Q: Why, in my Macintosh login Keychain list, are there certificates for individuals other than myself? Does that mean they could get to secure web sites using my Mac?

Answer

There are multiple ways another user's certificate can be added to your keychain, but the most common cause is a user sending you a secure email using the S/MIME standard. Messages sent with S/MIME include the sender's public certificate, which Mail automatically adds to your keychain when the message is viewed. A user's public certificate cannot be used to access secure websites and does not give them access to your machine or data.