We use AWS via Amazon as our cloud storage and it is protected by all the standard security measures. Amazon, of course, has top-of-the-line security protocol automatically integrated, but I assure you, all of your work is held in an encrypted database. Passwords are also encrypted and not stored as plain text.
To add an additional layer of safety for your content, if you ever find yourself working offline - such as a power outage or while you're traveling - all your work will backup on your local drive and save there until it can once again be synched with the server (when you're back online).
There is also a Save button that you can use at any time to force a back up of all your work and export a JSON file that can be used to restore your account if anything unplanned ever did occur.