Threat modeling: tools in practice

We’ve investigated two tools for our threat model. Here is an overview of both tools (from Microsoft) and our experience with them.

Threat Modeling Tool

The first tool supports system modelling with the definition of Entry Points, Trust Levels, Protected Resources, plus some general background information. Data Flows can be authored directory with the tool or imported from Visio. The tool main strength comes however from its Threat Tree modelling features. Threats can be identified and decomposed into a series of step following the same approach as Attack Tree. The tool supports AND and OR semantics when constructing advanced threat tree. E.g.: “Disclose password” can be achieved with either “Brute force password” or “Use default password”. Each step in the threat tree can be rated according to DREAD and mitigation notes can be added.

Microsoft Threat Analysis & Modeling

The second tool supports system modelling – called application decomposition in this case – with the definition of Roles, Data, Components and External dependencies. The traditional data flow are replaced with “application use cases” that can be modelled right form the tool using the predefined components, data and roles. The threat identification and decomposition follows the threat-attack-vulnerability-countermeasures model. The tool comes with existing base of attacks and vulnerability. E.g.:  Attack “SQL injection” is made possible because of “Usage of Dynamic SQL” vulnerability. Threats that have been identified can then be categorized using the CIA (confidentiality, integrity, availability) classification. Threats are not related to attacks or vulnerability but directly to countermeasures. This was a bit awkward to me and I don’t understand exactly the rationale behind this choice. It’s also interesting to note the existence of so-called “Relevancies” that can then be linked to attacks. E.g.: relevancy “Component utilizes HTTP” is linked to attack “Response Splitting”, “Session Hijacking”, and “Repudiation Attack”.
The tool’s ambitions are much bigger and go beyond a simple documentation of the system security. The tool contains analytics and visualization features. Analytics features aim at assessing the system automatically (e.g.: “Data access control matrix”), whereas visualization features help to understand the system from different view points (e.g.: “call flow”, “data flow”, etc.). These features make sense only if the complete system was modelled with the tool, especially the application use cases.
The tool is actively maintained and significant efforts have been invested in it in to promote the Threat Modelling practice.

Conclusion

Threat modelling is not easy. Several approaches can be used and it can be a time consuming activity. We didn’t model the system enough in detail to leverage the analytics and visualization features of the Microsoft Threat Analysis & Modeling tool. We can then not assess the relevance of such an analysis. Speaking generally about the process, our main problem was to decide to which level of details we wanted to go, and then how to organize our findings in a meaningful way.

The basic notion of threats, attack, vulnerability and countermeasures can be vague or overlapping and they must first be defined in the scope of the analysis in a way to ensure a consistency of the threat model. Sample questions to ask prior to start the analysis are:

  •  What will be the granularity of the threats to identify? E.g.: “disclose customer information” vs. “disclose customer document”, “disclose customer number”, “disclose credit card number”. Knowing to which extent the threat analysis will be performed must be defined in advanced and will drive the complete process.
  • What will be the granularity and the nature of the attacks to identify? An attack can be a concrete (e.g.: “password brute force attack”) or abstract (e.g.: “denial of service attack”). In the first case, an attack is a concrete technique that can be applied to exploit vulnerability. In the later case, an attack is an abstraction of a set of existing techniques that are related. Such abstract attack could be refined into a list of concrete attacks (e.g.: “SYN flood”, “XML bomb”, etc.).
  • How to capture generic attack/threats? This again related to the question of granularity of the analysis. Consider for instance the attack “Submit HTTP form twice”. Because it can be applied on all web pages, the list of threats will be an exhaustive listing of the application features: “Place wrong order”, “Rate the item more than once”, etc. Such attack can be capture into generic threats such as “Abuse web application” or “Disrupt system”.  Similarly, all the “Denial of service attacks” will lead to generic threat “Degrade service availability”.

The threat model must no be an exhaustive, useless document. Therefore, the analyst must find a balance between generic attack and threat (and the corresponding generic hardening best practice) and more detailed attacks and threats related to the specificities of the system under study.

Threat modeling: overview

Threat Modelling is a process of assessing and documenting a system’s security risks. The threat model identifies and describes the set of possible attacks to your system, as well as mitigation strategies and countermeasures. Your security threat modelling efforts also enable your team to justify security features within a system, or security practices for using the system, to protect your corporate assets.

Any threat modelling process will usually encompass the following steps:

1) A model of the system that is relevant for the threat analysis
2) A model of the potential threats
3) A categorization and rating of the threats
4) A set of countermeasures and mitigation strategies

There are however several approaches to perform each of the steps. We will now briefly give an overview of each step.

Step 1: Model your system

The system model is an abstraction of your system that fits the threat analysis. It differs from other traditional models in the sense that it is a mix between a deployment view, a data view and a use case view.

The system entry & exit points The entry & exit points are the ways through which data enter and leave the system, from and to the external environment.
The actors & external dependencies The actors and the external dependencies are the entities that legitimately interact with a system. The actors tend to represent real user roles whereas an external dependency refers usually to a third-party system. The distinction between both can sometimes be blurry: an external system could be considered as an actor in case it is the active participant in the interaction.
The trust levels & boundaries Trust levels define the minimal access granted to an actor within a system. For example, a system administrator actor may have a trust level that allows them to modify files in a certain directory on a file server, while another user entity may be restricted from modifying files.
The assets An asset is an item of value, an item security efforts are designed to protect. It is usually the destruction or acquisition of assets that drives malicious intent. A collection of credit card numbers is a high-value asset, while a database that contains candy store inventory is probably a lower-value asset. An asset is sometimes called a protected resource.
Use cases Identify the use case for operating on that data that the application will facilitate.
The assumptions All assumptions that were driving the modelling effort. Considering the cryptographic algorithm either public or private is for instance an assumption worth being mentioned.

 

Step 2: Model your threats

Let’s first define some concepts:

Threat – The possibility of something bad happening
Attack -A mean though which a threat is realized
Vulnerability – A flaw in the product
Countermeasure – A mean to mitigate the vulnerability

« Threats are realized through attacks which can materialize through certain vulnerabilities if they have not been mitigated with appropriate countermeasures »

A concrete example would be:

Threat  – Perform arbitrary query on the system
Attack – Access internal service exposed to the end-user
Vulnerabilities – (1) Firewall misconfigured (2) Lack of access control
Countermeasure – (1) Correct firewall rules (2) Secure the EJB correctly

« Arbitrary query can be executed through access to the back-end EJB  which can be possible because of wrong firewall configuration and lack of access control if the infrastructure and the application server were not configured correctly »

An advanced attack is frequently composed of a series of preliminary attacks which will exploit several vulnerabilities. The attacks can be represented as a tree, called an attack tree.

The level of details in the identification of threats, attacks and vulnerabilities is up to the analyst.

Step 3: Categorize and rate your threats

Once the threats, attacks and vulnerabilities have been identified, the threats can be categorized. Popular categorization schemes are STRIDE or CIA.

STRIDE:

Spoofing – To illegally acquire confidential information of someone and use it
Tampering – To modify maliciously information that is stored, in transit or otherwise.
Repudiation – A malicious used denying the fact of committing an action that he/she is unauthorised to do or that hampers security of an organisation, and the system has no trace of such action. This action cannot be proved.
Information Disclosure – To view information that is not meant to be disclosed.
Denial of Service – Sending or directing network traffic to a host or network that it cannot handle thus they become unusable to others.
Elevation of privileges – To increase the adversary’s system trust level, permitting additional attacks.

CIA:

Confidentiality – To ensure that information is accessible only to those authorized to have access
Availability – The ratio of the total time a functional unit is capable of being used during a given interval
Integrity – To ensure that the data remain an accurate reflection of the universe of discourse it is modelling or representing, that no inconsistencies exists.

Once categorized, the threats can be rated according to the risk the represent. The total risk can evaluated according to DREAD:

DREAD

Damage Potential – Defines the amount of potential damage that an attack may cause if successfully executed.
Reproducibility– Defines the ease in which the attack can be executed and repeated.
Exploitability– Defines the skill level and resources required to successfully execute an attack.
Affected Users – Defines the number of valid user entities affected if the attack is successfully executed.
Discoverability– Defines how quickly and easily an occurrence of an attack can be identified.

If attack trees have been modelled, the risk can be estimated based on the likelihood each step in the attack tree.

Step 4: set up countermeasures and mitigation strategies

Once the threats, attacks and vulnerabilities have been identified and documented, a set of countermeasure can be set up. Such strategies aim at reducing the risk surface and mitigating the potential effects of an attack. If the existence of the threat can not be removed altogether, the probability of such threat should be reduced to an acceptable threshold.

Wrap up

The following quote summarize well the rationale behind thread modeling. « Threat modeling is not a magic process/tool where you just throw stuff in and out comes goodness. Threat modeling is a structured way of thinking about and addressing the risks to what you are about to build rather than going about it randomly. »

References 

http://blogs.msdn.com/threatmodeling/archive/2007/10/30/a-discussion-on-threat-modeling.aspx
http://blogs.msdn.com/krishnanr/archive/2004/10/27/248780.aspx
http://www.devx.com/security/Article/37502/0/page/4
http://www.schneier.com/paper-attacktrees-ddj-ft.html#rf2
http://www.agilemodeling.com/artifacts/securityThreatModel.htm