This is something serious. If you have Viber app, there is a great potential that someone who can send message via Viber to you, will be able to access your locked phone quite easily.
This flaw was discovered by Bkav Internet Security and they have made a video to prove it. Watch this now.
As you can see, the flaw appears to be the Pop Up notification. According to the source, Viber had been informed and a patch has been promised without a delivery date. For now, to avoid such situation, you can disable the Pop Up notification for Viber.
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(source)