Reflection Attack in an Authentication Protocol

In a typical mutual authentication setup, both the client and server share a secret key. To prove identity without sending the key directly, they exchange random challenges that must be encrypted with that shared secret. The vulnerability arises when the same key is used across multiple sessions and the protocol design allows an attacker to use the server's response from one connection to answer a challenge in another. Here’s how the attack works in practice: An attacker initiates a connection to the server, posing as a legitimate user, and receives a challenge. Instead of solving it, the attacker opens a second connection to the server. In this new session, the attacker sends the server's original challenge as its own. The server helpfully encrypts it and sends the response back, which the attacker then uses to correctly answer the first connection's challenge. This bypasses authentication because the server essentially authenticates itself, granting the attacker access.