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Umbraco CmsApplication · Umbraco

CVE-2026-31833

MEDIUM · 6.7 CVSS v3.1 Published 2026-03-10
Fix available
A fix is available. Upgrade to 16.5.1 / 17.2.2 or later.
See remediation →
73/100
Remediation priority · Elevated
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Stored XSS vulnerability in Umbraco CMS allows authenticated backoffice users with Settings access to inject malicious HTML/event handler attributes (onclick, onload) into property type descriptions due to an overly permissive attributeNameCheck regex (/.+/) in the UFM DOMPurify instance that fails to filter these handlers within umb-*, uui-*, and ufm-* web component namespaces.

MitigationUpgrade Umbraco to version 16.5.1 or 17.2.2 or later to obtain the patched DOMPurify configuration that properly filters event handler attributes.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Umbraco CmsApplication
Affected:>= 16.2.0, < 16.5.1>= 17.0.0, < 17.2.2

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
High
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify the installed Umbraco CMS version
    Check the Umbraco assembly version by examining the UmbracoCms.dll file properties in the bin folder, or look for the version in the project's packages.config / .csproj file, or query the /umbraco/api/ServerApi/getversion endpoint if the backoffice is accessible
    Affected if The installed version falls within 16.2.0 to 16.5.0 (inclusive) or 17.0.0 to 17.2.1 (inclusive)
  2. Verify backoffice access is enabled
    Confirm that the Umbraco backoffice is publicly accessible and that user authentication is configured. Check if /umbraco/ is reachable and login page exists
    Affected if The backoffice is accessible without network restrictions, creating the precondition for an authenticated attacker
  3. Check for Settings section access
    Identify if any backoffice user accounts have Settings access permissions. This can be verified by logging into the backoffice and navigating to the Users section to examine user group permissions
    Affected if Any backoffice user or group possesses Settings access permissions, allowing the property type description injection vector
  4. Inspect UFM DOMPurify configuration
    Locate the DOMPurify configuration in the Umbraco Forms Maker (UFM) code or configuration files. Look for the attributeNameCheck regex pattern that should be set to /(.+)/ or similar permissive pattern that fails to filter event handlers in umb-*, uui-*, and ufm-* namespaces
    Affected if The attributeNameCheck regex in the UFM DOMPurify configuration is overly permissive (matching /.+/ ) and does not filter onclick, onload and similar event handler attributes within web component namespaces

You are affected if your Umbraco CMS version is between 16.2.0 and 16.5.0, or between 17.0.0 and 17.2.1, AND the backoffice is accessible to authenticated users with Settings permissions who could inject malicious event handlers into property type descriptions.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 16.5.1 / 17.2.2 or later
Fixed in 16.5.117.2.2
Interim mitigation

Upgrade Umbraco to version 16.5.1 or 17.2.2 or later to obtain the patched DOMPurify configuration that properly filters event handler attributes.

Recommended fix High confidence

16.5.1 for 16.x branch; 17.2.2 for 17.x branch

  1. 1. Identify the current Umbraco version by checking the assembly version or the Umbraco version file in the installation directory
  2. 2. For installations on the 16.x branch: upgrade to version 16.5.1 or later
  3. 3. For installations on the 17.x branch: upgrade to version 17.2.2 or later
  4. 4. Verify the upgrade by checking the Umbraco backoffice About section shows the patched version
  5. 5. Test that the backoffice Settings section still functions correctly after upgrade
Caveat Minor/patch upgrades typically have low risk but review Umbraco release notes for any configuration or API changes between your version and the target

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Umbraco Cms Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing3.0 h
  • Review / QA2.0 h
10.0 hours of engineering $1,750
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,800.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-31833 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data