Discourse Policy plugin gives the ability to confirm users have seen or done something. Prior to version 0.1.1, if there was a policy posted to a public topic that was tied to a private group then the group members could be shown to non-group members. This issue has been patched in version 0.1.1. A workaround involves moving any policy topics with private groups to restricted categories.
The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.