iOS 9 & Lets Encrypt

Today (1 October 2021) my old iOS 9 devices started getting a "Cannot Verify Server Identity" error. I still use a wall mounted original iPad Mini as a home automation dashboard.

The full error text:

The identity of "some.site" cannot be verified by Safari. Review the certificate details to continue.

The Solution

By browsing to https://letsencrypt.org/certificates/, and then clicking on "pem" link for the "Cross-signed by DST Root CA X3" certificate, you can install the current root certificate. Clicking on the link will take you to Settings to install a "profile", just follow the prompts to install it. You'll probably get an error about the certificate on that site because it's also signed by Lets Encrypt...

This will then fix any errors with any HTTPS site that's signed by the current Lets Encrypt certificate.