The Passster WordPress plugin before 3.5.5.9 does not properly check for password, as well as that the post to be viewed is public, allowing unauthenticated users to bypass the protection offered by the plugin, and access arbitrary posts (such as private) content, by sending a specifically crafted request.
https://wpscan.com/vulnerability/0967303d-ea49-4993-84eb-a7ec97240071