WP REST API version 1.2.3 and 2.0 Beta 4 address a security issue that affects sites running 1.2 or 2.0 beta. This release fixes a potential XSS vulnerability related to JSONP support in 1.2 and 2.0 of the API. Automatic updates are in progress for 1.2.3 but if your site hasn’t automatically updated, the team suggests updating manually as soon as possible.
In addition to the security release, 2.0 Beta 4 includes a number of enhancements, some of which break backwards compatibility. Developers and beta testers should read the detailed changelog and release notes.