Getting the same problem here since Friday as well. Auth.net log message is:
SSL certificate problem: unable to get local issuer certificate
I have tried to update ./lib/ssl/cert.pem from the GitHub project, as well as trying the big one from curl.haxx.se, neither one seems to work. I tried setting $VERIFY_PEER to false as well, but that apparently just shuts the whole connection down entirely. If I modify ./lib/shared/AuthorizeNetRequest.php to actually set CURLOPT_SSL_VERIFYPEER to false here, it does go through successfully. Obviously modifying the vendor code, and disabling the SSL verification, won't be ideal in the long term... likely fine in localhost/sandbox only, but I wouldn't want either one of those things in production.