In 2026, security is no longer a "feature". It is an architectural property.

Most .NET applications are secured reactively:

• we add [Authorize]

• we put a JWT

• we enable HTTPS

• we hope it is enough

But that is NOT Security by Design.

Security by Design means:

🔒 Security is thought of from day one, at the architecture, infrastructure, and code level.

🧠 What is Security by Design?

Security by Design involves:

• identifying risks before implementation

• reducing the attack surface

• multi-layered protections

• assuming the system can be compromised

• continuous monitoring and auditing

It is not middleware. It is an architectural mindset.

🧩 1. Threat Modeling

Threat modeling means:

Identifying the ways your application can be attacked.

Before writing code, ask yourself:

• What data is sensitive?

• Who has access to it?

• Which endpoints are publicly exposed?

• What happens if the JWT is stolen?

• What happens if the database is compromised?

A classic model is STRIDE:

• Spoofing

• Tampering

• Repudiation

• Information Disclosure

• Denial of Service

• Elevation of Privilege

Real example in .NET API:

You have an endpoint:

[HttpPost("invoice")]
public async Task<IActionResult> CreateInvoice(CreateInvoiceCommand cmd)

Threat modeling questions:

• Can a user create invoices for another CompanyId?

• Can I modify the payload?

• Can I send massive requests (DoS)?

• Can I inject dangerous data?

Threat modeling = preventive thinking.

🎯 2. Attack Surface

Attack surface = all the points through which the system can be attacked.

In a modern .NET application:

• API endpoints

• Swagger UI publicly exposed

• SignalR hubs

• File uploads

• Blazor WASM

• Admin panel

• Open ports in infrastructure

Reducing the attack surface means:

• Disabling Swagger in production

• Blocking unused endpoints

• Disabling error details

• Limiting CORS

• Implementing rate limiting

The fewer exposed points you have, the lower the risk.

🛡️ 3. Defense in Depth

Defense in Depth = multi-layered protection.

You don’t rely on a single mechanism. Real example:

Level 1 — HTTPS

Level 2 — correctly validated JWT

Level 3 — Policy-based authorization

Level 4 — Model validation

Level 5 — CompanyId verification in DB

Level 6 — Audit log

If one layer fails, another protects you.

🧨 4. Zero Trust

Zero Trust means:

You trust nothing by default. Not even inside the network.

In .NET this means:

• Every request is validated

• Every claim is verified

• Every service verifies the token

• mTLS between services

• There is no "we are in VPN so it’s safe"

Zero Trust is the enterprise standard in 2026.

⚠️ 5. OWASP Top 10

OWASP publishes annually the list of the most critical web vulnerabilities.

The most relevant for .NET applications:

1. Broken Access Control

2. Cryptographic Failures

3. Injection

4. Insecure Design

5. Security Misconfiguration

6. Vulnerable Components

7. Identification & Authentication Failures

8. Software Integrity Failures

9. Logging & Monitoring Failures

10. SSRF

Most enterprise applications fail at:

• access control

• misconfiguration

• poor logging

🔍 Security by Design applied concretely in a .NET application

If you have:

• ASP.NET Core Web API

• Blazor WebAssembly

• JWT

• External integration (e.g., e-Invoice)

• X509 Certificates

Security by Design would mean:

✅ JWT with asymmetric key

✅ Refresh token rotation

✅ Policy-based authorization

✅ Strict CompanyId validation

✅ Audit log for every critical action

✅ Rate limiting

✅ mTLS for external integration

✅ Correctly validated certificates (CRL / chain)

✅ Centralized logging

Not just "it works". But:

it is defensively built.

🔥 Common mistakes in .NET applications

1. JWT with hardcoded secret

2. No token expiration

3. Only role-based authorization

4. Lack of audit log

5. Returning full exceptions in production

6. No rate limiting

7. No model validation

8. Accepting any client certificate

🧱 Security by Design checklist for .NET

• Forced HTTPS

• HSTS enabled

• Correct JWT validation

• Secured refresh tokens

• Policy-based authorization

• Input validation

• Anti-forgery where applicable

• Rate limiting

• Secured logging

• Anomaly monitoring

• Dependency scanning

• Secrets in Azure Key Vault / env vars

🎯 Conclusion

Security by Design does NOT mean:

"We put a JWT and we are done."

It means:

We build the application as if someone is already trying to break it.