If you believe you've identified a security vulnerability in Mastodon (a bug that allows something to happen that shouldn't be possible), you can either:
You should not report such issues on public GitHub issues or in other public spaces to give us time to publish a fix for the issue without exposing Mastodon's users to increased risk.
A "vulnerability in Mastodon" is a vulnerability in the code distributed through our main source code repository on GitHub. Vulnerabilities that are specific to a given installation (e.g. misconfiguration) should be reported to the owner of that installation and not us.