General security

Quantitative risk analysis [updated 2021]

Dawid Czagan
May 19, 2021 by
Dawid Czagan

The goal of risk management is to deliver optimal security at a reasonable cost. When diving into quantitative risk analysis, you have to think about cost versus benefit, risk handling and types of countermeasures.

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Guide quantitative risk analysis with CIA Triad

Risk is related to vulnerabilities, which threaten confidentiality (C), integrity (I) and availability (A) of the assets. This is described as the CIA Triad.

Confidentiality is about not disclosing sensitive information to other people.

Integrity is about preserving the state of the system — we don’t want attackers to change our data.

We do want our systems to be up and running. Hence availability is considered.

What is quantitative analysis?

Quantitative analysis is about assigning monetary values to risk components. Let’s analyze the example of hard drive failure to better understand how it works.

Let’s first describe the threat, vulnerability and risk:

  1. Threat — hard drive failure
  2. Vulnerability — backups done rarely
  3. Risk — loss of data

The asset is data. The value of the asset (AV) is assessed first — $100,000, for example.

Let’s discuss the single loss expectancy (SLE). It contains information about the potential loss when a threat occurs (expressed in monetary values). It is calculated as follows: SLE = AV x EF, where EF is the exposure factor.

Exposure factor describes the loss that will happen to the asset as a result of the threat (expressed as percentage value). SLE is $30,000 in our example, when EF is estimated to be 0.3.

Let’s continue this case. Annualized rate of occurrence (ARO) is described as an estimated frequency of the threat occurring in one year. ARO is used to calculate ALE (annualized loss expectancy). ALE is calculated as follows: ALE = SLE x ARO. ALE is $15,000 ($30,000 x 0.5), when ARO is estimated to be 0.5 (once in two years).

As we can see, the risk is about the impact of the vulnerability on the business and the probability of the vulnerability to be exploited.

Cost and benefit analysis for risk

Let’s continue the example from the previous section. Annualized loss expectancy (ALE) is $15,000. This means that the potential loss is $15,000 in one year, when the data is lost as a result of the hard drive failure. A countermeasure can be used to reduce the potential loss. It happens when the management decides to reduce the risk. This countermeasure should not cost more than $15,000 per year. Otherwise it wouldn’t be logical from a business point of view (we don’t want to spend more money than we can potentially lose). This is basically how cost and benefit analysis works.

Let’s see how the annual value of the countermeasure to the company (COUNTERMEASURE_VALUE) can be calculated:


ALE_PREVIOUS: ALE before implementing the countermeasure

ALE_NOW: ALE after implementing the countermeasure

COUTERMEASURE_COST: annualized cost of countermeasure (please note that it’s not only purchasing cost — maintenance cost is included).

Risk handling for security

Risk can be handled in the following ways:

  • Risk reduction — risk is reduced to an acceptable level (countermeasures implemented; types of countermeasures are described in the next section).
  • Risk avoidance — stopping the activity, which leads to the risk.
  • Risk transference — the risk is transferred to the insurance company.
  • Risk acceptance — accepting the cost of potential loss (no countermeasures).

Countermeasures for risk reduction

Let’s discuss the types of countermeasures (also called controls) that are implemented in the case of risk reduction. There are three types of countermeasures:

  • Administrative (security awareness training should not be forgotten, because people are the weakest point in the security chain)
  • Technical (firewall)
  • Physical (locks)

Countermeasures are implemented to reduce the risk. We talk about total risk when no countermeasure is implemented. Let’s assume now that the countermeasure is implemented. Perfect security doesn’t exist and there is some risk left. This is a residual risk.

What should you learn next?

What should you learn next?

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Figuring out risk and using quantitative risk analysis

Quantitative risk analysis is important for every business. Single loss expectancy (SLE), exposure factor (EF), annualized rate of occurrence (ARO) and annualized loss expectancy (ALE) are all key parts of figuring out the cost and benefit associated with risk. Learning how to handle and countermeasure risk is important. 

Dawid Czagan
Dawid Czagan

Dawid Czagan (@dawidczagan) has found security vulnerabilities in Google, Yahoo, Mozilla, Microsoft, Twitter, BlackBerry and other companies. Due to the severity of many bugs, he received numerous awards for his findings.

Dawid is founder and CEO at Silesia Security Lab, which delivers specialized security auditing services with a results-driven approach. He also works as Security Architect at Future Processing.

Dawid shares his bug hunting experience in his workshop entitled "Hacking web applications - case studies of award-winning bugs in Google, Yahoo, Mozilla and more". To find out about the latest in Dawid's work, you are invited to visit his blog ( and follow him on Twitter (@dawidczagan).