Eval injection is a security vulnerability that occurs when untrusted user input is passed to functions like eval() in programming languages such as JavaScript, PHP, or Python. These functions interpret and execute string inputs as code. If input is not properly sanitized, attackers can inject malicious code, leading to unauthorized actions, data theft, or system compromise. Eval injection is especially dangerous because it can allow attackers to execute arbitrary commands, access sensitive data, or escalate privileges. Secure coding practices, such as avoiding eval() and validating all user input, are essential to prevent this vulnerability.