Command injection is a type of security vulnerability that occurs when an attacker is able to execute arbitrary commands on a server or system through a vulnerable application. This often happens when user input is not properly validated or sanitized, allowing malicious commands to be executed alongside legitimate ones. Attackers can exploit command injection vulnerabilities to gain unauthorized access, manipulate data, or even take control of the entire system. It is crucial for developers to implement proper input validation and use secure coding practices to protect applications from such attacks, ensuring that user inputs are treated safely and do not compromise the system's integrity.