Using the Windows Task Scheduler allows non-administrator users to gain access to administrative rights. Normally, Windows prevents execute applications which need to have more rights. A message appears where the user has to confirm it. There is an easy way to exploit the UAC dialog with just few easy steps. Here for example is a tutorial how to do that: http://www.raymond.cc/blog/task-scheduler-bypass-uac-prompt/
To be honest, this is so easy that everyone can do that without having a strong knowledge of security. I know that this security issue is there for a while now. That makes me wonder! This security issue is there and everyone knows it, and it so so easy. Why does this security issue exist? Is there a official statement from Microsoft?