what is d27m6tt163gu45.cloudfront.net

Download and run Malwarebytes

Free Antivirus Replacement & Anti-Malware Tool | Malwarebytes

if you would rather not use Malwarebytes

How to block pop-ups in Safari - Apple Support

if none of that helps contact apple support by phone

1800MYAPPLE

Is this on your iPad?

Try double clicking the Home button and swipe Safari upwards. Go to Settings/Safari and clear History and Website Data. Open Safari and test.

Safari - Clear the history and cookies on your iPhone, iPad, or iPod touch

Safari website data on your iPhone or iPad - How to clear

Safari - Stop pop-up ads in

Are you subscribed to an Amazon service?

https://en.wikipedia.org/wiki/Amazon_CloudFront

Hello. Same problem on an Imac running High Sierra 10.13.4. Every time I open Safari it just keeps filling up with Cloudfront.net windows. Not sure how I can download any anti-virus tool?