A new type of attack on quantum cryptography systems is proposed. In this attack, Eve utilizes various optical imperfections in Bob's scheme and constructs light pulses so that Bob does not distinguish his detection results from normal, whereas they give Bob the basis and bit value chosen at Eve's discretion. Applying this attack to systems with passive basis choice on Bob's side is considered. Also, a general workflow of breaking into a running quantum cryptolink using this or Trojan horse attack is discussed.