WhatsApp is one of the most popular private messaging apps out there, so it's a little surprising it didn't have support for two-step authentication. Now, the beta version of the app has added the feature so, like always, turn it on now.
Two-step verification adds a six-digit passcode to your login information, so when you attempt to log into WhatsApp on another device you'll need that passcode alongside your phone number. To enable two-step verification, open WhatsApp, then tap Settings > Account > Two-step verification > Enable. You can optionally enter an email address here that allows you to disable two-step verification if you ever forget your passcode.