Hey.
there's been no security holes identified recently. I assume as we don't have any '3 strikes and you're out' configuration for logon attempts then they can try to brute force username / passwords.
Please check that you have restrictions on your upload directory and that executables cannot be run (i.e. they can't upload a script then try to call it)
Also - for additional security you can configure an htaccess / htpasswd logon for your back-end admin page if you expose that to the internet.
I also have an antivirus scan (clamav) configured to run relularly on my attachments directory and the upload directory.
If you're concerned with ddos protection in particular, you could look to use Cloudflare.
For security testing, I have been looking at fuzz testing the codebase.. but unfortunately I'm fairly backlogged and could do with reviewing and approving some overdue commits before I get into new stuff!