Some applications use operating system commands to execute certain functionalities by using bad coding practices, say for instance, usage of functions such as system(),shell_exec(), etc. This allows a user to inject arbitrary commands that will execute on the remote host with the privilege of web server user. An attacker can trick the interpreter to execute his desired commands on the system.
Read more about Command Injection
https://www.owasp.org/index.php/Command_Injection
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