what's up with a rash of popups and security warnings

Hi,

More than likely adware and or malware.

You can use the Malwarebytes free utility to remove it.

Click here > https://www.malwarebytes.com

Then click Download.

Some more details would help: Writing an effective Apple Support Communities question.

Explain what's going on with your Mac, as if we can't see what you're seeing, because we can't.

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Popup remover – Scam Zapper