CVE-2026-31833
Umbraco has Stored XSS in UFM Rendering Pipeline via Permissive DOMPurify Attribute Filtering
Impact
### 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
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| nuget | Umbraco.Cms | >= 16.2.0, < 16.5.1 | 16.5.1 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Umbraco | Umbraco Cms | from 16.2.0 before 16.5.1 from 17.0.0 before 17.2.2 |