White hats are making significant contributions to cybersecurity by submitting vulnerability discovery reports to public vulnerability disclosure programs and company-initiated vulnerability reward programs. In this paper, we study white hat behaviors by analyzing a 3.5-year dataset which documents the contributions of 3254 white hats and their submitted 16446 Web vulnerability reports. Our dataset is collected from Wooyun, the predominant Web vulnerability disclosure program in China. We first show that Wooyun is continuously attracting new contributors from the white hat community. We then examine white hats' contributions along several dimensions. In particular, we provide evidence about the diversity inside Wooyun's white hat community and discuss the importance of this diversity for vulnerability discovery. Our results suggest that more participation, and thereby more diversity, contributes to higher productivity of the vulnerability discovery process.