D-Link DFL-2500 User Guide - Page 31
functionality of a firewall does not normally provide such protection.
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12 Chapter 3. Firewall Principles apart from better written software, is to disable such services or limiting surfing to less sensitive computers. • HTML pages that link in the contents of local files when they are opened without scripts. Such pages can, often with the help of unsuspecting local users who are lured into "helping" the page by clicking on a button, send the linked file onwards to an unknown Internet server. • Documents sent by email that contain hostile scripts which are activated once the document is opened. Possible ways to protect your system against this form of attack include avoiding using browser-based email software or disabling scripting and introducing mail gateways that can block scripts and other executable code. • Buffer overruns, which firewalls only rarely provide protection against. Buffer overruns can occur in any application, with a net result of intruders being able to coax protected computers into executing any command. Here, the only solution is to ensure that only well-written applications, which are specifically designed to be immune to this form of attack are installed and used. Unfortunately, most current software is not written with this problem in mind. At the time of writing, we are of the opinion that this poses the greatest technical threat of all forms of network-based attack, as almost all software is susceptible to buffer overruns. • Viruses and Trojan horses. A firewall can of course be connected to virus scanners, mail gateways and other similar devices in order to increase security, but it should be noted that the fundamental functionality of a firewall does not normally provide such protection. • Even if the firewall is connected to a virus scanner, it is possible that attacking viruses could be so well hidden that the scanner would be unable to detect them. In addition, a virus scanner can only detect viruses it recognizes. If somebody designs a virus specifically for attacking your systems or those of a small group of people, or if the trojan or virus in question has not been in circulation long enough for it to become well known, the virus scanner will not recognize it. At present, the most common targets for data-driven attacks are: • Public servers such as mail servers, DNS servers and web servers. Web servers are clearly over-represented in this category due to their enormous complexity. D-Link Firewalls User's Guide
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