I suggest you to take a look at the papers published in IEEE Transactions on Cloud Computing (TCC). 

http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=6245519

There are lots of research going on security over cloud computing. As cloud related to shared/distributed manner, security is important. I would like to suggest you to go through IaaS part and virtualized infrastucture of the cloud computing as security mainly maintains under this category; you will surely get an idea.

Please see our paper as might help:

Security of Cloud Computing Environment; In proceeding of: The 23rd IBIMA Conference Vision 2020: Sustainable Growth, Economic Development, and Global Competitiveness, At Valencia, Spain, Volume: pp. 2202-2215.      

An area of active research that I'm engaged in and have found to be interesting is semantic storage and its use for intrusion detection in Cloud computing systems and data centers.  There has been fundamental research on semantic memory as an alternative to traditional memory in computing, some simple examples in use like content-addressable memory, research into semantic storage and the semantic web.  All are interesting, but for the Cloud, I think you might find semantic storage interesting.  Here's a couple of overview papers to start with: http://research.cs.wisc.edu/adsl/Publications/sdsdb-fast05.pdf, http://eprints.soton.ac.uk/261985/1/owens-semanticStorageOverview-ecs.pdf