Memory management unit is a common attribute of any modern CPU architecture. Not only does it provide a virtual memory mechanism, it is also involved in isolation and protection of execution environments. Understanding the implementation of the MMU is not that difficult, but still requires some effort. Combined with modern complex memory managers of the operating systems, it becomes a potential source of security issues. Let’s discuss the key properties of MMU with Intel x86 architecture as an example, common pitfalls in building the page tables and see if modern operating systems miss anything in terms of securing the memory mappings.