This paper presents a robust data authentication scheme for protecting data integrity and availability in unattended wireless sensor networks. Such networks are vulnerable to several types of attacks. In particular, attackers can compromise a subset of nodes and use these nodes to transmit modified data or to prevent genuine data from being verified. The presented scheme combines security against data modification and denial of service attacks with traffic and storage efficiency. This is achieved by involving all sensor nodes in the network in the authentication process, implementing cooperative authentication with multiple authenticators, and using dual storage. Detailed analysis and extensive simulation tests show that our scheme achieves better performance compared to related schemes published in the literature in terms of traffic, storage, security against DoS attacks, and security against data replacement attacks.