I have a general, not Avast related question:
Is auto-update feature of programs like Ccleaner, Clasic Shell, Notepad++ etc. safe? Or is it vulnerable and can be exploited?
Thank you!
The programs you’ve listed only check and notify you if there is a newer version available.
It saves you the hassle of doing that task yourself. If you have a computer that’s lacking ram and resources,
you might not want that service which is safe since it doesn’t update unless you do the download and install.
Since almost none of the auto-updates are using a secure connection (https/ftps) there is always a (small) risk.
But the same goes for downloading a update manually.
As long as you have the application(update) directly from the developer(s), you should not worry about it.
That’s also what you have avast! for. It will check the file to be installed before the installation actually happens. 
That is a valid point about program updates as, provided the program you have is from a legitimate source, the program will (should !!)look to a legitimate update site for the update which could save you from downloading a very nasty piece of unwanted rubbish from a non legit site that you found and thought it was legitimate!!