Threat Model

Overview

Based on Threat Modeling: Designing for Security (Adam Shostack). Threat modeling is structured adversarial thinking applied before code ships. Shostack's standard: the goal is not to find every bug - it's to find the worst things that can happen to your system and decide what to do about each one. A threat model is not a security audit; it is a design artifact.

STRIDE framework (Shostack):

• Spoofing - impersonating a user or system
• Tampering - modifying data or code
• Repudiation - denying an action occurred
• Information disclosure - exposing data to unauthorized parties
• Denial of service - degrading or blocking availability
• Elevation of privilege - gaining access beyond what's authorized

Workflow

Step 1: Define scope

System: [name of the system or feature being modeled]
Scope: [what is in scope - specific components, APIs, data flows]
Out of scope: [what is explicitly excluded]
Assumptions: [what we assume is already secured, e.g. OS-level security, network perimeter]
Assets: [what we're protecting]
  - [asset 1, e.g. user PII]
  - [asset 2, e.g. payment data]
  - [asset 3, e.g. service availability]

Step 2: Draw the data flow diagram (DFD)

Describe or draw a simple DFD with:

• External entities (users, third-party systems)
• Processes (services, functions)
• Data stores (databases, caches, queues)
• Data flows (connections between components)
• Trust boundaries (lines where authorization is required)

External entity: [Browser / Mobile client]
  → [API Gateway] (trust boundary crossed - auth required)
    → [Service A] (internal trust)
      → [Database] (internal trust)
    → [External Payment API] (trust boundary - third party)

Every trust boundary crossing is a threat surface.

Step 3: Apply STRIDE per component

For each component and data flow, enumerate threats systematically:

Component: [API Gateway]

Spoofing:
  T1: Attacker sends requests with forged JWT tokens
  Mitigation: Validate token signature with public key. Check expiry and issuer claim.
  Status: Mitigated

Tampering:
  T2: Attacker modifies request body in transit
  Mitigation: TLS 1.2+ enforced. Request signing for critical operations.
  Status: Mitigated

Repudiation:
  T3: Authenticated user denies making a request
  Mitigation: Structured access logs with user_id, IP, timestamp, action. Logs are immutable (append-only S3).
  Status: Mitigated

Information Disclosure:
  T4: Error responses leak internal stack traces or DB queries
  Mitigation: Generic error messages in production. Internal details logged server-side only.
  Status: Mitigated

Denial of Service:
  T5: Request flooding exhausts API capacity
  Mitigation: Rate limiting per user_id and IP at gateway. [Rate limits: X req/min]
  Status: Partial - no burst protection for authenticated users

Elevation of Privilege:
  T6: User A accesses User B's resources via IDOR
  Mitigation: Authorization check on every resource endpoint verifies ownership.
  Status: Mitigated

Repeat for each component and trust boundary crossing.

Step 4: Rate and prioritize threats

Use DREAD or simplified risk scoring for unmitigated/partial threats:

Likelihood: Low / Medium / High Impact: Low / Medium / High / Critical

Step 5: Document mitigations and residual risk

For each threat rated P1 or P2:

Threat: [T5 - Burst flooding for authenticated users]
Current state: Rate limit is per-minute but no burst throttle
Proposed mitigation: Token bucket algorithm at gateway, limit 10 concurrent requests per user
Owner: [team]
Target resolution: [sprint or date]
Residual risk if not fixed: Authenticated users can amplify load during peak, causing latency SLA breach
Accepted residual risk: [yes/no and by whom if yes]

Step 6: Out-of-scope and assumptions

Document what is explicitly not covered and why:

Not modeled:
- Physical security of data centers (assumed: cloud provider responsibility)
- Insider threat from admins with DB access (assumed: handled by HR and access review processes)
- DDoS at network layer (assumed: CDN/WAF handles this)

These assumptions should be reviewed annually or when the architecture changes.

Anti-Patterns

1. Threat model as checkbox exercise Bad: "We did a threat model." (5-bullet doc, never revisited) Good: Threat model lives with the design doc, is updated when architecture changes, and P1/P2 threats have owners and dates.

2. No trust boundary analysis Bad: Threat model lists threats but doesn't identify where trust changes hands. Good: Every external entity, third-party integration, and auth boundary is explicitly called out as a threat surface.

3. Mitigations without validation Bad: "Mitigation: use TLS." (without verifying it's actually enforced) Good: "TLS 1.2+ enforced at load balancer. Certificate pinning on mobile clients. Verified via [test or config reference]."

4. STRIDE applied globally instead of per component Bad: "Spoofing threats: attackers could impersonate users." Good: STRIDE applied to each component individually so threats are specific and actionable.

Quality Checklist

• Scope defined with explicit out-of-scope boundaries
• Assets being protected are listed
• Data flow diagram covers all external entities, processes, stores, and trust boundaries
• STRIDE applied to each component individually
• Every identified threat has a mitigation OR is explicitly accepted with owner
• Unmitigated threats are risk-rated (likelihood × impact) and prioritized
• P1/P2 threats have owners and target resolution dates
• Assumptions documented and flagged for periodic review