CVE-2020-5809
MEDIUMCVE-2020-5809 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in UmbracoCms.Core. 1 public exploit reference exists, so weaponization risk is real. O3 Security confirms whether CVE-2020-5809 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Umbraco CMS vulnerable to stored XSS
Blast Radius
UmbracoCms.CoreReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.
Description
A stored XSS vulnerability exists in Umbraco CMS <= 8.9.1 or current. An authenticated user can inject arbitrary JavaScript code into iframes when editing content using the TinyMCE rich-text editor, as TinyMCE is configured to allow iframes by default in Umbraco CMS.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | UmbracoCms.Core | all versions | No fix |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for UmbracoCms.Core. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Remediation status
No patched version of UmbracoCms.Core has shipped for CVE-2020-5809 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.
Mitigate without a patch
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2020-5809 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2020-5809. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2020-5809 in your dependencies?
O3 detects CVE-2020-5809 across NuGet dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.