threat-modeler

Purpose

Identify likely threats and attack surfaces for a feature or system before it's built, so mitigations are designed in from the start rather than patched on after a Security Auditor review finds them in finished code.

When to Use

• Designing a new feature that touches user data, authentication, payments, or external/untrusted input.
• Before finalizing an architecture that introduces a new trust boundary (a new service, a new external integration, a new user-facing input).
• The user asks for a "threat model" or "what could go wrong from a security angle" on something not yet built.

Differs from Security Auditor: that skill reviews existing code for concrete vulnerabilities; this skill reasons about a design before code exists, to catch classes of risk while they're still cheap to address.

What to Analyze / Do

1. Identify assets — what's actually worth protecting here (user credentials, PII, payment data, business logic that could be abused).
2. Identify trust boundaries — every place data crosses from one trust level to another (client → server, service → service, user input → database).
3. Apply STRIDE per boundary — for each boundary, consider: Spoofing, Tampering, Repudiation, Information disclosure, Denial of service, Elevation of privilege. Not every category applies everywhere — skip what's clearly irrelevant rather than padding.
4. Rank by realistic likelihood × impact — a theoretically possible but operationally implausible threat matters less than a highly likely one; say which is which.
5. Propose concrete mitigations — tied to the specific threat, not generic "add security" advice.

Output Format

• Assets and trust boundaries identified, briefly.
• A table or list: Threat → STRIDE category → Likelihood/Impact → Mitigation.
• Flag the 2-3 highest-priority threats explicitly, separate from the full list, so they don't get lost among lower-priority ones.

Avoid

• Generating a threat for every STRIDE category at every boundary regardless of relevance — that produces noise, not signal.
• Vague mitigations ("validate input better") — name the specific validation/control.
• Threat-modeling code that already exists and can be concretely reviewed — that's Security Auditor's job.