Hackers have successfully installed remote monitoring software and tracking phones and other devices, having benefited from the vulnerability of the WhatsApp app owned by Facebook.
The WhatsApp team explained that a limited number of devices were being attacked.
The assault was detected at the beginning of the month, and last Friday the security breach was suspended.
WhatsApp urged users, who are about a billion and a half, to update the application. According to the Financial Times, the attack was the work of an American company based in Israel.
On documents there develop anti-terrorism software, but in the IT sector they call them "Cyber weapons dealers ".