Key Exchange without Entity Authentication

Description

Performing a key exchange will preserve the integrity of the information sent between two entities, but this will not guarantee that the entities are who they claim they are. This may enable an attacker to impersonate an actor by modifying traffic between the two entities. Typically, this involves a victim client that contacts a malicious server that is impersonating a trusted server. If the client skips authentication or ignores an authentication failure, the malicious server may request authentication information from the user. The malicious server can then use this authentication information to log in to the trusted server using the victim's credentials, sniff traffic between the victim and trusted server, etc.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

Many systems have used Diffie-Hellman key exchange without authenticating the entities exchanging keys, allowing attackers to influence communications by redirecting or interfering with the communication path. Many people using SSL/TLS skip the authentication (often unknowingly).

See Also