A command-line injection vulnerability exists in the core .NET class System.Windows.Forms.Help::ShowHelp function allowing an attacker without “UnmanagedCode” permission to nevertheless directly control arguments passed to a “ShellExecute” invocation of the users’ default browser. This vulnerability allows an attacker who is able to run arbitrary .NET code within a .NET PartialTrust sandbox including the “WebPermission” permission for any URL to inject arbitrary parameters after the first parameter into the command line of the users’ default browser.