https://daniel.haxx.se/blog/2024/03/08/the-apple-curl-security-incident-12604/
The curl command line option --cacert provides a way for the user to say to curl that this is the exact set of CA certificates to trust when doing the following transfer. If the TLS server cannot provide a certificate that can be verified with that set of certificates, it should fail and return error. This particular behavior and functionality in curl has been established since many years (this option was added to curl in December 2000) and of course is provided to allow users to know that it communicates with a known and trusted server. A pretty fundamental part of what TLS does really.

When this command line option is used with curl on macOS, the version shipped by Apple, it seems to fall back and checks the system CA store in case the provided set of CA certs fail the verification. A secondary check that was not asked for, is not documented and plain frankly comes completely by surprise. Therefore, when a user runs the check with a trimmed and dedicated CA cert file, it will not fail if the system CA store contains a cert that can verify the server!

This is a security problem because now suddenly certificate checks pass that should not pass.