The Popup Builder by OptinMonster WordPress plugin before 2.12.2 does not ensure that the campaign to be loaded via some shortcodes is actually a campaign, allowing any authenticated users such as subscriber to retrieve the content of arbitrary posts, like draft, private or even password protected ones.
The OptinMonster WordPress plugin is vulnerable to sensitive information disclosure and unauthorized setting updates due to insufficient authorization validation via the logged_in_or_has_api_key function in the ~/OMAPI/RestApi.php file that can used to exploit inject malicious web scripts on sites with the plugin installed. This affects versions up to, and including, 2.6.4.