As the level of data hacking deepens, the requirement of security is inevitable in that it is the only means to defend them. Since it is also inevitable that security often contradicts with realtimeness, a comprise must be taken to balance them. Little insight leads us that privacy as well as conflicts against security. Privacy itself is equivalent to be described as identity for technical reason. Note that security is equivalent to identity concealment. Having security comprise with such other factors forced us to focus to communication level of security due mainly to speed of transaction processing. Although network security is concern these days, other aspects of security cannot be totally ignored. These include operating system-level security and database-level security. Considering all the three levels of security into a single security policy leads us to develop a sound system security infrastructure. Thus far, however, OS security just comes into our sight as a solution of authorized certifier in an area of digital signature. Although the main target of attack attempted in hacking are micro-level data, data security or database security is still perceived by many people a far-fetched idea. Note that even concrete security in terms of network level and OS level do not always guarantee blockage of illegal data flow, which could be induced by conspiratorial data access behavior. Such illegality is attempted in a form of either data inference or micro-data level conspiracy of silence. Preventing or detecting this sort of covert channel is not an easy task. If not impossible, however, any breaking effort against this type of unauthorized accesses must be made within territory of database system. In this paper, issues to network and database security are respectively reviewed for readers to have a generalized picture.