Secure Internet Applications in .NET

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Secure Internet Applications
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Application servers, a platform on which the application s code runs, typically in the form of Web components such as servlets and business components such as EJBs.
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The first scenario shows a successful client request for some business functionality. The client browser request is filtered by the external router to ensure that it is destined for a valid server. The request is forwarded to the firewall to undergo more rigorous checking. If the firewall is happy with the protocol use, the request goes onwards to the server requested by the client.
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:Client Browser
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:External Router
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:Firewall Check address
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:Web Server
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OK: server Forward Check protocol
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OK Forward
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Filtering a client request in a DMZ
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The second scenario shows a malicious client call being blocked by the firewall. The client browser request is again filtered by the external router to ensure that it is destined for a valid server. The request is then forwarded to the firewall to undergo more rigorous checking. At this stage, the firewall detects invalid protocol use maybe some form of protocol-based attack, or an attempt to flood the server. The request is rejected and the suspicious activity is logged. See figure on page 453.
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Since the request handling and business functionality must be separated by a filter, it is best to use DEDICATED WEB and APPLICATION SERVERS [Dys04] where any
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Demilitarized Zone 453
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:Client Browser
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:External Router
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:Firewall Check address
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:Web Server
OK: server Forward Check protocol
BAD Reject Reject
Rejecting a client request in a DMZ
programmatic functionality, whether business or presentation, is deployed on an application server that is physically separate from the Web server. These application servers can be placed on a more protected network than the Web servers. This protected network will have easier (possibly direct) access to the corporate information and services required by the Web-based application. The external router should be configured to deny any attempted access to any network addresses outside of those known in the DMZ. To increase security, any requests with a destination address that does not match the Web server address (or that of the Web server cluster) may be rejected. The external router may also reject requests based on the port number of the request, for example rejecting any request that is not for port 80. The external router will therefore block direct attacks on the internal router, and possibly the firewall. The Web servers will be built solely for the purpose of delivering static Web content or proxying requests through to the application servers. These Web servers should be locked down (or hardened ) by removing unnecessary functionality. Such hardening helps to prevent other, unintended, access to the servers. The internal router will limit network traffic to connections between the Web servers on the DMZ and specific internal servers, such as the application servers, using a fixed set of protocols. This restriction reduces the risk of attack on other internal systems. The use of an internal router helps to reduce the risk of attack should the external router be breached. Because of this threat, no traffic should be allowed directly from the external router to the internal router.
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Secure Internet Applications
The whole operation of the routers and the traffic filtering may be controlled from a machine running specific firewall software. This makes it easier to apply consistent rules to the routers and to use statistical analysis to detect potential attacks. The firewall applies more sophisticated traffic filtering rules to detect more complex attacks. Depending on the type of firewall, the network traffic may or may not pass through the firewall itself. Because the number of servers exposed to the outside world is reduced, it means that fewer parts of the system need a high level of security. In the scenario described, the application servers will not need to be hardened to the same level as the Web servers. To access those servers not directly exposed (and hence less securely configured), any attacker will have to breach several security elements that form part of the DMZ. Hopefully, they will set off various intruder alerts as they do so if, indeed, they are capable of doing so. Applying a DMZ to a system is a good way to provide protection for the system. However, you must remember that protecting the platforms on which the system is built is only part of the solution. Since security is a matter of policy as well as technology, all protection mechanisms such as a DMZ must be backed up with appropriate procedures and processes to ensure that the level of security remains high see the patterns in 6, Enterprise Security and Risk Management. If there is a high level of concern about possible attacks on the system, an intrusion detection system (IDS) (see INTRUSION DETECTION REQUIREMENTS (388)) may also be used. An IDS monitors the traffic on the network, or on specific hosts, looking for suspicious activity. If the IDS identifies a pattern of network or host traffic that indicates an attack is underway, it will notify the system administrators. An IDS could be used on the DMZ itself, on the internal network, or both.