Progress Sitefinity 10.1.6536 does not invalidate session cookies upon logouts. It instead tries to overwrite the cookie in the browser, but it remains valid on the server side. This means the cookie can be reused to maintain access to the account, even if the account credentials and permissions are changed.
Telerik.Web.UI.dll in Progress Telerik UI for ASP.NET AJAX before R2 2017 SP1 and Sitefinity before 10.0.6412.0 does not properly protect Telerik.Web.UI.DialogParametersEncryptionKey or the MachineKey, which makes it easier for remote attackers to defeat cryptographic protection mechanisms, leading to a MachineKey leak, arbitrary file uploads or downloads, XSS, or ASP.NET ViewState compromise.