Wireless Sensor Networks have emerged as one of the most promising technologies and promoted research avenues due to their widespread applicability. Wireless Sensor Networks have found applications in critical information infrastructure like military surveillance, nuclear power plants, etc., hence there arises the need to restrict access to critical information of such systems. So as to maintain confidentiality, user authentication is required so that only legitimate users are allowed to retrieve the information. Several two-factor user authentication schemes have been suggested by the research community. In this paper, a brief review of various security issues, security attacks and authentication schemes pertaining to Wireless Sensor Networks has been presented.