Joomla! before 1.0.11 omits some checks for whether _VALID_MOS is defined, which allows attackers to have an unknown impact, possibly resulting in PHP remote file inclusion.
https://exchange.xforce.ibmcloud.com/vulnerabilities/28631
http://www.vupen.com/english/advisories/2006/3408
http://www.joomla.org/content/view/1843/74/