The security of a company's vital databases represents a common concern in this time of Web-enabled businesses. The general recommendation is against directly connecting from the Web server to the database because this setup would allow attackers easy access to critical data after they have seized control of the Web server.
Instead of this direct connection, an intermediate application server is introduced. This server is placed in a so-called demilitarized zone (DMZ), located between two firewalls. Firewall #1 only allows connections from the Web server to the app server, and Firewall #2 only allows connections from the app server to the databases.
Because the application server doesn't allow the execution of arbitrary SQL statements, yet provides object-oriented or function-based access to business logic, a security compromise of the Web server (which can only talk to the app server) is noncritical to a company's operations.
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