A mobile ad hoc network (MANET) is formed by a group of mobile wireless nodes, each of which functions as a router and agrees to forward packets for others. Many routing protocols (e.g., AODV, DSDV, etc) have been proposed for MANETs. However, most assume that nodes are trustworthy and cooperative. Thus, they are vulnerable to a variety of attacks. We propose a secure routing protocol based on DSDV, namely S-DSDV, in which a well-behaved node can successfully detect a malicious routing update with any sequence number fraud (larger or smaller) and any distance fraud (shorter, same, or longer) provided no two nodes are in collusion. We compare security properties and efficiency of S-DSDV with superSEAD. Our analysis shows that S-DSDV-R, a variation of S-DSDV with a risk window similar to that of superSEAD, offers better security than superSEAD with less network overhead.