Keeping in mind what we just discussed, you may now be surprised to find that this process is vulnerable to attack! However, the issue is not the core concept, but the practical implementation of the standard. As with most technical standards, sacrifices were made to the security of the solution in order to make it user-friendly. In specific, the sacrifice that was made to make the solution usable was making certain stages in the handshake replayable in the event of a missed message.
While this is not a huge issue for most of the process, Stage 3 is replayable and can have a dramatic effect on the security of the overall solution. By placing themselves in a Man-in-the-Middle (MITM) position during the authentication process, an attacker can block the correctly negotiated PTK and install their own in certain circumstances. The Key Replay Counter and associated nonce values are reset when a key is negotiated. So by blocking certain packets, an MITM attacker...