I’d recommend any WordPress users install the disable-JSON-API plugin. This will prevent anonymous access to WP REST API endpoints, such as /wp-json/wp/v2/users – which provides a list of usernames (API docs).
The REST API was introduced in WordPress 4.7 and is (unfortunately) on by default with no option in settings to turn it off – the idea is it will in time be used for AJAX in some of the admin system, not just external requests. There is a good change if you were running 4.7.0 or 4.7.1 and did not immediately upgrade to the 4.7.2 security release posts may have been defaced.
Also be aware that the WordPress readme.html file no longer displays complete version numbers – e.g. it will show 4.7 rather than 4.7.2. (a step backward, IMHO). Though you might not be aware however you can get the version number by clicking on the WordPress icon at the left of the admin bar (and of course, WP-CLI users can do:
wp core version)