Certain characteristics of MANETs, such as their lack of physical security and their cooperative nature, make them more vulnerable to inside (Byzantine) attacks coming from compromised nodes that behave arbitrarily to disrupt the network. We examine the survivability of ad hoc wireless routing protocols in the presence of several Byzantine attacks, under two communication models: unicast and multicast. We show that traditional secure routing protocols that assume authenticated nodes can always be trusted, fail to defend against such attacks. To fill this gap, we introduce the first secure routing protocols designed to withstand a large class of Byzantine attacks from colluding adversaries and show their effectiveness in mitigating the considered attacks. Our solutions are software-based and do not require additional or specialized hardware.