No, there is no vulnerability created because each HAF app is isolated to its own set of tables (its own schema). Each haf app runs as a separate role (like a separate user on a multi-user computer system), so it only has permission to write to its own tables.

About the worst thing that can happen is that a HAF app may be implemented inefficiently so that its queries are slow, slowing down access to the database to other apps.

But this can be mitigated by the new query_supervisor which kills queries that take too long or use too many resources. And if a HAF app adds too much load, HAF server operators will likely disable it until the app's devs make it faster.