iPhone 16 Pro stuck on iOS 18: how big is the security risk?

Which release of iOS 18 are you running? I believe that it needs to be iOS 18.7.4 or later to have received the fix for a key security certificate expiring at the end of next January (January 2027).

Apple security releases - Apple Support

The main problem you will likely encounter is that some apps that require higher security or simply only support newer OS versions may not update or work at all.

As for security I would say that the weakest item in security is the person and them doing foolish things. In 40 years of having used computers I have only once for a few months been full updated (e.g., currently running iOS 16) and I have yet to experience a security issue. Frankly even if I did have one, I do almost nothing on my old devices that anybody could benefit from (sure, see what recipe I looked up yesterday).

You'll hear the full range of opinions on this one. Here's one from Apple not long ago about their newest security, albeit mentioning that it was targeting "mercenary spyware" targeting the rich and famous. Are you rich and famous? If yes, then upgrade. https://security.apple.com/blog/memory-integrity-enforcement/

Well, if you have the latest version of iOS 18, that was updated in August 2026. So I think that's pretty good, actually, after all this is only October 2026.

I think the risk is very low from a security perspective.

If you still have access to the iPhone that was on OS 27, you could try what sberman suggests to see if it works for you. You could also visit an Apple Store and see if that offers any clarity on whether the latest version of iOS 27 still is disruptive for you. If it is, stay with the phone that works for you. My wife always resists change and she has stayed with ancient iPhones over the years that were quite a few versions out of date. I'm not glib about security but what you are doing does not sound risky at all to me, at least not for a year or two.

If your reluctance to upgrade your iPhone is related to Liquid Glass, you should immediately update to the latest iOS 27 version and adjust your Liquid Glass settings as shown here.

Adjust iPhone display and text settings - Apple Support

Then you will receive the latest security updates, feature improvements, defect repairs, etc.

Readers:

Marco_96 previously posted that the dithering around the edges of characters causes them massive eye and ultimately other health issues. BOTH iOS 26 and iOS 27 were non-starters.

Details in this discussion from (much less active) Accessibility forum:

Severe eye strain and dizziness from font… - Apple Community