A man-in-the-middle attack occurs when an attacker reroutes communication between two users through the attacker’s computer without the knowledge of the two communicating users. The attacker can monitor and read the traffic before sending it on to the intended recipient. Each user in the communication unknowingly sends traffic to and receives traffic from the attacker, all while thinking they are communicating only with the intended user. This can happen if an attacker can modify Active Directory Domain Services to add his or her server as a trusted server or modify Domain Name System (DNS) to get clients to connect through the attacker on their way to the server. A man-in-the-middle attack can also occur with media traffic between two clients, except that in Office Communications Server 2007 R2 point-to-point media streams are encrypted with SRTP, using cryptographic keys that are negotiated between the peers using Session Initiation Protocol (SIP) over TLS. Servers such as Group Chat and Communicator Web Access make use of HTTPS to secure Web traffic.