NukeCops - great hack for PHPNuke-security: get it at zulkiplyharun.com

are you interested in the improvement of PHPNuke:
in hacks, tricks and coding-work that is giving you some inspirations.

then travel over to zulkiplyharun.com and read this article

It was a known fact that PHPNUKE is using a simple form of CAPTCHA (called security code), which will try to resist against automated actions - login bruteforce, account creation DoS, ect. PHNUKE with the standard 6-digit CAPTCHA can be easily attacked not only by human, but also by any malicious script exploiting the design weakness.

This is a guide for you to change your captcha from 6 digits to 8. Be sure to make backup of your original copies of the files. Something might gone wrong at anytime, so you can revert to your original files.

It is a good move to use not-a-standard-phpnuke-captcha. However for a newbie, it is not an easy task. Changing a few line of codings in a few file might drowned him several couple of hours in front of his computer.
This guide will show you in a very simple way how to change the PHPNUKE captcha from 6 to 8 digits.

read this article