Cloud computing security is the set of control-based technologies and policies designed to adhere to regulatory compliance rules and protect information, data applications and infrastructure associated with cloud computing use. http://searchcompliance.techtarget.com/definition/cloud-computing-security https://cloudsecurityalliance.org/topthreats/csathreats.v1.0.pdf
Threats from cloud computing
IaaS providers offer their customers the illusion of unlimited compute, network, and storage capacity — often coupled with a ‘frictionless’ registration process where anyone with a valid credit card can register and immediately begin using cloud services. Some providers even offer free limited trial periods. By abusing the relative anonymity behind these registration and usage models, spammers, malicious code authors, and other criminals have been able to conduct their activities with relative impunity. PaaS providers have traditionally suffered most from this kind of attacks; however, recent evidence shows that hackers have begun to target IaaS vendors as well. Future areas of concern include password and key cracking, DDOS, launching dynamic attack points, hosting malicious data, botnet command and control, building rainbow tables, and CAPTCHA solving farms.

Threat #3: Malicious Insiders
Description
The threat of a malicious insider is well-known to most organizations.
This threat is amplified for consumers of cloud services by the convergence of IT services and customers under a single management domain, combined with a general lack of transparency into provider process and procedure. For example, a provider may not reveal how it grants employees access to physical and virtual assets, how it monitors these employees, or how it analyzes and reports on policy compliance.
To complicate matters, there is often little or no visibility