Abstract
Trojan horse attack is a common eavesdropping strategy which can attack various quantum secure communication systems. Its basic idea is to send auxiliary photons into a legitimate communicator's apparatuses and steal information by analyzing the reflected photons. In this paper, we consider a different kind of Trojan horse attack, the so-called counterfactual Trojan horse attack, which has not been studied in detail so far. In such an attack, the eavesdropper may steal the secret information by "ghost" photons, which can spontaneously avoid being detected, even if the detector is an ideal one. We present the details and requirements of such an attack. We also illustrate our results by considering two protocols, the ping-pong protocol and the counterfactual quantum cryptography. Furthermore, we discuss the nature of the counterfactual Trojan horse attack and the strategy to successfully deal with it. Our results indicate that additional resources may be required for the protection against such an attack.