Discourse is an open-source discussion platform. Prior to version 3.0.1 on the "stable" branch and version 3.1.0.beta2 on the "beta" and "tests-passed" branches, someone can use the "exclude_tag param" to filter out topics and deduce which ones were using a specific hidden tag. This affects any Discourse site using hidden tags in public categories. This issue is patched in version 3.0.1 on the "stable" branch and version 3.1.0.beta2 on the "beta" and "tests-passed" branches. As a workaround, secure any categories that are using hidden tags, change any existing hidden tags to not include private data, or remove any hidden tags currently in use.