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Attack Surface Reduction – An often overlooked element of web application security

In industry surveys ranging from the Symantec Threat Report to Gartner analyst reports, application security is constantly cited as the most significant area of risk for enterprises and the most prevalent threat vector for cyber crime. It certainly makes sense, why bother to spend time on reconnaissance when the front door is wide open?

Many organizations have begun to spend a great deal of energy and money to secure applications. Popular approaches include code review, threat modeling, source code analysis and black box testing. Often overlooked is the rather fundamental practice of reducing the attack surface of the application.

During development and configuration of a system and the associated application the software must typically expose both customer and business assets through network ports, database access, APIs, web services and the user interface. The entire collection of entry points in a product is called its Attack Surface. These form the ways in which an adversary can attack a system. A big attack surface generally means big security issues, or often more time and budget dollars dedicated to protecting the system. It’s also important to remember that channels to local resources are not the only vectors for attack, remote resources must also be kept in mind.

Generally, when a software system is architected, implemented and configured, the top of mind issue is about providing useful functionality that meets business goals. From a security point of view, however, the design and deployment teams must also think about turning things off as well as on. From a design standpoint this involves reducing the amount of code that is executing by default, running with user privileges rather system, reducing functionality and data accessible to unauthenticated users and limiting the damage if access points are exploited. From a system configuration point of view this involves turning off unnecessary services, providing access only to required authorized users on specific subnets, and using strong ACLs to control access to resources.

The security community has done a relatively good job with respect to understanding which attack vectors are more likely targeted by adversaries.  Given that perspective, keep the following in mind:

• Minimize the use of scripting engines and controls such as ActiveX, JavaScript or VBScript.
• Avoid symbolic links as these are likely targets.
• Restrict file permissions to the fullest extent possible.
• Minimize the number of services that must run as root.
• Keep up with vulnerability research and build an effective patch process.

A useful practice is to put together a design guideline for developers suitable to your design environment and the business and security requirements associated with your system. Further, at deployment time, a security configuration guide and checklist of security best practices is recommended. Interestingly, some in the industry such as SAP have invested even more heavily in this area. SAP Labs has developed and begun pilot deployment of an Eclipse extension that uses a more formal process to measure attack surfaces. Their method involves summing the damage and potential-effort ratios (DER) of relevant resources. The relevant resources of an application include its channels, such as TCP ports; methods, such as API calls; and data, whether persistent, in memory, or in transit. The DER of a resource is the ratio of potential damage to the effort required to breach the resource. The SAP tool discovers application resources and combines that data with DER numbers to generate attack surface metrics for software components. While the discovery of resources is fully automated, the tool requires context specific configuration based on experience, judgment, and a threat modeling process.

Given the complex nature of deploying SAP software securely it’s not surprising that they have invested in this area. However, all systems can benefit immediately from simply measuring the potential avenues of attack and understanding the impact. This practice can be particularly beneficial for complex systems with many configuration decisions. In the healthcare sector, where Redspin has done many information security assessment projects, a good example is healthcare information exchange systems. A further example with broad deployment across many sectors is CRM systems.

Whether through design reviews, deployment guides or development tools, the practice of reducing the attack surface associated with an application has the potential to quickly yield a high return on investment.

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