Which attack type takes advantage of insufficient validation of user input to modify application behavior?

The attack type that takes advantage of insufficient validation of user input to modify application behavior is command injection. This type of attack occurs when an application allows users to submit input that is not properly sanitized or validated, enabling an attacker to run arbitrary commands on the server.

In a command injection attack, the attacker can input malicious commands that the system executes, potentially granting them unauthorized access or control over the server. This occurs due to the application’s inability to adequately scrutinize incoming data, leading to unintended execution of commands that should not have been possible. An effective defense against such attacks involves implementing stringent input validation and ensuring that any input is thoroughly checked before processing it.

Other potential attack types listed, like LDAP injection or directory traversal, involve different mechanisms and targets within applications but are also based on exploiting validation weaknesses. However, command injection specifically relates to executing commands, making it the most fitting answer in this scenario. Zero-day attacks, while they do represent vulnerabilities exploited prior to a fix being available, do not focus on user input or command execution in the context described.