Spectrum handoff is an integral part of a cognitive radio-based network (CRN). It ensures the operational integrity of opportunistic spectrum access, the avoidance of harmful interference with licensed or primary users (PUs), and the delay requirement during a handoff. However, due to the random nature of PU activity, interference between primary and secondary users (SUs) are difficult to prevent. Proactive spectrum handoff aims to control this harmful interference between PUs and SUs by predicting the future activity of PUs and initiating spectrum handoff before a PU reappears. Though a few security aspects of CRNs attracted attention of researchers, vulnerabilities in the distributed proactive spectrum handoff process remain unstudied. In this paper, we introduce a vulnerability in the proactive spectrum handoff process and demonstrate how a selfish attacker can exploit this vulnerability to achieve personal gain. We name this covert spectrum handoff. To the best of our knowledge, this is the first work to consider security aspects of spectrum handoffs and to introduce an attack in the proactive spectrum handoff process.