Discourse is an open source platform for community discussion. In affected versions an attacker can learn that a secret subcategory exists under a public category which has no public subcategories. The issue is patched in the latest stable, beta and tests-passed version of Discourse. Users are advised to upgrade. There are no known workarounds for this vulnerability.
https://github.com/discourse/discourse/security/advisories/GHSA-3qh8-xw23-cq4x
https://github.com/discourse/discourse/commit/819361ba28f86a1347059af300bb5cca690f9193