i have just seen a notification at the top right of my mac screen saying ive been hacked

All you need to do is go to Safari > Settings > Websites > Notifications, and remove every website you see there.

There is nothing wrong with your computer, you just allowed a website to send you Notifications and that website decided to send you those. They got paid to do it.

MrHoffman wrote:

These are advertisements, and not any security issues with your Mac.

You're being generous. A message such as "Your iCloud is being hacked!" is a scam, presented by criminals. Maybe they "just" want you to buy some piece of (worthless) software or another, but it seems likely that they have some more thorough fraud in mind.

FTC – How To Spot, Avoid, and Report Tech Support Scams

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Servant of Cats wrote:

MrHoffman wrote:

These are advertisements, and not any security issues with your Mac.

You're being generous. A message such as "Your iCloud is being hacked!" is a scam, presented by criminals. Maybe they "just" want you to buy some piece of (worthless) software or another, but it seems likely that they have some more thorough fraud in mind.

Malware has gotten more difficult to create and spread, so getting folks to willingly install apps — and variously apps that can themselves be difficult to distinguish from malware — is easier.

Fear works, as advertisers, politicians, social media websites, and propagandists have long known.

The advertised app here quite possibly provides a payment to the advertiser, and the advertised app may well compromise user metadata or user data, sure. The prevalence of these adverts usually means money involved somewhere.

Many of the add-on “security’ apps are also sketchy, providing little or no benefit over the built-in anti-malware, often unnecessarily noisy, sometimes hazardous, and some of these have gotten caught selling personally-identified metadata.

Both replies above are correct.

Somewhere along the path, you were offered notifications, accepted the offer, and here we are.

These are advertisements, and not any security issues with your Mac.

The reply above from John Galt has a link that also covers how to block all websites from ever offering to send more of these advertisements. You’ll probably want to block these offers.

Please read Stop unwanted Notifications - Apple Community.