Movable Type has released a security upgrade that fixes XSS vulnerabilities, and recommends updating to 4.23 as a mandatory. The vulnerability hasn't been exploited yet, but this [1] update will ensure that your installation is secure.

I read one problem in particular on the [2] MT forum that I took into account when updating. If you've installed the recent update for [3] StyleCatcher, then do no update this plugin. The version included in the release is an older version than the one recommended for use with the [4] Sandbox Plugin version 1.1.

Remember to keep a backup of your old directory before updating. That way you have a copy of your previous version and can reinstate if anything goes wrong. My upgrade was smooth and hassle-free. I hope your is too.