This indicates an attack attempt to exploit a Remote Code Injection Vulnerability in Joomla! CMS.
The vulnerability is due to an input validation error while parsing a crafted HTTP request. A remote attacker could exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
Joomla! CMS versions 2.5.4 through 2.5.25
Joomla! CMS 3.2.5 and prior 3.x versions
Joomla! CMS 3.3.0 through 3.3.4
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.