To run your own code on a web server through a file upload feature, what special file type with a dangerous ending (like '.php') would you upload?

The special file type you would upload is a webshell, which is a malicious script file. When the question mentions a "dangerous ending like '.php'," it specifically refers to a PHP script file containing code designed for remote server control. A webshell is a script written in a server-side scripting language (such as PHP, ASP, or JSP) that an attacker uploads to a web server through a vulnerable file upload feature. The "dangerous ending" (for example, '.php') is a file extension that instructs the web server to execute the file using its corresponding interpreter (like the PHP interpreter) rather than simply serving it as a static file or inert data. If a web server's file upload feature lacks proper security validations, an attacker can upload this webshell. When this uploaded webshell is subsequently accessed via a web browser, the web server executes its code. This execution allows the attacker to send commands to the server through their web browser, effectively achieving Remote Code Execution (RCE) and gaining unauthorized control over the server's operating system, files, and resources.