  This article investigates the physical-layer security of cognitive radio (CR)
networks, which are vulnerable to various newly arising attacks targeting on
the weaknesses of CR communications and networking. We first review a range of
physical-layer attacks in CR networks, including the primary user emulation,
sensing falsification, intelligence compromise, jamming and eavesdropping
attacks. Then we focus on the physical-layer security of CR networks against
eavesdropping and examine the secrecy performance of cognitive communications
in terms of secrecy outage probability. We further consider the use of relays
for improving the CR security against eavesdropping and propose an
opportunistic relaying scheme, where a relay node that makes CR communications
most resistant to eavesdropping is chosen to participate in assisting the
transmission from a cognitive source to its destination. It is illustrated that
the physical-layer secrecy of CR communications relying on the opportunistic
relaying can be significantly improved by increasing the number of relays,
showing the security benefit of exploiting relay nodes. Finally, we present
some open challenges in the field of relays assisted physical-layer security
for CR networks.
