Abstract
The quantum secure direct communication protocol recently proposed by Cao and Song [Chin. Phys. Lett. 23 (2006) 290] (i.e., the C-S QSDC protocol) is revisited. A security leak is pointed out. Taking advantage of this leak, an eavesdropper may adopt the intercept–measure–resend strategy to attack the quantum channel such that in the C-S QSDC protocol the secret message can be completely eavesdropped. To fix the leak, the original version of the C-S QSDC protocol is revised. As a consequence, the security is improved and assured at least in the case of an ideal quantum channel.