Although cloud has been adopted by many organizations as their main infrastructure for IT delivery, there are still a large number of legacy applications running in non-cloud hosting environments. Thus, it is crucial to have migration techniques for such legacy applications so that they can benefit from many advantages of cloud such as elasticity, low upfront investment, and fast time-to-market. However, migrating large number of legacy applications into cloud in a timely manner is a daunting task. Common techniques such as redeveloping (i.e., modernizing) them or reinstalling from the scratch entails high costs. To mitigate these problems, we have developed a rapid migration technique, called AppCloak, that allows users to literally copy an already-installed application to cloud and run it without any modifications. The technique is based on intercepting a selected set of system calls and replacing the parameters and return values to hide any differences of environments to the application. We demonstrate that our technique works in Amazon EC2 and quantify the performance overhead.