FluentCRM <= 2.9.87 - Unauthenticated Blind Server-Side Request Forgery via 'SubscribeURL' Parameter

The FluentCRM – Email Newsletter, Automation, Email Marketing, Email Campaigns, Optins, Leads, and CRM Solution plugin for WordPress is vulnerable to Blind Server-Side Request Forgery in all versions up to, and including, 2.9.87 via the 'SubscribeURL' parameter. This makes it possible for unauthenticated attackers to make web requests to arbitrary locations originating from the web application and can be used to query and modify information from internal services. Exploitation requires that the SES bounce handling key ('_fc_bounce_key') has never been stored (i.e., the site is in its default/unconfigured state with respect to SES bounce handling) as visiting the bounce configuration page auto-generates and stores a random key that causes the authentication check to evaluate correctly and reject unauthenticated requests.