The Cloud Computing concept offers dynamically scalable resources provisioned as a service over the Internet.Economic benefits are the main driver for the Cloud, since it promises the reduction of capital expenditure and operational expenditure.In order for this to become reality, however, there are still some challenges to be solved. Amongst these are security and trust issues, since the user data has to be released to the Cloud and thus leaves the protection sphere of the data owner. Most of the discussions on these topics are mainly driven by arguments related to organisational means. This paper focuses on various security issues arising from the usage of Cloud services and especially by the rapid development of Cloud computing arena. It also discusses basic security model followed by various High Level Security threats in the industry.