Hosting ads for an Ad network on your website is a great way to spread malware and phishing attacks to your readership.
The popup below greeted me when I was attempting to browse a meme in order to repurpose it for a client.
Most people presented with a login prompt like this would likely enter in their corporate login details.
After that, it’s childs play to combine the login details with the likely static IP the person is surfing from and you have the makings of a remote logon to a user’s email and/or VPN. Depending on how high up in the organization that person is, the data breach could be very serious indeed.