CVE-2026-31859: Craft CMS

Medium severity, CVSS 6.1. EPSS: 0.3% chance of exploitation in the next 30 days.

Craft is a content management system (CMS). The fix for CVE-2025-35939 in craftcms/cms introduced a strip_tags() call in src/web/User.php to sanitize return URLs before they are stored in the session. However, strip_tags() only removes HTML tags (angle brackets) -- it does not inspect or filter URL schemes. Payloads like javascript:alert(document.cookie) contain no HTML tags and pass through strip_tags() completely unmodified, enabling reflected XSS when the return URL is rendered in an href attribute. This vulnerability is fixed in 5.9.7 and 4.17.3.

Affected products

• Craft CMS Craft CMS: from 4.15.3, before 4.17.3 (fixed in 4.17.3); from 5.7.5, before 5.9.7 (fixed in 5.9.7)

Published 2026-03-11. Last modified 2026-06-17.