CVE-2026-31859: CraftCMS vulnerable to reflective XSS via incomplete return URL sanitization

March 11, 2026 (updated May 25, 2026)

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.

References

Code Behaviors & Features

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