We utilize our knowledge of the quantum cloning to attack quantum cryptographic communications. The principle of quantum cryptography is robust, the eavesdropper is always revealed. But in real conditions, using lossy components some amount for transmission errors is tolerated. That is the way for eavesdropper to gain some information without being detected. Our research has clarified the limits for error rate for two well known protocols proposed by Bennett and Brassard (BB84) and Renes (R04). For both protocols the maximum allowed loss of communication line is 1/6 ≈ 16,7 %.