CVE-2026-31833: Umbraco CMS

Medium severity, CVSS 6.7. EPSS: 0.5% chance of exploitation in the next 30 days.

Umbraco is an ASP.NET CMS. From 16.2.0 to before 16.5.1 and 17.2.2, An authenticated backoffice user with access to Settings can inject malicious HTML into property type descriptions. Due to an overly permissive attributeNameCheck configuration (/.+/) in the UFM DOMPurify instance, event handler attributes such as onclick and onload, when used within Umbraco web components (umb-*, uui-*, ufm-*) were not filtered. This vulnerability is fixed in 16.5.1 and 17.2.2.

Affected products

• Umbraco Umbraco CMS: from 16.2.0, before 16.5.1 (fixed in 16.5.1); from 17.0.0, before 17.2.2 (fixed in 17.2.2)

Published 2026-03-10. Last modified 2026-06-17.