Add ISRG Root X1 certificate to legacy macOS

gistfile1.txt

# One-line Terminal command to add the ISRG Root X1 certificate to old versions of macOS / OS X.

# Run this command while connected to the internet via a trustworthy source.
# An example of a trustworthy source would (probably) be a password-protected home WiFi network.
# An example of an untrustedworthy source would be the public WiFi at your local coffee shop.

curl 'http://x1.i.lencr.org' > /tmp/ISRGRootX1.pem && sudo security -v add-trusted-cert -d -r trustRoot -k /Library/Keychains/System.keychain /tmp/ISRGRootX1.pem