Recently, major portal sites are suffering from a number of attacks and it is growing exponentially. July 2009, there has been system failure on government sites and some of the major portal sites due to the DDoS (Distributed Denial of Service) attack. Moreover, portal sites are exploited by a cross-site scripting vulnerability in 2010. To solve these problems, each portal site made an effort to eliminate the security vulnerability of the website and to protect personal information such as ID and password. However, portal sites still have the security vulnerabilities against ARP (Address Resolution Protocol) poisoning attack and the certificate spoofing attack. In this paper, we show the results of our penetration test and present the countermeasures on the ARP (Address Resolution Protocol) poisoning attack and the certificate spoofing attack.