11.2. Types of Assessments
Now that you have ensured that your Kali environment is ready, the next step is defining exactly what sort of assessment you are conducting. At the highest level, we may describe four types of assessments: a vulnerability assessment, a compliance test, a traditional penetration test, and an application assessment. An engagement may involve various elements of each type of assessment but it is worth describing them in some detail and explaining their relevance to your Kali Linux build and environment.
Before delving into the different types of assessments, it is important to first note the difference between a vulnerability and an exploit.
A vulnerability is defined as a flaw that, when taken advantage of, will compromise the confidentiality, integrity, or availability of an information system. There are many different types of vulnerabilities that can be encountered, including:
- File Inclusion: File inclusion vulnerabilities in web applications allow you to include the contents of a local or remote file into the computation of a program. For example, a web application may have a "Message of the day" function that reads the contents of a file and includes it in the web page to display it to the user. When this type of feature is programmed incorrectly, it can allow an attacker to modify their web request to force the site to include the contents of a file of their choosing.
- SQL Injection: A SQL injection attack is one where the input validation routines for the program are bypassed, allowing an attacker to provide SQL commands for the targeted program to execute. This is a form of command execution that can lead to potential security issues.
- Buffer Overflow: A buffer overflow is a vulnerability that bypasses input validation routines to write data into a buffer's adjacent memory. In some cases, that adjacent memory location may be critical to the operation of the targeted program and control of code execution can be obtained through careful manipulation of the overwritten memory data.
- Race Conditions: A race condition is a vulnerability that takes advantage of timing dependencies in a program. In some cases, the workflow of a program depends on a specific sequence of events to occur. If you can alter this sequence of events, that may lead to a vulnerability.
An exploit, on the other hand, is software that, when used, takes advantage of a specific vulnerability, although not all vulnerabilities are exploitable. Since an exploit must change a running process, forcing it to make an unintended action, exploit creation can be complex. Furthermore, there are a number of anti-exploit technologies in modern computing platforms that have been designed to make it harder to exploit vulnerabilities, such as Data Execution Prevention (DEP) and Address Space Layout Randomization (ASLR). However, just because there is no publicly-known exploit for a specific vulnerability, that does not mean that one does not exist (or that one can not be created). For example, many organizations sell commercialized exploits that are never made public, so all vulnerabilities must be treated as potentially exploitable.
A vulnerability is considered a weakness that could be used in some manner to compromise the confidentiality, integrity, or availability of an information system. In a vulnerability assessment, your objective is to create a simple inventory of discovered vulnerabilities within the target environment. This concept of a target environment is extremely important. You must be sure to stay within the scope of your client's target network and required objectives. Creeping outside the scope of an assessment can cause an interruption of service, a breach of trust with your client, or legal action against you and your employer.
Due to its relative simplicity, a vulnerability test is often completed in more mature environments on a regular basis as part of demonstrating their due diligence. In most cases, an automated tool, such as the ones in the Vulnerability Analysis and Web Applications categories of the Kali Tools site and Kali desktop Applications menu, is used to discover live systems in a target environment, identify listening services, and enumerate them to discover as much information as possible such as the server software, version, platform, and so on.
This information is then checked for known signatures of potential issues or vulnerabilities. These signatures are made up of data point combinations that are intended to represent known issues. Multiple data points are used, because the more data points you use, the more accurate the identification. A very large number of potential data points exist, including but not limited to:
- Operating System Version: It is not uncommon for software to be vulnerable on one operating system version but not on another. Because of this, the scanner will attempt to determine, as accurately as possible, what operating system version is hosting the targeted application.
- Patch Level: Many times, patches for an operating system will be released that do not increase the version information, but still change the way a vulnerability will respond, or even eliminate the vulnerability entirely.
- Processor Architecture: Many software applications are available for multiple processor architectures such as Intel x86, Intel x64, multiple versions of ARM, UltraSPARC, and so on. In some cases, a vulnerability will only exist on a specific architecture, so knowing this bit of information can be critical for an accurate signature.
- Software Version: The version of the targeted software is one of the basic items that needs to be captured to identify a vulnerability.
These, and many other data points, will be used to make up a signature as part of a vulnerability scan. As expected, the more data points that match, the more accurate the signature will be. When dealing with signature matches, you can have a few different potential results:
- True Positive: The signature is matched and it captures a true vulnerability. These results are the ones you will need to follow up on and correct, as these are the items that malicious individuals can take advantage of to hurt your organization (or your client's).
- False Positive: The signature is matched; however the detected issue is not a true vulnerability. In an assessment, these are often considered noise and can be quite frustrating. You never want to dismiss a true positive as a false positive without more extensive validation.
- True Negative: The signature is not matched and there is no vulnerability. This is the ideal scenario, verifying that a vulnerability does not exist on a target.
- False Negative: The signature is not matched but there is an existing vulnerability. As bad as a false positive is, a false negative is much worse. In this case, a problem exists but the scanner did not detect it, so you have no indication of its existence.
As you can imagine, the accuracy of the signatures is extremely important for accurate results. The more data that are provided, the greater the chance there is to have accurate results from an automated signature-based scan, which is why authenticated scans are often so popular.
With an authenticated scan, the scanning software will use provided credentials to authenticate to the target. This provides a deeper level of visibility into a target than would otherwise be possible. For instance, on a normal scan you may only detect information about the system that can be derived from listening services and the functionality they provide. This can be quite a bit of information sometimes but it can't compete with the level and depth of data that will be obtained if you authenticate to the system and comprehensively review all installed software, applied patches, running processes, and so on. This breadth of data is useful for detecting vulnerabilities that otherwise may not have been discovered.
A well-conducted vulnerability assessment presents a snapshot of potential problems in an organization and provides metrics to measure change over time. This is a fairly lightweight assessment, but even still, many organizations will regularly perform automated vulnerability scans in off-hours to avoid potential problems during the day when service availability and bandwidth are most critical.
As previously mentioned, a vulnerability scan will have to check many different data points in order to get an accurate result. All of these different checks can create load on the target system as well as consume bandwidth. Unfortunately, it is difficult to know exactly how many resources will be consumed on the target as it depends on the number of open services and the types of checks that would be associated with those services. This is the cost of doing a scan; it is going to occupy system resources. Having a general idea of the resources that will be consumed and how much load the target system can take is important when running these tools.
When a vulnerability scan is finished, the discovered issues are typically linked back to industry standard identifiers such as CVE number, EDB-ID, and vendor advisories. This information, along with the vulnerabilities CVSS score, is used to determine a risk rating. Along with false negatives (and false positives), these arbitrary risk ratings are common issues that need to be considered when analyzing the scan results.
Since automated tools use a database of signatures to detect vulnerabilities, any slight deviation from a known signature can alter the result and likewise the validity of the perceived vulnerability. A false positive incorrectly flags a vulnerability that does not exist, while a false negative is effectively blind to a vulnerability and does not report it. Because of this, a scanner is often said to only be as good as its signature rule base. For this reason, many vendors provide multiple signature sets: one that might be free to home users and another fairly expensive set that is more comprehensive, which is generally sold to corporate customers.
The other issue that is often encountered with vulnerability scans is the validity of the suggested risk ratings. These risk ratings are defined on a generic basis, considering many different factors such as privilege level, type of software, and pre- or post-authentication. Depending on your environment, these ratings may or may not be applicable so they should not be accepted blindly. Only those well-versed in the systems and the vulnerabilities can properly validate risk ratings.
While there is no universally defined agreement on risk ratings, NIST Special publication 800-30 is recommended as a baseline for evaluation of risk ratings and their accuracy in your environment. NIST SP 800-30 defines the true risk of a discovered vulnerability as a combination of the likelihood of occurrence and the potential impact.
According to the National Institute of Standards and Technology (NIST), the likelihood of occurrence is based on the probability that a particular threat is capable of exploiting a particular vulnerability, with possible ratings of Low, Medium, or High.
- High: the potential adversary is highly skilled and motivated and the measures that have been put in place to protect against the vulnerability are insufficient.
- Medium: the potential adversary is motivated and skilled but the measures put in place to protect against the vulnerability may impede their success.
- Low: the potential adversary is unskilled or lacks motivation and there are measures in place to protect against the vulnerability that are partially or completely effective.
The level of impact is determined by evaluating the amount of harm that could occur if the vulnerability in question were exploited or otherwise taken advantage of.
- High: taking advantage of the vulnerability could result in very significant financial losses, serious harm to the mission or reputation of the organization, or even serious injury, including loss of life.
- Medium: taking advantage of the vulnerability could lead to financial losses, harm to the mission or reputation of the organization, or human injury.
- Low: taking advantage of the vulnerability could result in some degree of financial loss or impact to the mission and reputation of the organization.
Once the likelihood of occurrence and impact have been determined, you can then determine the overall risk rating, which is defined as a function of the two ratings. The overall risk can be rated Low, Medium, or High, which provides guidance to those responsible for securing and maintaining the systems in question.
- High: There is a strong requirement for additional measures to be implemented to protect against the vulnerability. In some cases, the system may be allowed to continue operating but a plan must be designed and implemented as soon as possible.
- Medium: There is a requirement for additional measures to be implemented to protect against the vulnerability. A plan to implement the required measures must be done in a timely manner.
- Low: The owner of the system will determine whether to implement additional measures to protect against the vulnerability or they can opt to accept the risk instead and leave the system unchanged.
With so many factors making up the true risk of a discovered vulnerability, the pre-defined risk ratings from tool output should only be used as a starting point to determine the true risk to the overall organization.
Competently-created reports from a vulnerability assessment, when analyzed by a professional, can provide an initial foundation for other assessments, such as compliance penetration tests. As such, it is important to understand how to get the best results possible from this initial assessment.
Kali makes an excellent platform for conducting a vulnerability assessment and does not need any special configuration. In the Kali Applications menu, you will find numerous tools for vulnerability assessments in the Kali Linux Tools Listing, The Kali Linux Official Documentation site, and the free Metasploit Unleashed course provide excellent resources for using Kali Linux during a vulnerability assessment., , and categories. Several sites, including the aforementioned
The next type of assessment in order of complexity is a compliance- based penetration test. These are the most common penetration tests as they are government- and industry-mandated requirements based on a compliance framework the entire organization operates under.
While there are many industry-specific compliance frameworks, the most common would likely be Payment Card Industry Data Security Standard (PCI DSS), a framework dictated by payment card companies that retailers processing card-based payments must comply with. However, a number of other standards exist such as the Defense Information Systems Agency Security Technical Implementation Guides (DISA STIG), Federal Risk and Authorization Management Program (FedRAMP), Federal Information Security Management Act (FISMA), and others. In some cases, a corporate client may request an assessment, or ask to see the results of the most recent assessment for various reasons. Whether ad-hoc or mandated, these sorts of assessments are collectively called compliance-based penetration tests, or simply "compliance assessments" or "compliance checks".
A compliance test often begins with a vulnerability assessment. In the case of PCI compliance auditing, a vulnerability assessment, when performed properly, can satisfy several of the base requirements, including: "2. Do not use vendor-supplied defaults for system passwords and other security parameters" (for example, with tools from the menu category), "11. Regularly test security systems and processes" (with tools from the category) and others. Some requirements, such as "9. Restrict physical access to cardholder data" and "12. Maintain a policy that addresses information security for all personnel" don't seem to lend themselves to traditional tool-based vulnerability assessment and require additional creativity and testing.
Despite the fact that it might not seem straight-forward to use Kali Linux for some elements of a compliance test, the fact is that Kali is a perfect fit in this environment, not just because of the wide range of security-related tools, but because of the open-source Debian environment it is built on, allowing for the installation of a wide range of tools. Searching the package manager with carefully chosen keywords from whichever compliance framework you are using is almost certain to turn up multiple results. As it stands, many organizations use Kali Linux as the standard platform for these exact sorts of assessments.
A traditional penetration test has become a difficult item to define, with many working from different definitions, depending on the space they operate in. Part of this market confusion is driven by the fact that the term "Penetration Test" has become more commonly used for the previously mentioned compliance-based penetration test (or even a vulnerability assessment) where, by design, you are not delving too deep into the assessment because that would go beyond the minimum requirements.
For the purposes of this section, we will side-step that debate and use this category to cover assessments that go beyond the minimum requirements; assessments that are designed to actually improve the overall security of the organization.
As opposed to the previously-discussed assessment types, penetration tests don't often start with a scope definition, but instead a goal such as, "simulate what would happen if an internal user is compromised" or, "identify what would happen if the organization came under focused attack by an external malicious party." A key differentiator of these sorts of assessments is that they don't just find and validate vulnerabilities, but instead leverage identified issues to uncover the worst-case scenario. Instead of relying solely on heavy vulnerability scanning toolsets, you must follow up with validation of the findings through the use of exploits or tests to eliminate false positives and do your best to detect hidden vulnerabilities or false negatives. This often involves exploiting vulnerabilities discovered initially, exploring the level of access the exploit provides, and using this increased access as leverage for additional attacks against the target.
This requires critical review of the target environment along with manual searching, creativity, and outside-the-box thinking to discover other avenues of potential vulnerability and ultimately using other tools and tests outside those found by the heavier vulnerability scanners. Once this is completed, it is often necessary to start the whole process over again multiple times to do a full and complete job.
Even with this approach, you will often find that many assessments are composed of different phases. Kali makes it easy to find programs for each phase by way of the Kali Menu:
- Information Gathering: In this phase, you focus on learning as much as possible about the target environment. Typically, this activity is non-invasive and will appear similar to standard user activity. These actions will make up the foundation of the rest of the assessment and therefore need to be as complete as possible. Kali's category has dozens of tools to uncover as much information as possible about the environment being assessed.
- Vulnerability Discovery: This will often be called "active information gathering", where you don't attack but engage in non-standard user behaviour in an attempt to identify potential vulnerabilities in the target environment. This is where the previously-discussed vulnerability scanning will most often take place. The programs listed in the , , , and categories will be useful for this phase.
- Exploitation: With the potential vulnerabilities discovered, in this phase you try to exploit them to get a foothold into the target. Tools to assist you in this phase can be found in the , , , and categories.
- Pivoting and Exfiltration: Once the initial foothold is established, further steps have to be completed. These are often escalating privileges to a level adequate to accomplish your goals as an attacker, pivoting into other systems that may not have been previously accessible to you, and exfiltrating sensitive information from the targeted systems. Refer to the , , , and categories to help with this phase.
- Reporting: Once the active portion of the assessment is completed, you then have to document and report on the activities that were conducted. This phase is often not as technical as the previous phases, however it is highly important to ensure your client gets full value from the work completed. The category contains a number of tools that have proven useful in the reporting phase.
In most cases, these assessments will be very unique in their design as every organization will operate with different threats and assets to protect. Kali Linux makes a very versatile base for these sorts of assessments and this is where you can really take advantage of the many Kali Linux customization features. Many organizations that conduct these sorts of assessments will maintain highly customized versions of Kali Linux for internal use to speed up deployment of systems before a new assessment.
Customizations that organizations make to their Kali Linux installations will often include:
- Pre-installation of commercial packages with licensing information. For instance, you may have a package such as a commercial vulnerability scanner that you would like to use. To avoid having to install this package with each build, you can do it once and have it show up in every Kali deployment you do.
- Pre-configured connect-back virtual private networks (VPN). These are very useful in leave-behind devices that allow you to conduct "remote internal" assessments. In most cases, these systems will connect back to an assessor-controlled system, creating a tunnel that the assessor can use to access internal systems. The Kali Linux ISO of Doom is an example of this exact type of customization.
- Pre-installed internally-developed software and tools. Many organizations will have private toolsets, so setting these up once in a customized Kali install saves time.
- Pre-configured OS configurations such as host mappings, desktop wallpaper, proxy settings, etc. Many Kali users have specific settings they like to have tweaked just so. If you are going to do a re-deployment of Kali on a regular basis, capturing these changes makes a lot of sense.
While most assessments have a broad scope, an application assessment is a specialty that is narrowly focused on a single application. These sorts of assessments are becoming more common due to the complexity of mission-critical applications that organizations use, many of which are built in-house. An application assessment is usually added on to a broader assessment, as required. Applications that may be assessed in this manner include, but are not limited to:
- Web applications: The most common externally-facing attack surface, web applications make great targets simply because they are accessible. Often, standard assessments will find basic problems in web applications, however a more focused review is often worth the time to identify issues relating to the workflow of the application. The kali-linux-web metapackage has a number of tools to help with these assessments.
- Compiled desktop applications: Server software is not the only target; desktop applications also make up a wonderful attack surface. In years past, many desktop applications such as PDF readers or web-based video programs were highly targeted, forcing them to mature. However, there are still a wide number of desktop applications that are a wealth of vulnerabilities when properly reviewed.
- Mobile applications: As mobile devices become more popular, mobile applications will become that much more of a standard attack surface in many assessments. This is a fast moving target and methodologies are still maturing in this area, leading to new developments practically every week. Tools related to the analysis of mobile applications can be found in the menu category.
Application assessments can be conducted in a variety of different ways. As a simple example, an application-specific automated tool can be run against the application in an attempt to identify potential issues. These tools will use application-specific logic in an attempt to identify unknown issues rather than just depending on a set of known signatures. These tools must have a built-in understanding of the application's behaviour. A common example of this would be a web application vulnerability scanner such as Burp Suite, directed against an application that first identifies various input fields and then sends common SQL injection attacks to these fields while monitoring the application's response for indications of a successful attack.
In a more complex scenario, an application assessment can be conducted interactively in either a black box or white box manner.
- Black Box Assessment: The tool (or assessor) interacts with the application with no special knowledge or access beyond that of a standard user. For instance, in the case of a web application, the assessor may only have access to the functions and features that are available to a user that has not logged into the system. Any user accounts used would be ones where a general user can self-register the account. This would prevent the attacker from being able to review any functionality that is only available to users that need to be created by an administrator.
- White Box Assessment: The tool (or assessor) will often have full access to the source code, administrative access to the platform running the application, and so on. This ensures that a full and comprehensive review of all application functionality is completed, regardless of where that functionality lives in the application. The trade-off with this is that the assessment is in no way a simulation of actual malicious activity.
There are obviously shades of grey in between. Typically, the deciding factor is the goal of the assessment. If the goal is to identify what would happen in the event that the application came under a focused external attack, a black box assessment would likely be best. If the goal is to identify and eliminate as many security issues as possible in a relatively short time period, a white box approach may be more efficient.
In other cases, a hybrid approach may be taken where the assessor does not have full access to the application source code of the platform running the application, but user accounts are provisioned by an administrator to allow access to as much application functionality as possible.
Kali is an ideal platform for all manner of application assessments. On a default installation, a range of different application-specific scanners are available. For more advanced assessments, a range of tools, source editors, and scripting environments exist. You may find the Web Application and Reverse Engineering sections of the Kali Tools website helpful.