Presence of restricted personal Discourse messages may be leaked if tagged with a tag

Vulnerability Description

Discourse is an open-source messaging platform. In versions 3.0.1 and prior on the `stable` branch and versions 3.1.0.beta2 and prior on the `beta` and `tests-passed` branches, the count of personal messages displayed for a tag is a count of all personal messages regardless of whether the personal message is visible to a given user. As a result, any users can technically poll a sensitive tag to determine if a new personal message is created even if the user does not have access to the personal message. In the patched versions, the count of personal messages tagged with a given tag is hidden by default. To revert to the old behaviour of displaying the count of personal messages for a given tag, an admin may enable the `display_personal_messages_tag_counts` site setting.

References

• https://github.com/discourse/discourse/security/advisories/GHSA-rf8j-mf8c-82v7
• https://github.com/discourse/discourse/commit/f31f0b70f82c43d93220ce6fc0d4f57440452f37