OptinMonster is a WordPress plugin/service to collect leads from customers visiting a website. The plugin is free to download from wordpress.org and it claims there are 20,000+ active installs. The service does require a membership, so presumably they have contacted all of their customers to tell them to upgrade.
I discovered an issue that allows any non-authenticated user to execute arbitrary WordPress shortcodes. Generally this type of access is only allowed by users with the ability to edit content on a site. Most of the default shortcodes aren’t very exciting and don’t provide much leverage for an attacker, but many third-party plugins expect that anyone who is able to execute shortcodes has sufficient permission that they don’t need to provide any further security.