Discussions about cloud computing security often fail to distinguish general issues from cloud-specific issues.
The article starts off defining vulnerability in general and then goes on to establish the vulnerabilities that are inherent in cloud computing models.
It really boils down to access:
Of all these IAAA vulnerabilities, in the experience of cloud service providers, currently, authentication issues are the primary vulnerability that puts user data in cloud services at risk
...which is really no different than anything in traditional IT.