It’s a bit of a personal security flaw, that by default Viber will show your SMS messages on your phone as a notification popup, even if the phone is locked. To stop Viber popup notifications on Android:
- Log into Viber on your Android
- Click the Menu button then More Options
- Click Settings
- Deselect New message popup (Popup is disabled)
This will stop Viber from displaying your personal messages as popup notifications even if though the phone is locked.