React applications which rendered to HTML using the ReactDOMServer API were not escaping user-supplied attribute names at render-time. That lack of escaping could lead to a cross-site scripting vulnerability. This issue affected minor releases 16.0.x 16.1.x 16.2.x 16.3.x and 16.4.x. It was fixed in 16.0.1 16.1.2 16.2.1 16.3.3 and 16.4.2.