In this paper, we deal with backoff cheating technique in IEEE802.11 based MANETs and propose a novel scheme, dubbed HsF-MAC (Hash Function based MAC protocol), to cope with it. In contrast to the existing solutions, HsF-MAC allows MANET nodes to re-calculate the backoff value used by their 1-hop neighbors and immediately detect the misbehaving ones. Moreover, the colluding behavior of two cheating nodes is also considered along with effective countermeasures. A reconciliation based reaction mechanism is finally designed. The simulation results, under different topologies and network conditions, have validated the effectiveness of HsF-MAC.