security alert

• If that message appeared at the upper right of your Mac, it is a Notification: Stop unwanted Notifications - Apple Community.
• If you received a text message that was forwarded to your Mac, it's spam. To learn how to block and report spam messages, please read Block, filter, and report messages on iPhone - Apple Support.
• If a web page appeared alleging that your Mac is "infected" with something, read Phony "tech support" / "ransomware" popups and web pages - Apple Community.

In all cases, the answer is yes it's a scam, no different than an unwanted phone call. Hang up.

Hey totty67,

We're glad you asked! This pop-up is a scam. There's some great information in the section "Avoid interacting with pop-ups" about these types of pop-ups as well as other great tips throughout, here: Block pop-up ads and windows in Safari - Apple Support

We also thought you'd find these resources helpful and informative too:

Protect your Mac from malware - Apple Support

Protecting against malware in macOS - Apple Support

Ways to avoid malware and harmful apps on Mac - Apple Support

We hope this helps.

Take care!