Threat Risk Modeling

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When you start a web application design, it is essential to apply threat risk modeling; otherwise you will squander resources, time and money on useless controls that fail to focus on the real risks.

The method used to assess risk is not nearly as important as actually performing a structured threat risk modeling. Microsoft notes that the single most important factor in their security improvement program was the corporate adoption of threat risk modeling .

OWASP recommends Microsoft’s threat modeling process because it works well for addressing the unique challenges facing web application security and is simple to learn and adopt by designers, developers, code reviewers, and the quality assurance team.

The following sections provide some overview information (or see Section 6.9, Further Reading, for additional resources).

Threat Risk Modeling

Threat risk modeling is an essential process for secure web application development. It allows organizations to determine the correct controls and to produce effective countermeasures within budget. For example, there is little point in spending $100,000 for fraud control for a system that has negligible fraud risk.

Performing threat risk modeling using the Microsoft Threat Modeling Process

The threat risk modeling process has five steps , enumerated below and shown graphically in Figure 1. They are:

  1. Identify Security Objectives
  2. Survey the Application
  3. Decompose it
  4. Identify Threats
  5. Identify Vulnerabilities

Figure 1: Threat Model Flow

Let’s consider the steps in more detail.

Identify Security Objectives

The business (or project management) leadership, in concert with the software development and quality assurance teams, all need to understand the security objectives. To facilitate this, start by breaking down the application’s security objectives into the following categories:

  • Identity: Does the application protect user identity from abuse? Are there adequate controls in place to ensure evidence of identity (as required for many banking applications?)
  • Financial: Assess the level of risk the organization is prepared to absorb in remediation, as a potential financial loss. For example, forum software may have a lower estimated financial risk than an Internet banking application.
  • Reputation: Quantify or estimate of the loss of reputation derived from the application being misused or successfully attacked.
  • Privacy and Regulatory: To what extent will the application have to protect user data? Forum software by its nature is public, but a tax preparation application is subject to tax regulations and privacy legislation requirements in most countries.
  • Availability Guarantees: Is the application required to be available per a Service Level Agreement (SLA) or similar guarantee? Is it a nationally protected infrastructure? To what level will the application have to be available? High availability techniques are significantly more expensive, so applying the correct controls up front will save a great deal of time, resources, and money.

This is by no means an exhaustive list, but it gives an idea of some of the business risk decisions leading into selecting and building security controls.

Other sources of risk guidance come from:

  • Laws (such as privacy or finance laws)
  • Regulations (such as banking or e-commerce regulations)
  • Standards (such as ISO 17799)
  • Legal Agreements (such as payment card industry standards or merchant agreements)
  • Corporate Information Security Policy

Application Overview

Once the security objectives have been defined, analyze the application design to identify the components, data flows, and trust boundaries.

Do this by surveying the application’s architecture and design documentation. In particular, look for UML component diagrams. Such high level component diagrams are generally sufficient to understand how and why data flows to various places. For example, data movement across a trust boundary (such as from the Internet to the web tier, or from the business logic to the database server), needs to be carefully analyzed, whereas data that flows within the same trust level does not need as much scrutiny.

Decompose Application

Once the application architecture is understood then decompose it further, to identify the features and modules with a security impact that need to be evaluated. For example, when investigating the authentication module, it is necessary to understand how data enters the module, how the module validates and processes the data, where the data flows, how the data is stored, and what fundamental decisions and assumptions are made by the module.

Identify Threats

It is impossible to write down unknown threats, but it is likewise unlikely that new malware will be created to exploit new vulnerabilities within custom systems. Therefore, concentrate on known risks, which can be easily demonstrated using tools or techniques from Bugtraq .

Microsoft suggests two different approaches for writing up threats. One is a threat graph, as shown in Figure 2, and the other is a structured list, as shown in Figure 3.

Figure 2: Threat Graph

Typically, a threat graph imparts more information quickly but it takes longer to construct, while a structured list is easier to create but it will take longer for the threat impacts to become obvious.

  1. Attacker may be able to read other user’s messages
  2. User may not have logged off on a shared PC
  3. Data validation may allow SQL injection
  4. Implement data validation
  5. Authorization may fail, allowing unauthorized access
  6. Implement authorization checks
  7. Browser cache may contain contents of message
  8. Implement anti-caching directive in HTTP headers
  9. If eavesdropping risk is high, use SSL

Note that it takes a motivated attacker to exploit a threat; they generally want something from your application or to obviate controls. To understand the relevant threats, use the following categories to understand who might attack the application:

  • Accidental Discovery: An ordinary user stumbles across a functional mistake in your application, just using a web browser, and gains access to privileged information or functionality.
  • Automated Malware: Programs or scripts, which are searching for known vulnerabilities, and then report them back to a central collection site.
  • The Curious Attacker: a security researcher or ordinary user, who notices something wrong with the application, and decides to pursue further.
  • Script Kiddies: Common renegades, seeking to compromise or deface applications for collateral gain, notoriety, or a political agenda, perhaps using the attack categories described in the OWASP Web Application Penetration Checklist.
  • The Motivated Attacker: Potentially, a disgruntled staff member with inside knowledge or a paid professional attacker.
  • Organized Crime: Criminals seeking high stake payouts, such as cracking e-commerce or corporate banking applications, for financial gain.

It is vital to understand the level of attacker you are defending against. For example, a motivated attacker, who understands your internal processes is often more dangerous than script kiddies.

STRIDE

STRIDE is a methodology for identifying known threats. The STRIDE acronym is formed from the first letter of each of the following categories.

Spoofing Identity

“Identity spoofing” is a key risk for applications that have many users but provide a single execution context at the application and database level. In particular, users should not be able to become any other user or assume the attributes of another user.

Tampering with Data

Users can potentially change data delivered to them, return it, and thereby potentially manipulate client-side validation, GET and POST results, cookies, HTTP headers, and so forth. The application should not send data to the user, such as interest rates or periods, which are obtainable only from within the application itself. The application should also carefully check data received from the user and validate that it is sane and applicable before storing or using it.

Repudiation

Users may dispute transactions if there is insufficient auditing or recordkeeping of their activity. For example, if a user says, “But I didn’t transfer any money to this external account!”, and you cannot track his/her activities through the application, then it is extremely likely that the transaction will have to be written off as a loss.

Therefore, consider if the application requires non-repudiation controls, such as web access logs, audit trails at each tier, or the same user context from top to bottom. Preferably, the application should run with the user’s privileges, not more, but this may not be possible with many off-the-shelf application frameworks.

Information Disclosure

Users are rightfully wary of submitting private details to a system. If it is possible for an attacker to publicly reveal user data at large, whether anonymously or as an authorized user, there will be an immediate loss of confidence and a substantial period of reputation loss. Therefore, applications must include strong controls to prevent user ID tampering and abuse, particularly if they use a single context to run the entire application.

Also, consider if the user’s web browser may leak information. Some web browsers may ignore the no caching directives in HTTP headers or handle them incorrectly. In a corresponding fashion, every secure application has a responsibility to minimize the amount of information stored by the web browser, just in case it leaks or leaves information behind, which can be used by an attacker to learn details about the application, the user, or to potentially become that user.

Finally, in implementing persistent values, keep in mind that the use of hidden fields is insecure by nature. Such storage should not be relied on to secure sensitive information or to provide adequate personal privacy safeguards.

Denial of Service

Application designers should be aware that their applications may be subject to a denial of service attack. Therefore, the use of expensive resources such as large files, complex calculations, heavy-duty searches, or long queries should be reserved for authenticated and authorized users, and not available to anonymous users.

For applications that do not have this luxury, every facet of the application should be engineered to perform as little work as possible, to use fast and few database queries, to avoid exposing large files or unique links per user, in order to prevent simple denial of service attacks.

Elevation of Privilege

If an application provides distinct user and administrative roles, then it is vital to ensure that the user cannot elevate his/her role to a higher privilege one. In particular, simply not displaying privileged role links is insufficient. Instead, all actions should be gated through an authorization matrix, to ensure that only the permitted roles can access privileged functionality.

DREAD

The DREAD acronym is formed from the first letter of each category below.

DREAD modeling influences the thinking behind setting the risk rating, and is also used directly to sort the risks. The DREAD algorithm, shown below, is used to compute a risk value, which is an average of all five categories.

Risk_DREAD = (DAMAGE + REPRODUCABILITY + EXPLOITABILITY + AFFECTED USERS + DISCOVERABILITY) / 5

The calculation always produces a number between 0 and 10; the higher the number, the more serious the risk.

Here are some examples of how to quantify the DREAD categories.

Damage Potential

  • If a threat exploit occurs, how much damage will be caused?
    • 0 = Nothing
    • 5 = Individual user data is compromised or affected.
    • 10 = Complete system or data destruction

Reproducibility

  • How easy is it to reproduce the threat exploit?
    • 0 = Very hard or impossible, even for administrators of the application.
    • 5 = One or two steps required, may need to be an authorized user.
    • 10 = Just a web browser and the address bar is sufficient, without authentication.

Exploitability

  • What is needed to exploit this threat?
    • 0 = Advanced programming and networking knowledge, with custom or advanced attack tools.
    • 5 = Malware exists on the Internet, or an exploit is easily performed, using available attack tools.
    • 10 = Just a web browser

Affected Users

  • How many users will be affected?
    • 0 = None
    • 5 = Some users, but not all
    • 10 = All users

Discoverability

  • How easy is it to discover this threat?
    • 0 = Very hard to impossible; requires source code or administrative access.
    • 5 = Can figure it out by guessing or by monitoring network traces.
    • 9 = Details of faults like this are already in the public domain and can be easily discovered using a search engine.
    • 10 = The information is visible in the web browser address bar or in a form.

Note: When performing a security review of an existing application, “Discoverability” will often be set to 10 by convention, as it is assumed the threat issues will be discovered.



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