Basic attestation user sub ca1 certificate not trusted

The certificate is a valid Apple one. There were reports of this happening prior to macOS 11.3. Apparently, macOS 11.3 fixed the problem after upgrading to 11.3 users reported opening App Store and rebooting a couple times and the certificate was removed and they no longer saw it being referenced on contact records. Appears to have been an Apple bug.

Make sure you upgrade to the latest macOS 11.6 and it should be resolved.

https://discussions.apple.com/thread/252340614?page=3

Could be a certificate attached to an email you received which utilizes S/MIME verified or encrypted email and the certificate was copied into your keychain when you read the email. This happens from time to time with corporate users who have S/MIME configured by their IT staff who also manage their computers. Other employees would automatically trust certificates from the company domain name, etc. But sending it out to 3rd parties can result in you as the 3rd party not trusting the certificate.

It's either that or some software uses the certificate, etc.

Can you open the Certificate, take a screenshot and attach it in reply? It contains a lot of data.

Expand the Trust and Details and stretch the vertical size of the window to include all the technical details.

Then press CMD + Shift + 4 and then position the crosshair mouse cursor over the certificate window. Press Spacebar and click the window. A large thumbnail will appear in the bottom-right corner of your screen click it and then click Done in the upper-right. The screenshot will be saved on your desktop. Click the Image Insertion button when replying and select the screenshot from your desktop to attach it in reply.

Double-click the certificate it will open a window with a lot more detail, that's what I am looking for.

It is highly unlikely that Apple Pay was hacked. But it is up to the bank to ensure you are who you say you are when you add a credit or debit card to your Apple Pay Wallet. Not to mention the million other ways someone could obtain your credit / debit card information. Many retailers have been hacked and customers cards put at risk.

Please reply and attach a screenshot of the certificate in question. Without that information I have no idea what you are looking at. Certificates are used for many things.