When　the　secondary　receiver　has　no　channel　state　information,　a　novel　physical　layer　security　scheme　based　on　random　beamforming　is　proposed　for　a　cognitive　radio　network　where　the　secondary　transmission　from　a　multiple-antenna　secondary　transmitter　to　a　single-antenna　secondary　receiver　is　eavesdropped　by　a　multiple-antenna　passive　eavesdropper.　The　design　criteria　of　the　random　beamforming　are　to　make　the　received　symbols　change　randomly　at　the　eavesdropper　and　satisfy　the　interference　constraint　of　the　primary　receiver.　The　number　of　eavesdropping　antennas　is　not　constrained　in　our　scheme.　Simulation　results　validate　the　superiority　of　the　proposed　scheme　when　compared　with　the　competing　candidate.　©　2018　IEEE.