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.
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
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.
nuget
Umbraco.Cms (>= 16.2.0, < 16.5.1)Umbraco.Cms (>= 17.0.0, < 17.2.1)Umbraco.Cms → 16.5.1 (nuget)Umbraco.Cms → 17.2.2 (nuget)Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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 instead of chasing every advisory.
Kodem's runtime-powered SCA identifies whether CVE-2026-31833 is reachable in your applications. Explore open-source security for your team.
See if CVE-2026-31833 is reachable in your applications. Get a demo
Upgrade the following packages to resolve this vulnerability:
Umbraco.Cms to 16.5.1 or laterUmbraco.Cms to 17.2.2 or laterKodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
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.
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.
Umbraco.Cms (nuget) versions >= 16.2.0, < 16.5.1 is affected.
Yes. CVE-2026-31833 is fixed in 16.5.1, 17.2.2. Upgrade to this version or later.
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
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.
Umbraco.Cms to 16.5.1 or laterUmbraco.Cms to 17.2.2 or later