A flaw in secret communications which are based on quantum cryptography is revealed in Nature Communications this week. The findings highlight the need for further security updates on communication systems which are thought of as being highly secure.A system called quantum key distribution provides a means of secure communication allowing two remote parties to communicate in secret via a shared key that encrypts and decrypts messages. Such a protocol has potential applications in the exchange of secret information for the military, commerce, government and private affairs. Qin Liu and colleagues demonstrate a complete and undetected eavesdropping attack on a quantum key distribution connection. The system is breached by an eavesdropper accessing the secret key code while remaining undetected by the other parties.Various imperfections in quantum key distribution systems can be exploited to implement an attack; thus, efforts are needed to improve quantum cryptography for implementation in the real world.