In this paper, we present the security implications of x86 processor bugs or backdoors on operating systems and virtual machine monitors. We will not try to determine whether the backdoor threat is realistic or not, but we will assume that a bug or a backdoor exists and analyze the consequences on systems. We will show how it is possible for an attacker to implement a simple and generic CPU backdoor in order--at some later point in time--to bypass mandatory security mechanisms with very limited initial privileges. We will explain practical difficulties and show proof of concept schemes using a modified Qemu CPU emulator. Backdoors studied in this paper are all usable from the software level without any physical access to the hardware.