threat-modeling – Security skill for AI agents

Threat Modeling

Identify and mitigate security threats during system design.

When to Use This Skill

Use this skill when:

• Designing a new system, service, or feature
• Making significant architectural changes to existing systems
• Onboarding a new third-party integration or dependency
• Preparing for security audits or compliance reviews
• Responding to a security incident to improve defenses
• Reviewing infrastructure changes that affect trust boundaries

Prerequisites

• System architecture documentation or design diagrams
• Access to development and operations teams for context
• Understanding of the system's data classification (PII, PHI, financial, etc.)
• OWASP Threat Dragon or Microsoft Threat Modeling Tool (optional but helpful)
• Whiteboard or diagramming tool for collaborative sessions

STRIDE Methodology

| Threat | Description | Property Violated | Mitigation Examples |

|--------|-------------|-------------------|---------------------|

| Spoofing | Pretending to be another user or system | Authentication | MFA, mTLS, API key validation, certificate pinning |

| Tampering | Modifying data in transit or at rest | Integrity | HMAC, digital signatures, authenticated integrity checks, tamper-evident logs |

| Repudiation | Denying having performed an action | Non-repudiation | Audit logging, digital signatures, tamper-evident logs |

| Information Disclosure | Exposing data to unauthorized parties | Confidentiality | Encryption (TLS, AES), access controls, data masking |

| Denial of Service | Making service unavailable | Availability | Rate limiting, autoscaling, CDN, circuit breakers |

| Elevation of Privilege | Gaining unauthorized higher access | Authorization | RBAC, principle of least privilege, input validation |

STRIDE Worksheet Template

# stride-worksheet.yaml - Fill out one per component/trust boundary crossing
component:
  name: "API Gateway"
  owner: "Platform Team"
  data_classification: "Confidential"
  trust_boundary: "External -> Internal"

threats:
  - id: T001
    category: Spoofing
    description: "Attacker forges JWT tokens to impersonate users"
    attack_vector: "Stolen signing key or weak algorithm (HS256 with guessable secret)"
    likelihood: Medium
    impact: Critical
    risk_score: 15  # likelihood(3) x impact(5)
    existing_controls:
      - "JWT validation on every request"
      - "RS256 algorithm with rotated keys"
    gaps:
      - "No token binding to device/IP"
    recommended_mitigations:
      - "Use a supported proof-of-possession protocol when required; a copied claim alone does not bind a token"
      - "Implement short-lived tokens (15 min) with refresh"
      - "Monitor for token reuse from different IPs"
    status: "Mitigated (partial)"
    owner: "Auth Team"

  - id: T002
    category: Tampering
    description: "Man-in-the-middle modifies API requests"
    attack_vector: "Compromised network between client and gateway"
    likelihood: Low
    impact: High
    risk_score: 8
    existing_controls:
      - "TLS 1.3 enforced"
      - "HSTS enabled"
    gaps: []
    recommended_mitigations:
      - "Certificate pinning for mobile clients"
    status: "Mitigated"
    owner: "Platform Team"

  - id: T003
    category: Information Disclosure
    description: "Verbose error messages leak internal details"
    attack_vector: "Triggering errors returns stack traces, internal IPs, DB schema"
    likelihood: High
    impact: Medium
    risk_score: 12
    existing_controls:
      - "Generic error pages in production"
    gaps:
      - "Some microservices return raw exceptions"
    recommended_mitigations:
      - "Centralized error handling middleware"
      - "Error response schema validation"
    status: "Open"
    owner: "Backend Team"

  - id: T004
    category: Denial of Service
    description: "API rate limiting bypass through distributed requests"
    attack_vector: "Botnet sending requests below per-IP threshold"
    likelihood: Medium
    impact: High
    risk_score: 12
    existing_controls:
      - "Per-IP rate limiting at WAF"
    gaps:
      - "No aggregate rate limiting"
      - "No bot detection"
    recommended_mitigations:
      - "Add aggregate rate limiting per endpoint"
      - "Deploy bot detection (Cloudflare Bot Management)"
      - "Implement circuit breaker pattern"
    status: "Open"
    owner: "Platform Team"

  - id: T005
    category: Elevation of Privilege
    description: "IDOR allows accessing other users' data"
    attack_vector: "Manipulating resource IDs in API calls"
    likelihood: Medium
    impact: Critical
    risk_score: 15
    existing_controls:
      - "Authentication required"
    gaps:
      - "Authorization checks inconsistent across endpoints"
    recommended_mitigations:
      - "Enforce ownership checks on all resource access"
      - "Opaque IDs reduce guessing but never replace object-level authorization"
      - "Add authorization integration tests"
    status: "Open"
    owner: "Backend Team"

Data Flow Diagram

Text-Based DFD Notation

                    Trust Boundary: Internet
                    ==========================
                           |
                    [External User]
                           |
                      HTTPS/443
                           |
                    ==========================
                    Trust Boundary: DMZ
                    ==========================
                           |
                    (WAF / CDN)
                           |
                    [API Gateway]---->[Auth Service]--->[Identity DB]
                           |
                    ==========================
                    Trust Boundary: Internal
                    ==========================
                           |
                    [App Service]
                       /       \
                      /         \
               [Cache]       [Message Queue]
                                  |
                           [Worker Service]
                                  |
                    ==========================
                    Trust Boundary: Data
                    ==========================
                                  |
                           [Primary DB]--->[Replica DB]
                                  |
                           [Object Store]

Legend:
  [Box]     = Process
  (Parens)  = External entity / proxy
  ====      = Trust boundary
  --->      = Data flow

Threat Model Concept Sketch (JSON; not an importable Threat Dragon model)

{
  "summary": {
    "title": "E-Commerce Platform",
    "owner": "Security Team",
    "description": "Threat model for the e-commerce API platform"
  },
  "detail": {
    "diagrams": [
      {
        "title": "API Data Flow",
        "diagramType": "STRIDE",
        "cells": [
          {
            "type": "tm.Actor",
            "name": "Web Client",
            "threats": []
          },
          {
            "type": "tm.Process",
            "name": "API Gateway",
            "threats": ["T001", "T002", "T003", "T004"]
          },
          {
            "type": "tm.Process",
            "name": "Order Service",
            "threats": ["T005"]
          },
          {
            "type": "tm.Store",
            "name": "Orders Database",
            "threats": ["T006"]
          },
          {
            "type": "tm.Boundary",
            "name": "DMZ"
          },
          {
            "type": "tm.Boundary",
            "name": "Internal Network"
          }
        ]
      }
    ]
  }
}

Threat Library

# threat-library.yaml - Reusable threat patterns
categories:
  authentication:
    - id: TL-AUTH-001
      name: "Credential stuffing"
      description: "Attacker uses leaked credential databases to attempt logins"
      applicable_to: ["login endpoints", "API authentication"]
      mitigations: ["MFA", "rate limiting", "credential breach monitoring", "CAPTCHA"]

    - id: TL-AUTH-002
      name: "Session hijacking"
      description: "Attacker steals session tokens via XSS or network sniffing"
      applicable_to: ["web applications", "APIs with session tokens"]
      mitigations: ["HttpOnly cookies", "TLS", "session binding", "short TTL"]

    - id: TL-AUTH-003
      name: "OAuth token theft"
      description: "Access tokens stolen from logs, URLs, or insecure storage"
      applicable_to: ["OAuth/OIDC integrations"]
      mitigations: ["PKCE", "short-lived tokens", "token binding", "secure storage"]

  injection:
    - id: TL-INJ-001
      name: "SQL injection"
      description: "Malicious SQL in user input executes unauthorized queries"
      applicable_to: ["database-backed endpoints", "search functionality"]
      mitigations: ["parameterized queries", "ORM", "input validation", "WAF"]

    - id: TL-INJ-002
      name: "Command injection"
      description: "User input passed to system commands without sanitizatio

… (full content is in the download package)