Bug 2572750 (CVE-2026-126203)

Summary: CVE-2026-126203 loofah: loofah: Cross-site scripting via unescaped attribute values in HTML5 sanitization
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akostadi, amasferr, dmayorov, ehelms, eshamard, jlledo, jpasqual, juwatts, jvasik, kaycoth, lblicha, mhulan, ogajduse, osousa, pantinor, rblanco, rchan, smallamp, tsedmik, zhunting
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in loofah. A remote attacker can exploit this vulnerability by submitting specially crafted HTML markup containing unescaped characters within allowed attributes. When processed by the HTML5 sanitizer, the output can be reparsed by a client web browser as active HTML instead of safe content. This flaw allows an attacker to execute arbitrary script code in the context of the user's session, leading to Cross-Site Scripting (XSS).
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Description OSIDB Bzimport 2026-11-12 18:52:00 UTC
Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. From 2.21.0 until 2.25.3, Loofah's HTML5 APIs Loofah.html5_fragment, Loofah.html5_document, and the scrub_html5_* methods can emit sanitized output that a browser reparses as different markup when used with Nokogiri before 1.19.5, because those Nokogiri versions do not escape angle brackets in attribute values. MathML can place markup inside an allowed attribute so the reparsed output becomes live HTML. The :strip scrubber is affected in every browser, while :prune and :escape are affected in WebKit browsers. Loofah's HTML4 APIs, Loofah::Helpers.sanitize, :whitewash, rails-html-sanitizer's default configuration, and JRuby are not affected. This issue is fixed in version 2.25.3.