Building requirements is one of the first steps in the SDLC, where we define the goals and objectives of our future application. Usually, at this phase, we collect relevant stakeholders and start discussing their needs and expectations. We talk to people who will use the application, those who will manage it, and anyone else who might be affected by it to understand what they want the application to do and how it should work.

After reading this article, we will understand how to:

• Prepare Security Requirements
• Properly classify and rate threats
• Use a language of money in the risk assessment and prioritization
• Make sure, that defined security requirements will be implemented during other steps of SDLC

During requirements engineering, we need to separatefunctional andsecurity requirements. While functional requirements show what an applicationshould do, security requirements show what an applicationshouldn't do.

It's important to carefully consider building security requirements because a large part of the system's security depends on them. Good security requirement followsSMART principle:

Usually, stakeholders involved in security requirements engineering are:

• Developers
• Security experts
• Project Managers/Architects

All of them can participate in one of the ways to build security requirements:Security andAbuser stories.

Security Stories

As a [role], I want [security feature], so that [benefit]

Security stories are a special type of user stories, that focus on the security of the system.

You should look at the application from the perspective of users and stakeholderswho need protection.

Examples:

1. As a developer, I want to ensure that all passwords are stored with strong hashing algorithm so that even if the database is compromised, the passwords remain secure.
2. As a system administrator, I want to log all access to sensitive data so that we can audit and identify any unauthorized access.

Abuser Stories

As a [bad guy], I want to [do something bad]

Abuser stories are the opposite of security stories. Here you need to think like anattacker, finding the ways you canexploit an application.

Examples:

1. As an attacker, I want to perform a brute force attack on the login page to gain access to user accounts.
2. As an attacker, I want to intercept data transmitted over the network to steal sensitive information.

So, security and abuser stories allow us to look at the application from both points of view: the user's and the attacker's.

It is aproactive approach, that provides a detailed and scenario-based understanding of security requirements.

Now we need a comprehensive way to ensure our critical assets and potential risks are managed.
For this, we can use theFAIR model:

Factor analysis of information risk (FAIR)

FAIR is a methodology, that helps to assess and manage informational risk in afinancial terms.

It includes the following core steps:

1. Threat andCritical Asset identification - defining valuable assets of the application and identifying related threats for them.
2. Contact Frequency (СF) assessment - calculating how frequently the vulnerability interacts with critical asset
3. CalculatingProbability of Action (PoA) - finding the probability that asset would be attacked
4. Threat Event Frequency (TEF) assessment - multiplication ofCF andPOA
5. Vulnerability (Vuln) assessment - the probability that the attack on the asset would be successful
6. Loss Event Frequency (LEF) assessment - multiplication ofTEF xVuln
7. DefiningLoss Magnitude (LM) - calculatingPrimary Losses (actual damage in a result of the attack)andSecondary Losses (reputation, litigation losses)
8. CalculatingOverall risk - multiplication ofLEF xLM

Sounds a bit hard, right? AsFAIR is justmethodology, notframework, there are no concrete ways ofhow you should calculate risks.

But using simpleThreat Modeling techniques, we can cover most of these steps:

Threat Modeling

Threat modeling allows us toidentify andrate threats.

Identifying threats helps us to understand which security aspects are at risk, while rating ensures we prioritize them in the right way.

To properly identify threat we will useSTRIDE framework:

As we see, it allows us to classify a threat in one or more of 6 categories, defining which security aspects are affected.
After the identification we can calculate the risk by using theDREAD framework:

Each category in the model is scored from 0 to 10. The sum of the scores in all categories istotal risk score. Maximum risk score is50.

As an example let's create a STRIDE and DREAD analysis forSQL Injection:

According to STRIDE classification, we can easily calculate DREAD scores:

Thus, the total risk score for SQL Injection is9 + 8 + 10 + 10 + 8 =45.

After this, we can useSecurity & Abuser stories,STRIDE andDREAD frameworks to structure our approach withFAIR methodology:

› Threat and Critical Asset identification

UsingSecurity & Abuser stories, we can find critical assets.

• As a developer, I want all user input to be validated to prevent SQL Injection in the database.

From this security story we see, thatdatabase is our critical asset.
To identify the influence vectors of the threat, we will useSTRIDE.

› Contact Frequency (СF) assessment

This criteria fully depends on the functional requirements.
For instance, if our critical asset is the database and vulnerability relates to it, the frequency is actually how often will the user interact with the database.

› Calculating Probability of Action (PoA)

We can useReproducibility andExploitability scores from theDREAD framework.
For instance, SQL Injection'sReproducibility score is9, and theExploitability's score is10.

Then ourPoA would be(9 + 10) / 20 =0.95

› Threat Event Frequency (TEF)

As mentioned before,TEF =CF xPoA.
For example, if there are 100 user-side interactions with the database, then for SQL Injection:

TEF =100 x 0.95 =95 threat events per day.

› Vulnerability (Vuln) assessment

For theVulnerability assessment we can use the finalDREAD score.
SQL Injection's DREAD score is9 + 8 + 10 + 10 + 8 = 45/50, or0.99.

› Loss Event Frequency (LEF)

LEF =TEF xVuln
Then for our scenario,LEF =95 x 0.99 =94 loss events per day.

› Loss Magnitude (LM)

Loss Magnitude is calculated by summing potentialprimary andsecondary losses. At this step we don't use any threat modeling approaches, cause it requires more specific analysis.

For instance, let's calculate the imaginarySQL Injection's Loss Magnitude:

Primary Losses:
The potential cost of stolen data:50.000$
Cost of restoration works:30.000$
System downtime:10.000$
Total :90.000$

Secondary Losses:
Legal and regulatory losses:60.000$
Increased security costs:20.000$
Total:80.000$

Thus, Total Loss Magnitude is90.000$ + 80.000$ =170.000$

› Overall Risk

Potential Overall Risk = LEF(94) x LM(90.000) =8.460.000$ per day

By usingSTRIDE,DREAD,Security & Abuser Stories, andFAIR, we learned how to develop strong security requirements.

The great thing aboutFAIR is that in the end, it translates these risks intofinancial terms, making it much easier formanagement to understand the importance of each security measure. This is especially helpful since it's oftenchallenging to convey the significance of security risks totop executives.

Now after we have our security requirements and know their financial impacts, we can ensure we don't miss anything by using aSecure Requirements Traceability Matrix (SRTM).

Security Requirements Traceability Matrix (SRTM)

SRTM is a detailed document that links security requirements to their implementation and testing.

It makes sure that all security needs are handled during development, showing a clear path from the start to the final tests.

Let's imagine that after using the FAIR framework with Security & Abuser Stories, we identified the following security requirements:

• Implement 2FA to follow PCI-DSS
• Using input validation to prevent XSS Injection
• Logging access to sensitive assets

In this case, our matrix will look like this:

For building requirements traceability matrix you will use such tools likeYouTrack orJira.

Summary

In this article, we learned how important it is to build security requirements early in the SDLC. By talking to stakeholders and using methods likeSecurity & Abuser Stories, we can spot critical assets and potential threats from both user and attacker perspectives.

We usedSTRIDE to identify threats,DREAD to assess them, andFAIR methodology to look at these threats fromall angles and translate their impact intofinancial terms, making it easier for management to understand theirimportance.

Finally, we talked about the Secure Requirements Traceability Matrix (SRTM), which helps ustrack security requirements from start to finish.
This ensures thatnothing is missed and all security needs are properly addressed.

Finding and fixing security issues during the requirements phase cansave millions of dollars later on. It’smuch cheaper to address these problems early rather than after the application is built or at later SDLC steps.

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