Stimulsoft Server: Unauthenticated SYSTEM RCE via Signup and Report-Script Compilation

Stimulsoft Server 2026.3.1: the default-open /1/signup endpoint (no email activation) hands an anonymous attacker a Supervisor account, who uploads a .mrt report with an embedded C# script; PUT /1/reporttemplates/{id}/run triggers full-trust CSharpCodeProvider compilation and runs the attacker-controlled constructor as NT AUTHORITY\SYSTEM.