Security Certificates / Unsafe Sites but they are..

Thanks you.

Our 21.5-inch Mid 2011 iMac runs os10.13 High Sierra as well has it has any OS before it with no compatibility issues.

The link to 10.13 High Sierra is in this Apple article:

How to get old versions of macOS - Apple Support

Be sure to use Safari for any downloads (Firefox tend to error on system download, AND back up you current installation.

If you see an error message about a damaged files, it only means the new installer had found an older outdated version of itself in your Applications folder. Trash the older verion and all should go well.

Please tell us which Mac model and year you’re having trouble with, and what macOS version you’re running. You can find that info by clicking the  (Apple) menu at the top-left of your screen and then select About This Mac.

(Note- for privacy and security please don’t post your serial number.)

It may very well be that your Mac is running outdated versions of macOS and Safari. If at all possible, you should consider upgrading the macOS to the most current version that your Mac can run.

Check the Compatibility of your Mac with newer versions of macOS:

➡️ macOS 10.13 High Sierra is compatible with the Macs listed here

➡️ macOS 10.14 Mojave is compatible with the Macs listed here

➡️ macOS 10.15 Catalina is compatible with the Macs listed here

➡️ macOS 11 Big Sur is compatible with the Macs listed here

➡️ macOS 12 Monterey is compatible with the Macs listed here

If you cannot upgrade your OS (and Safari with it) then consider using another browser app such as Firefox or Brave.

The retailer is partially correct. Your operating system is likely too old. You appear to be running macOS 10.11 "El Capitan." If so, there is your likely reason.

Pretty much all the reports here of seeing that advisory are from posters running macOS 10.11 El Capitan or older. And Apple did ot do this.

A major supplier of browser security certificates has allowed some of its certs to expire. This primarily affects OS versions like yours with outdated browsers. Here is that supplier's statement on the subject:

https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/

The issuing company has a replacement cert but older devices and/or their OS versions may not support technologies that the replacement uses.

Any 21.5-inch iMac can run at least macOS 10.13.6 which should have a new enough Safari version to avoid those errors. If you tell us your Mac's sub-model from "About this Mac..." we can give your very specific recommendations regarding your possible upgrade paths.

I used to advise users of macOS10.11 to switch to FireFox for browsing but Mozilla have given up on any macOS older that 10.13 High Sierra. Their current version for 10.11 is an ESR release that stopped getting security updates last summer

With the sub-model we can give you links to newer maOS versions

Because browser developers are dropping support for older version of both Windows and Mac OS, and the cert issuers are not likely to downgrade support, your options are either upgrade your existing OS to at least 10.13, or buy a new computer.

👍🏽

Lots of good info here. Thanks.