Current Denial-of-Service (DoS) attacks are directed towards a specific victim. The research community has devised several countermeasures that protect victim hosts against undesired traffic. We present Coremelt, a new attack mechanism, where attackers only send traffic between each other, and not towards a victim host. As a result, none of the attack traffic is unwanted. The Coremelt attack is powerful because among N attackers there are O(N2 ) connections, which can cause significant congestion in the network core. We demonstrate the attack based on simulations within a real Internet topology using realistic attacker distributions and show that attackers can induce a significant amount of congestion.