Secure your JIRA server by only serving logged in users.
Dashboards and filters can be shared with anyone. This also includes users which are not logged in (the "famous" anonymous users). While JIRA displays a warning message that such sharing may not have the intended consequences it is so far impossible for an admin to prevent the sharing or access without patching the JIRA source code.
A second problem is the quick search which can also be executed by an anonymous user and depending on the settings in the projects may or may not show some issues.
The "Prevent Anonymous Access" add-on prevents these problems by redirecting any requests not contained in a white list to the login form.