  As a novel technology, cloud computing attracts more and more people
including technology enthusiasts and malicious users. Different from the
classical network architecture, cloud environment has many its own features
which make the traditional defense mechanism invalid. To make the network more
robust against a malicious attack, we introduce a new method to mitigate this
risk efficiently and systematically. In this paper, we first propose a coupled
networks model which adequately considers the interactions between physical
layer and virtual layer in a practical cloud computing environment. Based on
this new model and our systematical method, we show that with the addition of
protection of some specific nodes in the network structure, the robustness of
cloud computing's network can be significantly improved whereas their
functionality remains unchanged. Our results demonstrate that our new method
can effectively settle the hard problems which cloud computing now is facing
without much cost.
