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The Internet is on the way to become a common infrastructure for everyone. But, it is hard to catch up the current situation for both applications and users. Many developers of applications, which depend on the Internet, like e-mail clients, did not consider about to protecting one's personal information. In this situation, scoundrels will attack people by means of to be famous, to show their skills, just for fun, and so on. There was no problem if users were doing a suitable defense countermeasure. But, now, network applications with poor consideration by their developers, users must consider about new style of attack "passive attack". In this paper, the author classifies the types of attacks observed on the Internet, and explains the current security levels on some network client applications as a practice.