Threat Model Skill

Security bugs are cheapest to fix at design time. Threat modeling asks, systematically, "what can go wrong here?" — before code exists. This skill runs a structured pass: map what you're protecting and the trust boundaries, enumerate threats with STRIDE, and prioritize mitigations by risk. It's for systems you own or are authorized to assess.

Required Inputs

Ask for these only if they aren't already provided:

• The system/feature — what it does, its components, and how data flows through it.
• Assets — what's worth protecting (data, credentials, funds, availability, reputation).
• Trust boundaries — where control changes hands (internet↔app, app↔DB, tenant↔tenant, user roles).
• Actors & entry points — users, admins, services, third parties; APIs, inputs, uploads, auth.

Output Format

Threat model: system/feature]

1. Scope & assets — what's in scope, and the assets ranked by what their compromise would cost.

2. Architecture & trust boundaries — the components, data flows, and where trust boundaries sit. (A Mermaid diagram helps — the playground renders it.)

mermaid
flowchart LR
    User -->|HTTPS| API
    API --> DB[(Data)]
    API -.->|boundary| ThirdParty[/3rd party/]

3. Threats (STRIDE) — walk each boundary/data-flow and enumerate threats by category:

| # | STRIDE category | Threat (how the attack works) | Asset at risk | Likelihood × Impact | Priority | |---|---|---|---|---|---|

Cover Spoofing, Tampering, Repudiation, Information disclosure, Denial of service, Elevation of privilege — skip a category only with a reason.

4. Mitigations (prioritized) — for the top threats, the concrete control (authn/authz, validation, encryption, rate-limiting, logging, least privilege) and where it goes. Note residual risk you're accepting.

5. Assumptions & out-of-scope — trust assumptions and what this model deliberately doesn't cover.

Quality Checks

• ] Assets and trust boundaries are explicit; the data-flow view makes the attack surface visible
• ] Threats are enumerated across all STRIDE categories (or a category is skipped with a stated reason)
• ] Each significant threat is rated by likelihood × impact and prioritized
• ] Top threats have concrete, placed mitigations — and accepted residual risk is named
• ] Trust assumptions and out-of-scope areas are stated

Anti-Patterns

• ] Do not list generic threats — tie each to a specific boundary/data-flow in this system
• ] Do not skip categories silently — at least consider each STRIDE class
• ] Do not rate everything "high" — prioritize by realistic likelihood × impact
• ] Do not propose vague mitigations ("add security") — name the specific control and where it lives
• ] Do not model an attack on a system you don't own or aren't authorized to assess

Based On

Threat-modeling practice (STRIDE, trust boundaries, data-flow diagrams, risk-ranked mitigations).