Security is a fundamental prerequisite for network survivability and reliability in mobile ad hoc networks (MANET). In the presence of selfish nodes that disobey the standard, the performance of well-behaved nodes will significantly degrade. In this paper, we focus on identifying potential threats in medium access control (MAC) layer introduced by selfish nodes, especially "smart" attack strategies that can defeat the existing detection and reaction systems against MAC layer selfish misbehavior. Furthermore, we propose predictable random backoff (PRB) algorithm that is capable of mitigating the impact of these vulnerabilities. PRB is based on minor modification of IEEE 802.11 binary exponential backoff (BEB) and forces each node to generate "predictable" random backoff intervals. Via computer simulations, we show that PRB is fairly efficient in ensuring reasonable throughput for well-behaved flows in the presence of selfish flows