CVE-2026-31833
Received Received - Intake
HTML Injection in Umbraco Backoffice via Improper DOMPurify Filtering

Publication date: 2026-03-10

Last updated on: 2026-03-18

Assigner: GitHub, Inc.

Description
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.
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Meta Information
Published
2026-03-10
Last Modified
2026-03-18
Generated
2026-06-16
AI Q&A
2026-03-11
EPSS Evaluated
2026-06-15
NVD
CVE-2026-31833
EUVD
EUVD-2026-10936
Affected Vendors & Products
Showing 2 associated CPEs
Vendor Product Version / Range
umbraco umbraco_cms From 17.0.0 (inc) to 17.2.2 (exc)
umbraco umbraco_cms From 16.2.0 (inc) to 16.5.1 (exc)
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-79 The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.
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Executive Summary

This vulnerability exists in Umbraco, an ASP.NET CMS, in versions 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. The root cause is an overly permissive attributeNameCheck configuration (/.+/) in the UFM DOMPurify instance, which fails to filter event handler attributes such as onclick and onload within Umbraco web components (umb-*, uui-*, ufm-*). This allows malicious scripts to be injected and potentially executed.

Impact Analysis

The vulnerability can lead to the injection and execution of malicious HTML and scripts by an authenticated backoffice user. This can result in unauthorized actions being performed, data compromise, or other malicious activities within the Umbraco CMS environment. The CVSS score indicates a high impact on confidentiality and integrity, with some impact on availability.

Compliance Impact

I don't know

Detection Guidance

I don't know

Mitigation Strategies

To mitigate this vulnerability, you should upgrade Umbraco to version 16.5.1 or 17.2.2 or later, where the issue has been fixed.

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