Threat Modeling

Threat Modeling

Threat modeling is structured anticipation: you reason about how a system can be attacked while it is still cheap to change — at design time, on paper or in a diagram, before the code exists. The output is not a document; it is a set of mitigations wired to tests and a habit of asking "what could go wrong?" continuously.

This skill runs on Shostack's Four Question Framework (Adam Shostack, Threat Modeling: Designing for Security): What are we working on? What can go wrong? What are we going to do about it? Did we do a good job? Everything else — STRIDE, DFDs, risk ranking — serves those four questions.

Read first (always)

List learnings/ and read every file relevant to the current system (the domain, the trust boundaries, the third parties, the compliance regime). Project-specific assets, known attacker personas, accepted risks, and "in this system X is out of scope because Y" decisions live there and override the defaults in this SKILL.md. If a learning conflicts with this file, the learning wins — mention it to the user.

If learnings/ holds only its README, proceed with the defaults below.

Why this matters

Most vulnerabilities are design flaws, not coding bugs — a missing trust boundary, an unverified assumption, an authorization check that lives on the client. Code review and tests catch implementation defects; they rarely catch "we never decided who is allowed to do this." Threat modeling is the only activity that surfaces the flaw before the architecture sets. The Threat Modeling Manifesto frames it as a culture: a team that continuously finds and fixes, values a journey over a one-time artifact, and treats the model as living.

The Four Question Framework (the workflow)

1. What are we working on?

Define scope and assets, then draw the system.

• Assets: what is worth protecting? Data (PII, credentials, payment data, secrets), capabilities (admin actions, money movement), and reputation/availability. If nothing is worth stealing or breaking, you have no threat model.
• Diagram: build a Data Flow Diagram (DFD) — external entities, processes, data stores, data flows — and draw trust boundaries where privilege or control changes (network edge, process boundary, third-party call, user↔server). The boundaries are where the threats concentrate.
• Altitude: scope the system at the right zoom — one feature or service, not the whole company. Too big and you boil the ocean; too small and you miss the boundary crossing that matters.

→ Full method, element types, boundary rules, and a worked example: references/dfd-and-scope.md. → verify: you can name the assets, every data flow crosses a labeled boundary or stays inside one, and the diagram fits on one screen.

2. What can go wrong?

Walk the diagram and apply STRIDE per element along each data flow and every boundary crossing.

• STRIDE maps each threat to the security property it violates: Spoofing↔authentication, Tampering↔integrity, Repudiation↔non-repudiation, Information disclosure↔confidentiality, Denial of service↔availability, Elevation of privilege↔authorization (Microsoft STRIDE).
• STRIDE-per-element: not every category applies to every element type — processes face all six; data stores rarely "spoof." Use the element-type table so you ask the right questions and don't waste effort.
• Abuse / misuse cases: flip each user story into an attacker's goal ("as an attacker, I want to…"). Add attacker personas to keep the analysis grounded in who would actually try.

→ STRIDE definitions, per-element applicability, example threats and mitigations: references/stride.md. → Abuse cases, attacker personas, and other lenses (LINDDUN for privacy): references/methodologies-and-risk.md. → verify: every element and every boundary crossing has been questioned against its applicable STRIDE categories; threats are written as concrete scenarios, not category names.

3. What are we going to do about it?

For each threat, decide a response and bind it to something verifiable.

• Response (pick one): mitigate (add a control), eliminate (remove the feature/flow), transfer (push to a provider/insurer), or accept (document who accepted it and why). Accepting is a valid answer — silently ignoring is not.
• Map threat → control: every mitigated threat names a concrete control (input validation, authz check, encryption in transit, rate limit, audit log, signed token…).
• Map control → test: every control names a test or abuse-case that would fail if the control regressed. A control with no test will rot.
• Track the rest: unresolved threats become work items with an owner and a risk rank. Prioritize by risk = likelihood × impact (see references/methodologies-and-risk.md for ranking options and the criticism of DREAD).

→ verify: no threat is left in "what can go wrong" without a response; every "mitigate" has a control and a test reference; accepted risks name an accepter.

4. Did we do a good job?

Review coverage and treat the model as living.

• Coverage: did you cover every element, every boundary, every applicable STRIDE category? Did the highest-risk threats get the strongest controls?
• Continuity: threat modeling is not done once. Redo it on significant change — a new data flow, a new trust boundary, a new third party, a new auth path, new categories of data (especially PII). Keep each pass lightweight.
• Feedback: did findings actually become controls and tests in the codebase? A model that never changes the code did nothing.

→ verify: the model is dated, the diff from last time is clear, and follow-through (controls + tests landed) is confirmed — not just promised.

Anti-patterns (these defeat the purpose)

• Boiling the ocean. Modeling the entire system at maximum detail. You will never finish and you will model things that don't matter. Scope tight, model the risky boundary.
• One-time-only with no follow-through. A threat model produced once, filed, and never revisited — and whose threats never became controls or tests. The Manifesto's core warning: value the journey, not the artifact.
• Modeling without a DFD or assets. "Brainstorming threats" with no diagram and no asset list produces a random list, not coverage. You can't reason about boundary crossings you never drew.
• Threats with no controls or tests. A long "what can go wrong" list and an empty "what are we going to do about it." The list is theater unless each item maps to a mitigation and a test.
• Wrong altitude. Modeling at the level of individual functions (too low) or the whole company (too high). Model a feature/service.

Capture a learning

After completing a threat model, ask: did I encounter an asset, attacker persona, trust boundary, accepted-risk decision, compliance constraint, or recurring threat pattern not already in this SKILL.md or references/? If yes, append a learnings/YYYY-MM-DD-slug.md (protocol in learnings/README.md). If no, skip — don't log the model you just produced.

See also

• references/stride.md — STRIDE category definitions, the security property each violates, STRIDE-per-element applicability, and example threats with example mitigations.
• references/dfd-and-scope.md — how to build a Data Flow Diagram (four element types + trust boundaries), how to scope assets and altitude, with a worked mini-example.
• references/methodologies-and-risk.md — when to reach for PASTA, Attack Trees, LINDDUN (privacy), VAST, OCTAVE; risk ranking (likelihood × impact, DREAD and its criticism, CVSS); abuse cases and attacker personas.
• developing-features skill — security implementation depth once threats are known; see its references/security.md for the controls (validation, authz, secrets, encryption) you map threats to.
• reviewing-code skill — the security checks that verify, at PR time, that the controls this model specified actually shipped.
• api-design skill — API security (authn/authz, input validation, rate limiting) for threats found on API trust boundaries.
• learnings/ — project-specific assets, attacker personas, accepted risks, and recurring threat patterns.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Cristhianzl
• Source: Cristhianzl/claude-skills-czl
• License: MIT

Install and usage instructions live in the source repository linked above.