Umbraco has Stored XSS in UFM Rendering Pipeline via Permissive DOMPurify Attribute Filtering
Date: 2026-03
Affected: nuget/Umbraco.Cms, nuget/Umbraco.Cms
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-31833
CWE: CWE-79
CVSS: 6.7 (CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L)

### Description 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. ### Impact As property type descriptions support Markdown/HTML via the UFM rendering pipeline, injected event handlers are rendered in the backoffice interface, resulting in a stored XSS affecting other backoffice users. ### Patches The issue is patched in 16.5.1 and 17.2.2. ### Workarounds There is no workaround other than upgrading. ### References https://docs.umbraco.com/umbraco-cms/reference/umbraco-flavored-markdown


Mitigations:

Architectural layers (MAESTRO):