Some existing families of potentially unwanted programs and adware have added browser push notifications to their weapons arsenal. Offering themselves up as browser extensions on Chrome and Firefox, these threats pose as useful plugins then haggle users with notifications.
A family of search hijackers
The first I would like to discuss is a large family of Chrome extensions that were already active as search hijackers, but have now added a notifications service from a provider hailing from a domain blocked for fraud by Malwarebytes. What that means is you can now expect browser notifications