allowed_uri? method that validates URI schemes. This flaw affects versions 2.25.0 through 2.25.2 and represents a classic input validation bypass mechanism where malicious URIs can evade security checks through carefully crafted encoding. The vulnerability exploits the difference between HTML entity decoding behavior in the validation routine versus actual browser interpretation, creating a discrepancy that allows dangerous javascript: and vbscript: URI schemes to pass through security filters.
The technical flaw stems from how Loofah processes HTML entities within URI validation. When URIs containing schemes like javascript: are encoded using numeric character references such as : for colon, 	 for tab, 
 for line feed, or 
 for carriage return, the allowed_uri? method fails to properly decode these sequences before validation. The CGI.unescapeHTML function processes these encodings but leaves them in their encoded form within the validation logic, causing the method to incorrectly approve malicious URIs that would be properly sanitized by browsers during actual rendering.
This vulnerability operates at the intersection of HTML parsing and URI validation, creating a security gap where attackers can inject malicious scripts through seemingly safe input. The issue specifically affects applications that directly pass HTML-encoded strings to the allowed_uri? method without first normalizing them, as opposed to using Loofah's standard sanitize() function which properly handles these encoding scenarios. This creates a vector for cross-site scripting attacks where malicious URIs can be embedded in HTML documents and executed when rendered by browsers.
The operational impact of this vulnerability extends beyond simple script execution to potentially enable full exploitation of web application security boundaries. When properly exploited, attackers can inject javascript: or vbscript: URIs that bypass security filters and execute arbitrary code within the context of the victim's browser session. This aligns with CWE-15 (External Control of System or Configuration Setting) and follows ATT&CK techniques related to command injection and script execution in web applications. The vulnerability demonstrates how seemingly innocuous HTML encoding can create significant security implications when validation logic doesn't properly account for different decoding behaviors between application layers and browser environments.
The fix implemented in version 2.25.2 addresses this by ensuring that HTML entity decoding occurs properly within the allowed_uri? method before URI scheme validation takes place. This change brings the validation behavior in line with actual browser interpretation, where encoded characters are decoded and properly validated against allowed schemes. Organizations using affected versions should immediately upgrade to 2.25.2 or later while also reviewing their application code patterns to ensure that direct calls to allowed_uri? are not being used without proper input sanitization. The vulnerability serves as a reminder of the importance of consistent encoding handling across different layers of web applications and the potential security implications of HTML entity processing discrepancies.