CVE-2026-31833: Umbraco.Cms Cross-Site Scripting Vulnerability

MODERATE
Advisory Identifier: GHSA-vrqc-59mw-qqg7
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CVSS 6.7
CWE-79
Published
Advisory Summary

CVE-2026-31833 is a MODERATE (CVSS 6.7) cross-site scripting vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Umbraco.Cms libraries in the NuGet ecosystem. The issue involves umbraco has stored xss in ufm rendering pipeline via permissive dompurify attribute filtering. Remediation is available by upgrading affected packages to version 16.5.1, 17.2.2.

CVE-2026-31833 at a Glance

CVE Identifier CVE-2026-31833
GHSA Identifier GHSA-vrqc-59mw-qqg7
Severity MODERATE
CVSS v3.1 Base Score 6.7
CWE Classification CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Affected Product Umbraco.Cms
Package Ecosystem NuGet
Published Date
Last Updated

Security Impact

CVSS: 6.7

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required High (PR:H)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality High (C:H)
Integrity High (I:H)
Availability Low (A:L)

How to Fix CVE-2026-31833

To remediate this vulnerability, upgrade all affected Umbraco.Cms dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

Umbraco.Cms fixed: 16.5.1 
Upgrade Umbraco.Cms to 16.5.1
Umbraco.Cms fixed: 17.2.2 
Upgrade Umbraco.Cms to 17.2.2
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for Umbraco.Cms

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-31833, inspect your project dependencies for Umbraco.Cms:

inspect dependencies for Umbraco.Cms

Analyze the terminal output to locate any version of Umbraco.Cms in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 16.5.1), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'), where 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. In Umbraco.Cms, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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

CVE-2026-31833 Affected Umbraco.Cms Versions

2 component(s)
Fixed Versions:
16.5.117.2.2
Product / Package Ecosystem Affected Versions Fixed Version
Umbraco.Cms
nuget/Umbraco.Cms
NuGet
≥ 16.2.0
< 16.5.1
16.5.1
Umbraco.Cms
nuget/Umbraco.Cms
NuGet
≥ 17.0.0
< 17.2.2
17.2.2

References

GitHub Advisory / Wiki ADVISORY
https://github.com/umbraco/Umbraco-CMS/security/advisories/GHSA-vrqc-59mw-qqg7
NVD — CVE-2026-31833 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-31833
Umbraco.Cms Source Repository REPOSITORY
https://github.com/umbraco/Umbraco-CMS

Frequently Asked Questions

What is CVE-2026-31833?
CVE-2026-31833 is a security vulnerability in Umbraco.Cms involving Umbraco has Stored XSS in UFM Rendering Pipeline via Permissive DOMPurify Attribute Filtering under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').
Which Umbraco.Cms versions are affected?
Affected versions include releases prior to 16.5.1, 17.2.2 across affected packages: Umbraco.Cms.
What version fixes CVE-2026-31833?
Upgrading to 16.5.1, 17.2.2 (or the latest patched release for your respective branch) resolves the vulnerability.
Is Umbraco.Cms affected?
Yes, Umbraco.Cms is affected in releases prior to 16.5.1, 17.2.2. Upgrade your dependency to 16.5.1, 17.2.2 or later.
What is the CVSS score of CVE-2026-31833?
CVE-2026-31833 has a CVSS v3.1 base score of 6.7 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L.
What CWE is associated with CVE-2026-31833?
CVE-2026-31833 is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (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.).
How can I check NuGet dependencies for CVE-2026-31833?
Run the dependency tree command: 'inspect dependencies for Umbraco.Cms'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vrqc-59mw-qqg7",
  "modified" : "2026-03-11T14:54:00Z",
  "published" : "2026-03-11T14:54:00Z",
  "aliases" : [ "CVE-2026-31833" ],
  "summary" : "Umbraco has Stored XSS in UFM Rendering Pipeline via Permissive DOMPurify Attribute Filtering",
  "details" : "### Description\nAn 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.\n\n### Impact\nAs 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.\n\n### Patches\nThe issue is patched in 16.5.1 and 17.2.2.\n\n### Workarounds\nThere is no workaround other than upgrading.\n\n### References\nhttps://docs.umbraco.com/umbraco-cms/reference/umbraco-flavored-markdown",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "Umbraco.Cms"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "16.2.0"
      }, {
        "fixed" : "16.5.1"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "Umbraco.Cms"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "17.0.0"
      }, {
        "fixed" : "17.2.2"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "< 17.2.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/umbraco/Umbraco-CMS/security/advisories/GHSA-vrqc-59mw-qqg7"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-31833"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/umbraco/Umbraco-CMS"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-11T14:54:00Z",
    "nvd_published_at" : "2026-03-10T22:16:21Z"
  }
}