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 isnt 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.
Many web applications use template engines that allow developers to insert externally-influenced values into free text or messages in order to generate a full web page, document, message, etc. Such engines include Twig, Jinja2, Pug, Java Server Pages, FreeMarker, Velocity, ColdFusion, Smarty, and many others - including PHP itself. Some CMS (Content Management Systems) also use templates. Template engines often have their own custom command or expression language. If an attacker can influence input into a template before it is processed, then the attacker can invoke arbitrary expressions, i.e. perform injection attacks. For example, in some template languages, an attacker could inject the expression “{{7*7}}” and determine if the output returns “49” instead. The syntax varies depending on the language. In some cases, XSS-style attacks can work, which can obscure the root cause if the developer does not closely investigate the root cause of the error. Template engines can be used on the server or client, so both “sides” could be affected by injection. The mechanisms of attack or the affected technologies might be different, but the mistake is fundamentally the same.