Although distributed control in microgrids is well-known for reliability and scalability, the absence of a global monitoring entity makes it highly vulnerable to cyber attacks. Considering that the detection of cyber attacks becomes fairly easy for distributed observers, a well-planned set of balanced attacks, commonly termed as stealth attack, can always bypass these observers with the control objectives being successfully met. In this letter, a mitigation step is thus introduced to remove stealth attack on the frequency control input in AC microgrids. The attack mitigation is carried out using a novel event-driven attack-resilient controller for N cooperative grid-forming converters (GfCs), which guarantees resilient synchronization for up to N-1 attacked units. Finally, the resilience capabilities and robustness of the proposed controller are discussed and verified under various scenarios.