We study data pollution attacks in wireless interflow network coding systems. Although several defenses for these attacks are known for intra-flow network coding systems, none of them are applicable to inter-flow coding systems. We classify pollution attacks in inter-flow coding systems based on the type of packets injected by the attacker and the attacks' impact on flows in the network. We identify a new attack, cross-flow pollution, which exploits the coding dependencies among flows to propagate pollution across flows. Our analysis shows that the effects of pollution attacks depend not only on the network topology, but also on the location and strategy of the attacker nodes. We propose a reactive attestation-based defense mechanism that uses efficient bit-level traceback and a novel cross-examination technique to unequivocally identify attacker nodes.