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