There seems to be a bug in the system where sometimes it reports an app as damaged when it&#x27;s perfectly capable of downloading a new receipt (this is why a reboot works, because the bug seems to only crop up after some period of usage). When I bought a new iMac and transferred all my stuff from my old computer, I had this exact same issue, all of my MAS apps were reported as damaged. After reinstalling one, I tried rebooting, and that worked; got a password prompt for the next MAS app, and the rest just worked fine at that point.<p>Hopefully they&#x27;re aware of what&#x27;s causing this bug and will fix it in an OS update.<p>Of course, when the certificate is actually expired, and they haven&#x27;t replaced it yet, then the &quot;damaged&quot; dialog is reasonable, because the system cannot get a valid replacement receipt. But once they replaced it, OS X should have just started working again. In fact, I&#x27;m going to go file a radar right now.