Cloud computing is a way to let users dynamically access unlimited resources anytime anywhere through Internet using various Internet-enabled devices. The cost and time of system implementation and maintenance can thus be reduced, the risk can be lowered. Moving user’s applications and sensitive data of individual users/enterprise users from closed environments to open cloud environments raises many new concerns about security and privacy. This paper analyzes the problems of the mostly widely used single sign-on mechanisms - OAuth, and then proposes solutions to improve the above-mentioned problems. Hope that the risks of user authentication in cloud computing should be understood in more detail, and the users’ confidence in adopting cloud services can thus be increased.