Check Point security researcher Kasif Dekel recently discovered significant vulnerabilities which exploit the WhatsApp Web logic and allow attackers to trick victims into executing arbitrary code on their machines in a new and sophisticated way.
All an attacker needed to do to exploit the vulnerability was to send a user a seemingly innocent vCard containing malicious code. Once opened, the alleged contact is revealed to be an executable file, further compromising computers by distributing bots, ransomware, RATs, and other malwares.
WhatsApp responded quickly and responsibly to deploy an initial mitigation against exploitation of this issue in all web clients and released a fix few days later.
More technical details are available here - http://blog.checkpoint.com/2015/09/08/whatsapp-maliciouscard-vulnerabilities-allowed-attackers-to-compromise-hundreds-of-millions-of-whatsapp-users/