OWASP Risk Rating Methodology

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Background
Application security risk analysis consists in the assessment of the levels of risk calculated for threats and vulnerabilities. Risk Assessment practitioners usually refer to application security risk analysis as technical risk assessment. In the technical risk analysis model, threat and vulnerability represents the two factors (i.e. multipliers) used for risk calculation. Depending on the model used, different parameters can be used to characterize threats and vulnerabilities and calculate the final risk value. By associating risk values to each of the finding it is possible to prioritize fixing vulnerabilities with the highest security risks: the ones characterized by with highest impact, ease of exploitation and exposure. On the other hand, information risk analysts also look to other risk factors to decide deciding the strategy for mitigating application security risks such the evaluation of business impacts derived by the exploitation of the vulnerability. Technical risks and business impact analysis are both evaluated as part of the information risk management strategy identification that includes selection and adoption of countermeasures justified by the identified risks to assets and the reduction of those risks to acceptable levels. For the purpose of this guide we limit the scope of risk evaluation to technical impact analysis that is the evaluation of risk for known threats and vulnerabilities.

Definitions
According to NIST publication 800-27 Risk is defined as the probability that a particular threat-source will exercise particular information system vulnerability and the resulting impact if this should occur. 

Risks Formulation
For the purposes of a security assessment, OWASP defines the risk associated with a security finding as a product of the Impact, Ease of Exploitation and Exposure as risk parameters of the following empirical formula:  '''Risk = Impact x Ease of Exploitation x Exposure By breaking down the security findings in this way, threats criticality and the implication of the vulnerability exposure may be better evaluated and risks can be prioritized and managed. The risk evaluation is taken out of the context of the nature of the application or host (e.g. server, firewall or workstation), and regardless of the type of data stored on it. The impact and the easiness of exploitation of a threat are usually considered the main factors in determining the criticality of threats. In this case, impact is taken as a measure of the implication of a discrete component being compromised (e.g. attack potency). The ease of exploitation for a threat can be further described as a factor of discoverability and reproducibility of a threat. Exposure quantifies the likelihood that the vulnerability can be compromised through its availability. For example, an easily exploited vulnerability on an internet facing host would be highly exposed, and therefore very likely to be compromised. An internal host with a vulnerability that was difficult to exploit would present a much lower threat.

Threat Categorizations
For the purpose of the OWASP guide, threats can be identified for each of the testing category using threat categorization models as part of a threat modeling framework (e.g. STRIDE, TRIKE, SecurityFrame etc). OWASP does not particular endorse any vendor specific threat categorization, but rather recommends to adopt a threat categorization and risk ranking that depends on specific objectives. In general categorizations that focus on threats rather than attacks are more suitable for the identification of countermeasures. Some threat categorizations also include risk rankings for the identified threats (e.g. STRIDE/DREAD) Example of specific objectives for the risk evaluation can be:
 * Regulatory and requirements compliance
 * Tactical prioritization for mitigation of vulnerabilities (e.g. high risk first)
 * Overall risk strategy selection

Risks Calculation
Before any kind of calculation can be done appropriate values for the impact, ease of exploitation and exposure have to be found. Every risk parameter can be assigned a qualitative value based on its importance (e.g. severity). Using qualitative values rather than quantitative ones can help avoid the ranking becoming overly subjective. The same argument holds true for the number of different qualitative values, hence, only three different values (high, medium, low) are used.

However while determining qualitative values for risk several criteria and qualitative ranking systems can be used:
 * Proprietary common vulnerability list risk ranking
 * Industry standard vulnerability severity and risk rankings (CVSS)
 * Security-enhancing process models vulnerability root cause categorization (CLASP)
 * Threat modeling risk rankings (DREAD, TRIKE, PTA, CORAS/SECURIS, PTA)

Example: A (TBD)
Risk Categories: Impact (imp) Ease of Exploitation (EoE) Exposure (Exp)
 * critical
 * high
 * medium
 * low
 * Easy
 * Moderate
 * Difficult
 * Very high
 * High
 * Medium
 * Low