The Joomla security team have just released a new version of Joomla to patch a critical remote command execution vulnerability that affects all versions from 1.5 to 3.4.
This is a serious vulnerability that can be easily exploited and is already in the wild. If you are using Joomla, you have to update it right now.
If you are using the old (unsupported) versions 1.5.x and 2.5.x, you have to apply the hotfixes from here. This article from OSTraining explains how to apply them. This vulnerability is already being exploited in the wild and has been for the last 2 days before there was a patch available.
If you are using Joomla, you should patch immediately or upgrade it to latest version immediately.