Cloud Computing has emerged as a model to process large volumetric data. Though Cloud Computing is very popular, cloud security could delay its adoption. Security of the cloud must provide data confidentiality and protection of resources. Such architecture seems to be vulnerable when confronted to distributed attacks also known as large-scale coordinated attacks. In this paper, we study the impact of large-scale coordinated attacks on Cloud Computing and its current security solutions. We experiment the open-source IDS Snort and a commercialized firewall using distributed portscan. Our results show that these security solutions are not designed to detect distributed attacks. Indeed, an attacker who controls about 32 hosts can easily achieve a distributed portscan without being detected.