Any WordPress Plugin or theme that leverages the genericons package is vulnerable to a DOM-based Cross-Site Scripting (XSS) vulnerability due to an insecure file included with genericons. So far, the JetPack plugin (reported to have over 1 million active installs) and the TwentyFifteen theme (installed by default) are found to be vulnerable. The exact count is difficult to grasp, but both the plugin and theme are default installs in millions of WordPress installs. The main issue here is the genericons package, so any plugin that makes use of this package is potentially vulnerable if it includes the example.html file that comes with the package.
The XSS vulnerability is very simple to exploit and happens at the Document Object Model (DOM) level. If you are not familiar with DOM attacks, the OWASP group explain it well:
DOM-Based XSS is an XSS attack wherein the attack payload is executed as a result of modifying the Document Object Model (DOM) “environment” in the victim’s browser used by the original client side script, so that the client side code runs in an “unexpected” manner. That is, the page itself (the HTTP response that is) does not change, but the client side code contained in the page executes differently due to the malicious modifications that have occurred in the DOM environment.