Description
Joomla! Core is prone to a vulnerability that lets attackers upload arbitrary files because the application fails to properly verify user-supplied input. An attacker can exploit this vulnerability to upload arbitrary code and run it in the context of the webserver process. This may facilitate unauthorized access or privilege escalation; other attacks are also possible. Joomla! Core versions 2.5.x ranging from 2.5.0 and up to and including 2.5.13 are vulnerable.
Remediation
Update to Joomla! Core version 2.5.14 or latest
References
http://niiconsulting.com/checkmate/2013/08/critical-joomla-file-upload-vulnerability/
https://www.exploit-db.com/exploits/27610/
https://developer.joomla.org/security/news/563-20130801-core-unauthorised-uploads