An executable file is a file that the operating system, interpreter, or web server can run as code. Common examples include binaries, scripts, and files with extensions that a server is configured to process as active content. In security, the file type is only part of the risk; location and permissions matter just as much.
Executable files matter because an upload that should be treated as data can become a path to remote code execution if it is stored in a web-accessible directory and the server is allowed to run it. Attackers look for weak upload handlers, missing allowlists, or misconfigured permissions so they can place a script or binary where the server will execute it. Defenders reduce this risk by blocking dangerous file types, storing uploads outside executable paths, disabling execution in upload folders, and monitoring for unexpected file creation. In short, an executable file becomes dangerous when a system trusts it in the wrong place.