Existing authentication methodologies involve three basic “factors”:
- Something the user knows (e.g., password, PIN);
- Something the user has (e.g., ATM card, smart card); and
- Something the user is (e.g., biometric characteristic, such as a fingerprint).
A known flaw in some T-FA systems is the server storage of a hash representation of the credentials contained on the smart card or token. With this in hand, the attacker can replay that data to the authentication system; in this case, that of the proxy server, without needing the smart card or token.
Two-factor authentication is still susceptible to man-in-the-middle attacks.