CUBRID uses Subversion to manage its source code at Sourceforge.net. Instructions on Subversion use can be found at http://svnbook.red-bean.com/.
You may browser the source repository using SF.net's interface at http://cubrid.svn.sourceforge.net or at http://svn.cubrid.org/websvn/.
The latest version of CUBRID source code can be checked out anonymously from SVN branch with the following command:
svn co http://cubrid.svn.sourceforge.net/svnroot/cubrid/cubrid/branches/RB-<CUBRID VERSION> cubrid
Replace <CUBRID VERSION> with the appropriate version number such as 8.4.1. Browse the repository at http://cubrid.svn.sourceforge.net/viewvc/cubrid/ to find available versions.
Alternatively, you can simply download .tar.gz CUBRID source code from each version at the Downloads page or the CUBRID FTP.