Just as errors in sequential programs can lead to security exploits, errors in concurrent programs can lead to concurrency attacks. In this paper, we present an in-depth study of concurrency attacks and how they may affect existing defenses. Our study yields several interesting findings. For instance, we find that concurrency attacks can corrupt non-pointer data, such as user identifiers, which existing memory-safety defenses cannot handle. Inspired by our findings, we propose new defense directions and fixes to existing defenses.