Cloud computing is a rapidly growing internet technology for facilitating various services to its consumers. It proposes many striking promises to the general public or big companies like Amazon, Google, Microsoft, IBM, etc., to maintain and upgrade their position in a fast-growing cloud computing environment and to enhance their services for a large number of users. However, with the fast development and enticing offerings, many issues associated with this technology also arise, which need to be addressed, with security being the strongest barrier to its adoption. Security concerns are an active area of research, which needs to be addressed properly to avoid security threats and attacks that are disasters for both service providers and service consumers. This paper highlights cloud computing architectural principles, cloud computing key security requirements, cloud computing security threats and cloud computing security attacks with their mitigation techniques, and future research challenges.