changedetection.io is an open source web page change detection, website watcher, restock monitor and notification service. There is a Server Side Template Injection (SSTI) in Jinja2 that allows Remote Command Execution on the server host. Attackers can run any system command without any restriction and they could use a reverse shell. The impact is critical as the attacker can completely takeover the server machine. This can be reduced if changedetection is behind a login page, but this isn't required by the application (not by default and not enforced).
The product uses a template engine to insert or process externally-influenced input, but it does not neutralize or incorrectly neutralizes special elements or syntax that can be interpreted as template expressions or other code directives when processed by the engine.