It is one of the numerous knockoffs of the akamaihd.net scam haunting the macOS ecosystem. This string replaces the default search engine setting in Google Chrome, Safari, or Firefox on a computer raided by the underlying adware application. The purpose of this unauthorized change is to narrow down the victim’s Internet search activity to a service that works as a dispatcher sending the traffic further. Although the resulting site is the authentic Yahoo Search, the route towards it includes several midway spots that can hardly be discerned with the naked eye. These pages in transit are traffic aggregator networks that enable the actors behind a.akamaihd.net to rake in profits. Going back to the original question, default21076355-a.akamaihd.net is just a small, yet important, link in a complex chain of the browser hijack campaign.