Multipath TCP (MPTCP) is an IETF standardized suite of TCP extensions that allow two endpoints to simultaneously use multiple paths between them. In this paper, we report vulnerabilities in MPTCP that arise because of cross-path interactions between MPTCP subflows. First, an attacker eavesdropping one MPTCP subflow can infer throughput of other subflows. Second, an attacker can inject forged MPTCP packets to change priorities of any MPTCP subflow. We present two attacks to exploit these vulnerabilities. In the connection hijack attack, an attacker takes full control of the MPTCP connection by suspending the subflows he has no access to. In the traffic diversion attack, an attacker diverts traffic from one path to other paths. Proposed vulnerabilities fixes, changes to MPTCP specification, provide the guarantees that MPTCP is at least as secure as TCP and the original MPTCP. We validate attacks and prevention mechanism, using MPTCP Linux implementation (v0.91), on a real-network testbed.