A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a security mechanism that can be used to distinguish between humans and machines. Most existing CAPTCHA systems are vulnerable against a so-called "third party human attack." The third party human attack employs hired human to solve challenges so that the CAPTCHA systems will no longer be effective. In this paper, we design an efficient and effective aspect to defend the attack. Following the aspect, we design and analyze a novel CAPTCHA system, Drag-n-Drop Interactive Masking CAPTCHA (DDIM CAPTCHA), to deal with both the traditional attacks and the third party human attack. The DDIM CAPTCHA retains the basic requirements of CAPTCHAs and adds the properties of interaction and masking. Through a series of analyses and experiments, the proposed Drag-nDrop CAPTCHA can be claimed to be a good approach for deployment to remedy the weaknesses of present CAPTCHA systems.