What firewalls can not do

Weaknesses within the system. While firewalls are a useful part of a security program of the network, not an impenetrable fortress. If properly managed, are useful, but will not do everything. If improperly used instead, the 'only thing that gave you a false sense of security. Firewalls are useless against attacks from' inside. An internal attack can come from a legitimate user, however, is that the attacker, or someone who has been given 'access to an internal machine via other means. The hostile code that is executed from an internal machine, maybe you got a virus through e-mail mail or by exploiting a buffer overflow on the machine, it can be seen as an internal attacker
Limited effectiveness of the firewall
If your firewall is the only security mechanism you have, and someone manages to enter through some other mechanism, you're in trouble. For example, if you run virus detection only on the e-mail gateway, security can go up in smoke if any part of such an infected disk, or download an executable from the Web Any backdoor connection that bypasses the filter may demonstrate the effectiveness Limited firewall.
The concept that makes it a good idea is that of a solid shell enclosing a soft filling this provides security only if there is no way to get to 'internal. Today, this may be unrealistic.
The virus scanning e-mail seems to be winning for Windows sites. If nothing else, throw away all the infected e-mail at the gateway can save a lot of bandwidth.
Instead try to scan the FTP download is useless for most of the sites.
Transitive trust
Moreover, another problem with the firewall is the concept of transitive trust, in every way you like it or not. The concept of transitive trust is very simple: if A trusts B through its firewall, and B in turn trusts C, willy-nilly (and always assuming that he knows it), then A trusts C.
Finally, the firewall may contain errors or not work as expected. The best management can do nothing to oppose a firewall that does not work as expected.

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