The easiest way to use viral videos legally is to use creative commons videos. You can also curate viral videos that are copyrighted but most likely usually you will not be able to monetize them. Either way you should always give credit to your sources in your descriptions.
Hi,
Viral videos are often protected by Viral Hog or other media companies. You will need a licence to use them. You can however use the Reuters footage in the dashboard, it isn't viral but does have some great footage.