Umbraco is a free and open source .NET content management system. In versions on the 13.x branch prior to 13.5.2 and versions on the 10.x branch prior to 10.8.7, during an explicit sign-out, the server session is not fully terminated. Versions 13.5.2 and 10.8.7 contain a patch for the issue.
https://github.com/umbraco/Umbraco-CMS/security/advisories/GHSA-wxw9-6pv9-c3xc