Diagram 1: Man in the Middle attack

Man in the Middle, or MITM for short, a hacking attack that is useful to achieve various goals. Attack commences usually from infecting victim's device or network, and attacker engage in initial Internet transaction called handshake, which aims to establish identities between user/client and server. Simplified, attacker act as a proxy between client and server without knowledge of both. User believes attacker is actually a server, and server consider attacker as an authenticated user.

At this point, attacker is able to capture all data even if connection is encrypted. This includes data originating from user such as credentials, and data from server. Also attacker is having full control over data transferred and is able to alter it in any way, freely changing response from web service for own convenience - for example after successfully capturing credentials display the message to the user that service is unavailable and user should try again in few minutes so attacker may disengage unnoticed and restore normal connection.