Using Mint 17.3 Cinnamon, but I downloaded the ISO about 7 days prior to the counterfeit IS being posted.
There were (possibly) security updates available.
I tried updating, but I got a warning in the update manager about my usual mirror (at uc davis) not having the most recent cache (might not be the exact phrase).
so, I changed settings in software sources, and tried again.
It updated, but then I got the update manager that said my apt-cache had been corrupted and not to update anything else it might break my system.
Ok, now what?
I would ask on the linux mint forums but... there are no linux mint forums.
I tried going to the linux mint BLOG (instead of the forums), and I got an SSL security certificate error, saying that the certificate is for mycloudproxy.com (whom I have never heard of).