PostNuke is free to download and use for both commercial and non-commercial use. It is licensed under the GNU/GPL version 2 license. The GPL allows you to modify the code without restrictions ensuring you can customize it to fit your project requirements. You can also redistribute your work, with the understanding credit must be given to the original authors and that you must also license the new modified code as GPL. At no time are you permitted to change the license terms.
The GPL license ensures you have full access to the source code. This gives peace of mind about security since nothing is hidden in the code. It improves security since there is no need to reverse engineer the code to find potential security issues, in fact this open-ness enables world wide peer review. Lastly it makes it easy for anyone to modify or tweak the software to their own particular requirement.