If you believe you've identified a security vulnerability in Mastodon (a bug that allows something to happen that shouldn't be possible), you should submit the report through our Bug Bounty Program. Alternatively, you can reach us at firstname.lastname@example.org.
You should not report such issues on GitHub 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.