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What it is
Threat modeling expertise covering STRIDE methodology, attack trees, data flow diagrams, trust boundaries, threat prioritization with DREAD scoring, mitigation strategies, threat modeling for APIs and microservices, and automated threat modeling tools for proactively identifying security risks during system design.
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SKILL.md
446 lines · 1,740 words
Threat modeling is the practice of identifying security threats to a system during the design phase, before code is written. It answers four fundamental questions: What are we building? What can go wrong? What are we going to do about it? Did we do a good enough job? This skill covers the methodologies, tools, and frameworks for systematic threat identification and risk assessment.
Threat modeling expertise covering STRIDE methodology, attack trees, data flow diagrams, trust boundaries, threat prioritization with DREAD scoring, mitigation strategies, threat modeling for APIs and microservices, and automated threat modeling tools for proactively identifying security risks during system design.
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What it covers
Overview
The Four Questions of Threat Modeling
Data Flow Diagrams (DFD)
STRIDE Methodology
Attack Trees
Trust Boundaries
Threat Prioritization (DREAD)
Threat Modeling for APIs
Microservices Threat Modeling
Automated Threat Modeling
When to Threat Model
When to Use
Output Format
Example
Edge Cases
1. WHAT ARE WE BUILDING?
-> Data Flow Diagrams (DFD)
-> Architecture diagrams
-> Asset inventory
2. WHAT CAN GO WRONG?
-> STRIDE analysis
-> Attack trees
-> Known vulnerability patterns
3. WHAT ARE WE GOING TO DO ABOUT IT?
-> Mitigations for each threat
-> Accept, mitigate, transfer, or avoid
4. DID WE DO A GOOD ENOUGH JOB?
-> Review completeness
-> Validate mitigations
-> Update as system evolvesElements:
[Rectangle] = External Entity (users, third-party services)
(Circle) = Process (application logic, services)
[== lines ==] = Data Store (database, file system, cache)
--> = Data Flow (network request, function call)
--- --- --- = Trust Boundary (network perimeter, auth boundary)
Levels:
DFD Level 0: Context diagram (entire system as one process)
DFD Level 1: Major components and their interactions
DFD Level 2: Detailed view of individual componentsDFD Level 1:
+------------------+
| Trust Boundary: |
| Internet |
| |
| [Web Browser] |
| [Mobile App] |
+--------|---------+
|
HTTPS (TLS 1.3)
|
+---------v---------+
| Trust Boundary: |
# ... (condensed) ...
+----------------------------------------------------+
|
External API
|
[Payment Processor]data_flows:
- id: DF-001
name: User Login
source: Web Browser
destination: Auth Service
protocol: HTTPS (TLS 1.3)
data: "username, password"
classification: confidential
trust_boundary_crossed: Internet -> DMZ -> Internal
authentication: None (pre-auth flow)
- id: DF-002
name: Session Token Return
source: Auth Service
# ... (condensed) ...
protocol: HTTPS
data: "payment token, amount, currency"
classification: restricted (PCI)
trust_boundary_crossed: Internal -> External
authentication: API key (server-side)STRIDE is a threat classification framework developed by Microsoft. Each letter represents a category of threat.
S - Spoofing:
definition: "Pretending to be someone or something else"
targets: Authentication mechanisms
examples:
- Forging authentication tokens
- IP address spoofing
- DNS spoofing
- Phishing to steal credentials
mitigations:
- Strong authentication (MFA)
- Certificate-based identity
- Mutual TLS for service-to-service
T - Tampering:
# ... (condensed) ...
mitigations:
- Principle of least privilege
- Input validation
- Secure coding practices
- Sandboxing and isolation# Apply STRIDE to each DFD element
element: API Gateway
spoofing:
threat: "Attacker spoofs requests as legitimate client"
likelihood: medium
impact: high
mitigation: "OAuth 2.0 + PKCE, API key validation"
tampering:
threat: "Attacker modifies request in transit"
likelihood: low
impact: high
mitigation: "TLS 1.3 enforced, HSTS"
# ... (condensed) ...
denial_of_service:
threat: "Expensive queries exhaust database resources"
likelihood: medium
impact: high
mitigation: "Query timeouts, connection pooling, read replicas"Attack trees model the steps an attacker might take to achieve a goal.
Goal: Steal Customer Payment Data
|
+-- 1. Compromise Application Server
| +-- 1.1 Exploit web vulnerability
| | +-- 1.1.1 SQL injection in search
| | +-- 1.1.2 RCE via file upload
| | +-- 1.1.3 SSRF to internal services
| +-- 1.2 Compromise employee credentials
| | +-- 1.2.1 Phishing attack
| | +-- 1.2.2 Credential stuffing (reused passwords)
| | +-- 1.2.3 Social engineering
| +-- 1.3 Supply chain attack
| +-- 1.3.1 Compromise npm/pip dependency
| +-- 1.3.2 Compromise CI/CD pipeline
# ... (condensed) ...
|
+-- 4. Access Backups/Logs
+-- 4.1 Unencrypted backup in public S3
+-- 4.2 Payment data in application logs
+-- 4.3 Database snapshot accessible# Annotate each leaf node with difficulty and impact
attack_tree:
goal: "Steal Customer Payment Data"
nodes:
- id: "1.1.1"
path: "SQL injection in search"
difficulty: medium # CVSS attack complexity
cost: low # Resources needed
detectability: medium
impact: critical
controls_in_place:
- parameterized_queries
- waf_sqli_rules
# ... (condensed) ...
impact: critical
controls_in_place:
- s3_bucket_policies
- encryption_at_rest
residual_risk: low # If controls are properly configuredtrust_boundaries:
- name: "Internet Boundary"
between: "External users/services" and "DMZ"
controls:
- TLS termination
- WAF
- DDoS protection
- Rate limiting
threats:
- All external attack traffic crosses here
- Must assume all input is malicious
- name: "DMZ to Internal Network"
between: "Edge services" and "Backend services"
# ... (condensed) ...
- Rate limiting outbound calls
threats:
- SSRF via internal services
- Data leakage to third parties
- Supply chain compromise of third-partyDREAD scores each threat on five dimensions (1-10 scale):
dread_scoring:
D - Damage:
question: "How much damage could this cause?"
scale:
1-3: "Minor inconvenience"
4-6: "Significant data loss or service disruption"
7-9: "Major breach, regulatory impact"
10: "Complete system compromise"
R - Reproducibility:
question: "How easy is it to reproduce the attack?"
scale:
1-3: "Requires specific conditions, timing, or insider access"
4-6: "Reproducible with some effort"
# ... (condensed) ...
7-9: "Easily found with automated scanning"
10: "Publicly known or obvious"
# Overall risk = average of all five scores
# Priority: > 8 Critical, 6-8 High, 4-6 Medium, < 4 Lowthreats:
- id: T-001
name: "SQL injection in user search"
stride: [Tampering, Information Disclosure, Elevation of Privilege]
dread:
damage: 9 # Full database access
reproducibility: 8 # Standard SQLi techniques
exploitability: 7 # Automated tools available
affected_users: 10 # All users' data at risk
discoverability: 7 # Found by automated scanners
dread_score: 8.2
priority: Critical
mitigation: "Parameterized queries + WAF + input validation"
status: mitigated
# ... (condensed) ...
discoverability: 6 # Requires API testing
dread_score: 7.4
priority: High
mitigation: "Server-side authorization check on every request"
status: openapi_threat_model:
asset: "REST API /api/v2/*"
threats:
authentication:
- broken_authentication:
description: "Missing or weak authentication on endpoints"
check: "Test each endpoint without auth token"
mitigation: "OAuth 2.0 + PKCE for all endpoints"
- jwt_manipulation:
description: "Tampered JWT accepted by server"
check: "Modify claims, change algorithm, use expired tokens"
mitigation: "Strict JWT validation (algorithm allowlist, signature, expiry, issuer)"
# ... (condensed) ...
- lack_of_rate_limiting:
description: "No rate limits enable abuse"
check: "Send 1000 requests rapidly"
mitigation: "Per-user and per-IP rate limiting"microservices_threats:
service_mesh:
- unencrypted_service_communication:
threat: "Internal traffic intercepted"
mitigation: "mTLS between all services (Istio/Linkerd)"
- service_impersonation:
threat: "Rogue service joins the mesh"
mitigation: "Service identity verification, SPIFFE"
- lateral_movement:
threat: "Compromised service accesses other services"
mitigation: "Network policies, least-privilege service accounts"
# ... (condensed) ...
mitigation: "RBAC, network policies, private API endpoint"
- pod_security:
threat: "Privileged pod runs malicious code"
mitigation: "Pod Security Standards (restricted), OPA/Gatekeeper"| Tool | Approach | Best For |
|---|---|---|
| Microsoft Threat Modeling Tool | DFD-based, STRIDE | Windows/.NET applications |
| OWASP Threat Dragon | DFD-based, open source | General web applications |
| IriusRisk | Questionnaire + library | Enterprise, compliance-driven |
| Threagile | Code-as-model (YAML) | Infrastructure/architecture |
| pytm | Python-based DFD | Developer-friendly, CI integration |
from pytm import TM, Server, Datastore, Dataflow, Boundary, Actor
tm = TM("E-Commerce Application")
tm.description = "Online store with payment processing"
# Define boundaries
internet = Boundary("Internet")
dmz = Boundary("DMZ")
internal = Boundary("Internal Network")
# Define elements
user = Actor("Customer", inBoundary=internet)
api_gw = Server("API Gateway", inBoundary=dmz)
auth_svc = Server("Auth Service", inBoundary=internal)
# ... (condensed) ...
payment_to_stripe.data = "Payment token, amount"
# Generate threat model
tm.process()
# Outputs: DFD diagram, threat list, and reportNew system design: Before writing code
Major feature additions: When architecture changes
Security incidents: After a breach, reassess the model
Regulatory requirements: Before compliance audits
Third-party integrations: When adding new external dependencies
Infrastructure changes: Cloud migration, new deployment model
Periodic review: Annually for all production systems
Use this skill when:
Designing or implementing threat modeler solutions
Reviewing or improving existing threat modeler approaches
Making architectural or implementation decisions about threat modeler
Learning threat modeler patterns and best practices
Troubleshooting threat modeler-related issues
Do NOT use this skill when:
The question is about a fundamentally different technology domain
A more specific sibling skill covers the exact topic needed
The user needs a complete hands-on tutorial rather than expert guidance
# Threat Modeler Analysis
## Context Assessment
[Situation summary and constraints]
## Recommended Approach
[Primary recommendation with rationale]
## Implementation Steps
1. [Step with specific details]
2. [Step with specific details]
3. [Step with specific details]
## Trade-offs and Considerations
- [Key trade-off 1]
- [Key trade-off 2]
## Next Steps
- [Immediate action item]
- [Follow-up action item]Input: "Help me implement threat modeler for a medium-scale production application"
Output: A structured analysis covering current state assessment, recommended threat modeler approach with specific patterns, implementation roadmap with milestones, and risk mitigation strategies tailored to the application scale and constraints.
Legacy system integration: When threat modeler must coexist with legacy approaches, provide a gradual migration path rather than a complete rewrite
Scale mismatch: When the solution complexity exceeds the project scale, recommend a simpler approach and note when to revisit
Team skill gaps: When the team lacks experience with the recommended approach, include learning resources and simpler alternatives
Conflicting requirements: When constraints conflict (e.g., performance vs. maintainability), explicitly state the trade-off and recommend based on stated priorities
SKILL.md
446 lines · 1,740 words
The text of SKILL.md is shown as it downloads, under the Apache-2.0 license, by Ferrox Labs. The license is the one in the file's own frontmatter.
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