What is a Man-in-the-Middle Attack (MITM)?

A Man-in-the-Middle Attack (MITM) is characterized by a third party intercepting communications between two entities without their knowledge. This interception allows the attacker to potentially eavesdrop, alter messages, or inject malicious content into the communication stream. The essence of a MITM attack lies in the attacker's ability to transparently sit between the communicating parties, making it appear to both that they are directly interacting with each other.

The other options outline concepts that do not adequately define a MITM attack. For instance, enhancing data integrity and protecting data confidentiality pertain to security measures rather than describing an attack method. Similarly, validating user identities focuses on authentication processes rather than interception of communication, which is the hallmark of a MITM scenario. Thus, the correct understanding of a MITM attack is rooted in its interceptive nature, making the choice that describes a device or method capable of intercepting communications the right answer.