Excerpt from Network Security Auditing

Chapter 1: The Principles of Auditing

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Risk in the Fourth Dimension

No, this isn’t the section about Euclidean geometry or a discussion about Einstein’s theory of relativity (I know you are disappointed!). Instead, the Fourth dimension as it applies to information security is about time. Time is important in security because it enables us to measure the effectiveness of countermeasures based on how long they are exposed to a particular event. When you think about the security of your data, how well do your countermeasures stand up to a sustained attack? When you want to purchase a safe, time is one of the most important factors in picking the correct one to do the job. If you go out and purchase a safe for your valuables, you have to decide what level of protection you need because no safe is impenetrable. Given enough time and the right resources someone can get into it. Safe manufacturers know this, so they rank their safes based on how long it takes an expert to break in. This is called Net Working Time and is displayed on a sticker or other label to indicate how well the safe did. If it is rated as a TL30, it should take an expert with grinding wheels, high-speed drills, saws, and hammers 30 minutes to get in. Of course, the time could be significantly less if you have a little C4, but you run the risk of an accident and destroying the items inside. A safe with TRL30 should withstand all of the hand tools and a gas-cutting and welding torch. The safe isn’t designed to prevent someone from getting in forever, just long enough for you to detect that he is there and hopefully catch him before he makes off with your valuables.

The concept of using time as a measurement tool for security is not new. Many people have contributed to this concept, but Winn Schwartau was the first to write a book applying these techniques to information security. Schwartau’s book, Time Based Security, details how to use time as a mechanism to determine whether or not the countermeasures are sufficient. This methodology—although not the only one to consider in risk assessment—gives us another tool that puts risk-management strategy into fairly simple terms that can help identify the areas that need the most attention. The following formula can be used to determine exposure time:

Exposure = Detection + Reaction

This technique uses detection and reaction as variables to measure potential exposure time to an incident. If you have a fire, the quicker you can detect it and react to it (put it out), the less damage you will have. Let’s work through a quick example.

Let’s say there is an organization selling vitamins on the Internet through an online shopping cart. The company operates its own web server and database and processes credit cards through its website. A new vulnerability in the web server is discovered and an exploit code is released that can enable an attacker to gain control of the server. On average the security analysts review vulnerability reports every 24 hours. After a vulnerability is detected, it takes about two hours to update the server and bring it back into operation. The formula to represent this scenario would look like this: Detection (24 hours) + Reaction (2 hours) = 26 hours. This means that there is an exposure time of roughly 26 hours. For those 26 hours, any customer placing an order can have his credit card information stolen, or the database itself can be siphoned off by the attacker. Many organizations do not have people dedicated to monitoring these types of events 24/7, so the detection time is drastically different. You can, however, easily see that you have a major problem that requires you to do something to tilt the equation back to the correct direction.

Prevention technologies can be used to decrease detection and reaction time, reducing exposure. An IPS appliance can decrease detection and also reaction time based on new signatures being deployed. If an IPS receives a new signature every 4 hours, then your exposure in the previous example would be cut by 22 hours. Using a web application firewall might completely prevent a successful attack of this nature in the first place, changing the organizations exposure time to zero.

Utilizing time-based security measurement provides a relatively simple method of determining how effective countermeasures are in real-world scenarios.

How, What, and Why You Audit

So far this chapter has spent a lot of time talking about the fundamentals of security, covering many areas that are essential knowledge for someone performing an audit. Auditing is most concerned with risk and how that risk is addressed. Are the controls put in place effective at protecting the assets? The only way to know is to test them. That is why the role of the auditor is so essential to good security. This section discusses the details of the audit process and provides an overview of the types of audits and key aspects that help make an audit successful.

Audit Charter

So what’s the difference between a hacker and an auditor? Permission. The auditing, by nature, includes having access to sensitive data and systems. This function is defined by an audit charter, which is a document that defines the purpose, responsibility, authority, and accountability of the auditing program. This document helps to clearly define the requirement for performing audits and provides justification as to why the auditor should be given access to critical systems. An audit charter usually applies to an internal corporate auditing organization and will include the following:

  • Purpose of the auditing function
    • Create a mission statement.
    • Set goal objectives.
    • Define the scope.
  • Authority
    • Access rights to audited systems.
    • Obtain support of personnel to accomplish audit goals.
    • Use technology to test auditing controls.
    • Perform risk assessment.
  • Responsibility
    • Develop audit plan.
    • Maintain professional expertise.
    • Issue reports on results of audits.
    • Make recommendations for reducing risk.
    • Maintain integrity and professional standards.
  • Accountability
    • Report deficiencies about control effectiveness to executives.
    • Provide reports about the current risk.
    • Ensure the organization complies with standards and legal requirements

Engagement Letter

When an outside party performs an audit, an engagement letter must be obtained. This document functions in a similar manner to the audit charter, but is usually written per project. It includes many of the same items as the audit charter but is more specific about the deliverables of the current engagement. The engagement letter includes:

  • Authority
    • Who contracted for the audit
    • Rights of access to systems
    • The signature of the company executive
  • Responsibility
    • Scope of the audit
    • Specific deliverables
  • Accountability
    • Who is to receive the final report
    • Agreed upon completion dates

Types of Audits

Audits can be broken down into a number of types, from the simple analysis of security architecture based on opinion, to a full-blown, end-to-end audit against a security framework such as ISO27001. The difference between types of audits is in what the auditor based the findings on and how detailed the audit’s scope is.

Security Review

A security review is when you examine the security posture of an organization based on professional experience and opinion. Think of a security review as a site survey. In this type of examination, you look for issues that stand out as a way to help define the starting point for further activities. Running a vulnerability scanner such as Nessus would fall under this category. The tool generates a list of potential security issues, but the data must be analyzed further to determine on what needs to be acted on. This is the most basic form of security analysis and the primary output is in the form of an opinion. Examples include:

  • Penetration test
  • Vulnerability scan
  • Architecture review
  • Policy review
  • Compliance review
  • Risk analysis

Security Assessment

Security assessments utilize professional opinion and expertise, but they also analyze the output for relevancy and criticality to the organization. The analysis aspect of an assessment attempts to quantify the risk associated with the items discovered to determine the extent of the problem. If you have two servers with the same vulnerability, but one is your financial server, and the other operates as a print server a security assessment would rank the financial server as a high risk and the print server as a lower risk based on the severity and damage potential. The biggest differentiator between an assessment and a review is the depth to which the auditor examines the system and analyzes the results. Examples include:

  • Vulnerability assessment
  • Risk assessment
  • Architecture assessment
  • Policy assessment

Security Audit

A security Audit examines the organization’s security posture against an industry standard (ISO27001 or COBIT) and/or regulatory compliance such as HIPAA or PCI. An audit includes review and assessment; it also conducts a gap analysis against standards to measure how well the organization complies. Audits take into account people, processes, and technologies, and it compares them to a benchmark in a standardized and repeatable way. Examples include:

  • Compliance audit
  • Policy audit
  • Procedure audit
  • Risk audit

The Role of the Auditor

The role of the auditor is to identify, measure, and report on risk. The auditor is not tasked to fix the problem, but to give a snapshot in time of the effectiveness of the security program. An auditor might be asked to make recommendations about what needs to be done to fix a deficiency in a control, but the objective of the auditor is to report on security weakness. Auditors ask the questions, test the controls, and determine if the security policies are followed in a manner that protects the assets the controls are intended to secure by measuring the organization’s activities versus its security best practices.

The auditor functions as an independent advisor and inspector. The auditor is responsible for planning and conducting audits in a manner that is fair and consistent to the people and processes that are examined. Auditors must have appropriate access and cooperation or the audit runs a risk of not being successful or worse, not identifying critical items that could jeopardize key systems. The auditing charter or engagement letter defines the conduct and responsibilities of an auditor.

Depending on how a company’s auditing program is structured, ultimate accountability for the auditor is usually to senior management or the Board of Directors. The auditor must be independent of the business entity being audited or the impartiality of the audit can be called into question. Auditors are usually required to present a report to management about the findings of the audit and also make recommendations about how to reduce the risk identified.

Conflicts of interest can preclude an auditor from conducting an assessment of a particular system or organization. If you were the one that installed the firewall, it doesn’t make sense for you to also be the one to audit it. Auditors are expected to excuse themselves from an audit if they feel that there is a potential for a conflict to exist.

Places Where Audits Occur

Depending on the scope of the audit, an auditor can be asked to examine many different systems and processes. When defining the scope, the specific items to be audited fall under the category of policy, procedure, or control. Some audits are concerned only with policy review and nothing else, whereas other audits might assess all aspects of security by looking at all three areas. Regardless of how detailed the audit becomes, the three categories are not islands unto themselves. They represent interlocking components of the overall security strategy.

Policy Level

Auditing policy entails examining current policy to ascertain whether or not the policy meets the objectives of the business. It should be specific enough while not being so specific that you can’t change your firewall rules without changing the policy. The policy itself should stay consistent regardless of how you accomplish executing the objectives of the policy. Of course an auditor might also find that the organization does not have a policy for a potentially risky business system, which would mean that the auditor would recommend the creation of a policy and give examples based on industry standards. Policy is the cornerstone of security, so care and attention need to be paid in the creation of these documents. Techniques for auditing policy include:

  • Categorize policies into Administrative, Operational, and Technical.
  • Ensure that the policy meets business objectives.
  • Check for compliance to ensure the policy is being followed and enforced.
  • Compare it against best practices (SANS, ISO, and COBIT).
  • Identify gaps.

Procedure Level

Procedures represent how a company implements policy. Here is where all of the detail resides on how the company will go about protecting its assets. From an auditor’s perspective, procedures provide a lot of information in creating checklists to measure how the business applies policy and controls. If a company has a policy that requires all systems to have a personal firewall installed, configured, and active at all times—but does not have a consistent procedure documenting how the firewall is to be configured—then the company will more than likely have a hard time enforcing this policy. These are the kinds of areas that auditors can help with by recognizing deficiencies in policy implementation and recommending solutions to improve security. Techniques for auditing procedures include:

  • Compare procedures to policies to ensure that the procedures follow the spirit of the policies.
  • Check for configuration compliance.
  • Compare procedures with industry standard practices.

Control Level

Many audits and assessments are focused on the control level. Controls can be technical, administrative, or physical, and they can represent a key component in reducing risk. The auditor is concerned with whether or not the control provides a level of protection greater than the level of risk. Techniques for auditing controls include:

  • Test control functionality.
  • Inspect configuration.
  • Inspect logs.

The Auditing Process

The auditing process can be easily broken down into a number of phases. Each phase builds on the last with the ultimate product being a report that documents the findings of the audit. Having a good framework to conduct an audit makes the process run smoothly and helps to eliminate opportunities for mistakes and inconsistencies that reduce the accuracy of the audit. The phases of an audit are:

  • Planning phase: Audit the subject, objective, and scope.
  • Research phase: Plan, audit procedures, and evaluate criteria.
  • Data gathering phase: Gather checklists, tools, and evidence.
  • Data analysis phase: Analyze, map, and recommend.
  • Audit report phase: Write, present, and file the audit report.
  • Follow up phase: Follow up, follow up, and follow up!
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