Threat Modeling for Backend Systems

Introduction

Threat modeling turns security assumptions into explicit, testable decisions. For backend systems, it helps teams identify trust boundaries, enumerate threats early, and prioritize mitigations before code reaches production.

Scope and Asset Inventory

Start with a clear inventory of what you are protecting.

• Sensitive data stores and encryption keys.
• Critical APIs that drive revenue or compliance.
• Administrative interfaces and background jobs.

Data Flow Diagrams and Trust Boundaries

Create data flow diagrams to identify the paths attackers can traverse.

• Mark ingress points for external traffic.
• Document service-to-service calls and queues.
• Highlight trust boundaries where identity changes.

STRIDE Threat Categories

STRIDE provides a consistent taxonomy for backend threats.

• Spoofing: Fake identity or token use.
• Tampering: Data modification in transit or at rest.
• Repudiation: Missing audit trails for sensitive actions.
• Information Disclosure: Leaking sensitive fields or logs.
• Denial of Service: Resource exhaustion or queue flooding.
• Elevation of Privilege: Escalation via flawed authorization checks.

C# Example: Modeling Threats as Code

A lightweight model helps teams keep threat coverage current.

public record Component(string Name, string Boundary, string DataType);
public record Threat(string Category, string Description, int Impact, int Likelihood);

var components = new List<Component>
{
    new("API Gateway", "External", "Token and Headers"),
    new("Order Service", "Internal", "Customer Orders"),
    new("Payments Queue", "Internal", "Payment Events")
};

var threats = new List<Threat>
{
    new("Spoofing", "Forged JWT with missing audience validation", 5, 4),
    new("Tampering", "Unsigned webhook payloads", 4, 3),
    new("DoS", "Unbounded fan-out on queue consumers", 3, 4)
};

Prioritization and Mitigation

• Use risk scoring to prioritize the highest-impact threats.
• Map mitigations to the owning service and sprint backlog.
• Re-evaluate models after architecture changes.

Conclusion

Threat modeling is a repeatable process, not a one-time exercise. Treat it as part of architectural reviews so new features ship with security built in.