How can I have my CMS upgrade itself?

Theres a bunch of ways to do this but the least complicated is just to have Git installedo n your client servers and set up a cron job that runs a git pull origin master every now and then. If your application uses Migrations it should be easy as hell to do.

You can do this as it sounds like you are in full control of your clients. For something like PyroCMS or PancakeApp that doesn't work because anyone can have it on any server and we have to be a little smarter. We just download a ZIP which contains all changed files and a list of deleted files, which means the file system is updated nicely.

We have a list of installations which we can ping with a HTTP request so the system knows to run the download, or the click can hit "Upgrade" when they log in.

You can use Git from your CMS: Glip. The cron would be a url on your own system, without installing Git.

@Obsidian Wouldn't a DNS poisoning attack also compromise most methods being mentioned in this thread?

Additionally SSH could be compromised by a man in the middle attack as well.

While total paranoia is a good thing when dealing with security, Wordpress being a GPL codebase would make it easy to detect an unauthorized code change in your code if such an attack did occur, so resolution would be easy.

SSH and Git does sound like a good solution, but what is the intended audience?