If you were tried to install OS X software updates this weekend, you might have noticed that you...can't. As MacRumors points out, OS X users who attempt to install software updates through the App Store app are getting error messages that complain about an invalid certificate. The reason? Apple apparently forgot to renew the SSL certificate in question.
The error message reads, "The certificate for this server is invalid. You might be connecting to a server that is pretending to be "swscan.apple.com" which could put you confidential information at risk." Some users (myself included) report seeing an "NSURLErrorDomain error -1012" alert instead; MacRumors speculates that the invalid certificate may be causing this error as well.
A quick check on the certificate status for Apple's update server shows that it expired Saturday, May 24th.
For the uninitiated, SSL certificates are a method of verifying a website's identity that are issued by organizations known as certificate authorities. Think of them as a sort of identification card, except for websites. OS X's Software Update mechanism looks for a certificate from Apple's update server to verify its identity, and when that certificate is missing or invalid, OS X will refuse to connect to the update server.
There's no word on when Software Update will be up and running again, so in the meantime, your best bet is to head to the downloads page on Apple's support site to get the updates to install manually.