Whatsapp has had long-standing security issues (see the substantial section on security flaws on their Wikipedia page), with a team that appears either disinterested or incompetent to handle them, despite repeatedly being notified by researchers. It’s immensely popular, which makes awareness of these issues much more relevant. Along with lax authentication systems, easily sniffable incoming/outgoing messages, sending spoofing and phone numbers/contacts/IMEI’s leaking like a sieve, there’s more.
_”The IMEI can be obtained if you have physical access to the phone or if you control an app installed on the users device. The WLAN MAC address can be found using a network sniffer. Congratulations, you can now take over a users WhatsApp account¹. But how? Well, some people have done a excellent job reverse engineering the WhatsApp protocol. There is a working PHP class available that contains everything needed to build your own WhatsApp client: https://github.com/venomous0x/WhatsAPI“_*