Joomla! 3.0.3 isn't the latest version in your case. Joomla! has currently two lines:
- STS - this is a Joomla! 3.0.* line
- LTS - this is a Joomla! 2.5.* line
So in your case the latest version is 2.5.9 not 3.0.3 - if you're using the latest version of Joomla! 2.5.* then you are safe if your configuration is safe.
Practically I recommend two things:
* Upgrade your Joomla! to 2.5.9
* Make an automatic security audit using this page:
http://myjoomla.com/Then you will know about potential configuration problems and vulnerable extensions. Using of the latest version of Joomla! doesn't mean that you're safe. Hackers can use vulnerablities in your installed extensions, too big permissions in your directories/files, vulnerabilities in the hosting configuration etc.