WordPress Plugins or themes that leverages the genericons package is vulnerable to a DOM-based Cross-Site Scripting (XSS) vulnerability due to an insecure file included with genericons.

JetPack plugin 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.