Are security updates available for High Sierra 10.13.6

Generally not. See the "Available for" column in the following Apple Support article for information on versions that have security updates: Apple security updates - Apple Support

You may be able to upgrade your Mac from 10.13.6 to a higher version. What is the model year of your Mac and what model (e.g., iMac, MacBook Air, etc) is it? Both are available at Apple menu > About this Mac.

Sheriff1973 wrote:

Are older versions of MacOS like High Sierra 10.13.6 updated with the latest security software?

No.

Most of the added security features and updates simply cannot be back-ported to macOS High Sierra. A whole lot has changed within macOS since 2017 and High Sierra, including around integrated security.

Recent macOS versions have the read-only system volume, the updates to XProtect and the addition of XProtect Remediator, the password recommendations, etc.

A Mac that can't be updated to Monterey, Ventura, or whatever is probably arriving later this year, is also old hardware, and that hardware is also missing security-related features that have been added more recently. (Check the model and model year to see if yours can be upgraded.)

What to do? A whole lot of the attacks are not against macOS itself, but against users. Some well-known add-on "security" apps were selling personally-identified web activity and web purchasing info, others scan traffic, and more than a few efforts toward compromising users' password through phishing, and other scams. Use unique and robust passwords, and—as High Sierra is AFAIK too old for the password recommendations—check the website HaveIBeenPwned.Com for known password compromises of your logins.

Just to add...

Mostly you just need, or most you can do if you can't upgrade is a more modern Browser than Safari on Mojave, or High Sierra like Firefox or Brave.

https://brave.com/

Thanks to dialabrain…

Firefox 111.0.1 is now the current version. runs in High Sierra.

https://www.mozilla.org/en-US/firefox/new/