iPhone untrusted certificate warning My iPhone 7 iOS 10.3.1 randomly started receiving untrusted certificate pop-ups from onboard.info (but I’ve seen it on devices going back to iPhone 5):

This was at home last night and I had just unlocked my phone. I tried to get a screenshot of the rest of the certificate details but it was registered as just a home button press and there is no way to switch back into the certificate warning dialogue by changing apps.

I can’t apply any logic to the root cause of this or recreate it. I’ve never clicked Trust for obvious reasons.

Can anyone explain?

21/04/17 - Just happened again.  The irony of the backdrop in this picture is not lost on me.  Is this some sort of MITM attack?
Yes, it seems to be a MITM attack. The pcr.apple.com has a certificate issued by Symantec for Apple - but you're getting an entirely different, self-signed certificate. 

Note that MITM doesn't mean that it must be a "malicious" attack as such.

You do not mention the internet source you're connected to when the problem occurs. Some connections, particular public WiFi such as in restaurants, on trains and busses, etc. will use seemingly MITM for presenting sign-on pages for their WiFi, or for the purpose of injecting ads into web pages.

The certificate you're getting is from an Icomera system. They supply systems for passenger WiFi on trains and busses. If you're using such WiFi, this is the source of your problems.

And yes, it is a terrible practice for Icomera and others to be building their systems like this.