Since the application level is the one being directly attacked, one might think that it is the appropriate point to thwart the attacks at. However, to assure the complete absence of security vulnerabilities expensive manual verification of every single application has to be carried out. Taking into account the sheer size of the application space and the rate of its growth reveals the impossibility of such an audit.
The short analysis of host based security attacks in Section 3.2 pointed at the common aim of most of the exploits -- to gain the privileges of the ``trusted'' applications. The fact that overall result is the same no matter which application is exploited, or the way in which it was exploited, points at a problem at the layer below the application layer - the Operating System layer [9,23]. This conclusion is not intended to eliminate security concerns when developing applications. Although addressing security problems solely in the application space does not suffice, it is still a valuable aspect of the overall system security.
The following mechanisms and aspects of the most commercial and server side Operating Systems enable the described attack behavior [9,23,24,22,30]: