CVE-2026-31833: 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
Other sources
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.
— MITRE
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-31833?
CVE-2026-31833 has a medium severity rating due to the potential for stored XSS attacks.
How do I fix CVE-2026-31833?
To fix CVE-2026-31833, upgrade Umbraco to version 16.5.1 or later.
What versions of Umbraco are affected by CVE-2026-31833?
Umbraco versions from 16.2.0 up to but not including 16.5.1 and version 17.2.2 are affected by CVE-2026-31833.
Who can exploit CVE-2026-31833?
An authenticated backoffice user with access to Settings can exploit CVE-2026-31833.
What type of vulnerability is CVE-2026-31833?
CVE-2026-31833 is a stored cross-site scripting (XSS) vulnerability.