Umbraco has Stored XSS in UFM Rendering Pipeline via Permissive DOMPurify Attribute Filtering

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