Unlike some other messaging services, WhatsApp only lets you register one phone number per account. That means when you log into your WhatsApp account on another phone using that number, it logs you out of the app on your existing device. So if you open WhatsApp on your usual phone to find yourself logged out—even though you never logged into another phone in the first place—you might be a little freaked out.
An unexpected WhatsApp logout is scary
Let’s be clear: your concern is completely understandable. After all, it’s unusual to open WhatsApp to the following unexpected message: “Your phone number is no longer registered with WhatsApp on this phone. This might be because you registered it on another phone. If you didn’t do this, verify your phone number to log back into your account.” That certainly sounds like someone logged into your account on a strange device.
Why you probably don’t need to worry about this logout
Fortunately, that likely isn’t the case this time. As it turns out, a known bug with WhatsApp for Android is logging users out of their accounts. According to WABetaInfo, if you find yourself logged out of your Android’s WhatsApp account when you didn’t log into another phone, all you need to do is simply log back into WhatsApp—no need to change your password or take other security measures.
WABetaInfo doesn’t say whether this bug is something that WhatsApp can fix server-side (which would solve the problem without the need for an app update), or if the bug will need to be addressed with an upcoming patch.
If you’re on iPhone, stay vigilant
It’s important to note that WABetaInfo specifically says that this bug affects WhatsApp for Android, and not iOS. If you were unexpectedly logged out of WhatsApp on your iPhone, we don’t have an innocent explantation for that, and highly recommend you change your password at the very least (and make sure to make it a strong one).