Mac running High Sierra 10.13.6

The 2013 iMac can run macOS 10.15.7 Catalina.

However, Catalina will break any 32-bit software that you are currently running so you’d have to update or find replacements for those.

Securing this High Sierra iMac on the internet has become difficult because the common browsers are no longer supported.

There is little to do to secure it but perhaps dump macOS and install some flavor of Linux if you wish to keep the hardware. Of course this will also require you to replace your apps.

If it works great, why buy a new one?

You are going to encounter increasing problems with recent websites or even using anything Internet-related due to the commensurately diminishing support for older operating systems. That would be reason to buy a new Mac. Until then, milk it for all it's worth.

Linux Mint can breathe new life into old Macs: https://www.linuxmint.com/

Linux can run the latest versions of most popular products and services.

... what I can do to secure it ...

Not quite sure what you mean by that, but Effective defenses against malware and other threats - Apple Community applies to Macs of all ages, and everything Internet-connected for that matter. Its Appendix even addresses Linux systems, including Mint.

To see which apps you have that will not work with Catalina download and run GO64. That will tell you want apps will not be useable with Catalina. Then you can check to see if there will be updated versions of your software that you can get or alternatives available.

If you decide to put that Apple obsolete iMac out to pasture and get a new Mac consider the following: a new 10 Core Mac Mini M4 with 16 GB RAM and 1 TB SSD ($1084) with a 32" LG 4k monitor ($331) from Amazon.com)  is $1415 which is $541 less than a similarly configured new 24" iMac ($1956)  re 07/2025. Keep your keyboard and mouse from your current Mac for use with the Mini. 

I got the above Mini with a 32" LG 4K monitor w/speakers for only $60 more than the 27" (didn't know the Mini had a speaker).  There are many monitors available in 27" size on up for $100 to $500 depending on what features you want.  The monitor prices are before tariffs.

Just some food for thought.

If you can put up with Catalina breaking 32-bit applications, it would be enough to let you run current versions of Firefox and LibreOffice.

However, the next version of LibreOffice will not run on Catalina, and it is unclear how long Catalina will continue to get current versions of Firefox as opposed to ESR ones.

12 years is a pretty good run for a computer, so you might want to start thinking about replacing it with a new Apple Silicon Mac, such as a 24” M4 iMac or a M4 Mac mini.

As long as you adhere to the recommendations in these user tips you will be fine: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

Do you have a question for this community?

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