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.

Workarounds

There is no workaround other than upgrading.

References

https://docs.umbraco.com/umbraco-cms/reference/umbraco-flavored-markdown

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.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-31833 has a CVSS score of 6.7 (Medium). The vector is network-reachable, high privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (16.5.1, 17.2.2); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

The issue is patched in 16.5.1 and 17.2.2.

1. What is CVE-2026-31833? CVE-2026-31833 is a medium-severity cross-site scripting (XSS) vulnerability in Umbraco.Cms (nuget), affecting versions >= 16.2.0, < 16.5.1. It is fixed in 16.5.1, 17.2.2. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-31833? CVE-2026-31833 has a CVSS score of 6.7 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of Umbraco.Cms are affected by CVE-2026-31833? Umbraco.Cms (nuget) versions >= 16.2.0, < 16.5.1 is affected.
4. Is there a fix for CVE-2026-31833? Yes. CVE-2026-31833 is fixed in 16.5.1, 17.2.2. Upgrade to this version or later.
5. Is CVE-2026-31833 exploitable, and should I be worried? Whether CVE-2026-31833 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-31833 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-31833?
   • Upgrade Umbraco.Cms to 16.5.1 or later
   • Upgrade Umbraco.Cms to 17.2.2 or later

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