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

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