Threat Model STRIDE

A threat found at design review costs a paragraph in a doc; the same threat found in production costs an incident, a credential rotation, and sometimes a disclosure. STRIDE catches whole classes of flaws - spoofed callers, tamperable payloads, privilege leaks - before there is code to review. Apply it to any feature touching auth, data storage, external calls, or privilege changes; skip it and the same flaws surface later in secure-code-review or, worse, in vulnerability-triage.

Operating procedure

Follow the steps in order. Threats enumerated before trust boundaries are explicit come out vague and unactionable, and mitigations written before scoring waste effort on Low findings.

Step 1: Gather inputs

Collect these before modeling. Where the user cannot answer, use the default and label the assumption a guess.

• Feature description in 3-5 sentences: what it does, who calls it, what it stores.
• Actors: users, internal services, external APIs, admin roles. Default: anonymous internet user, authenticated user, one backend service.
• Data assets and a sensitivity tier for each: credentials > financial > PII > internal > public.
• Deployment context: internet-facing or internal-only; behind a WAF or VPN or not.
• Existing controls: auth mechanism (session, JWT, mTLS), rate limiting, audit logging.
• Compliance regime in scope (SOC 2, PCI, HIPAA, GDPR), if any.

Step 2: Map the data flow in prose

Establish a minimal data-flow diagram in prose: name every actor, every trust boundary (browser/server, service/DB, internal/external network), and every data asset with its sensitivity tier. Do not proceed until boundaries are explicit - vague scope produces vague threats.

Step 3: Apply STRIDE per boundary-crossing component

For each component that crosses a trust boundary, evaluate all six categories. State "no credible threat" explicitly rather than skipping a category.

• Spoofing: can an attacker impersonate a user, service, or token? Check auth mechanisms at every entry point.
• Tampering: can inputs, stored records, or in-flight data be modified without detection? Check integrity controls and audit logs.
• Repudiation: can an actor deny an action? Check whether sensitive operations are logged with actor identity and timestamp.
• Information Disclosure: can data leak to unauthorized parties? Check access controls, error messages, logs, and caches.
• Denial of Service: can an attacker exhaust resources or degrade availability? Check rate limiting, queue depth limits, and timeouts.
• Elevation of Privilege: can an actor gain permissions beyond their role? Check authorization at every privilege boundary, not just at entry.

Step 4: Score with DREAD-light

Score each threat on two axes only.

• Exploitability: 3 = triggerable by an anonymous internet user with public tooling; 2 = requires an authenticated low-privilege account or nontrivial setup; 1 = requires insider access, physical access, or chaining other flaws.
• Impact: 3 = credential theft, remote code execution, regulated-data exposure, or full account takeover; 2 = single-user data exposure or degraded service; 1 = marginal information or brief nuisance.

Severity = the sum: 6 is Critical, 5 is High, 4 is Medium, 3 or below is Low. Drop Low findings from the table unless they chain into a higher-severity path - a padded table trains reviewers to skim.

Step 5: Recommend one concrete control per Critical and High

• Prefer existing platform primitives (JWT validation middleware, ORM parameterization, IAM roles) over custom code - custom security code is new attack surface.
• Specify the control, not the category: "add HMAC-SHA256 signature on the webhook payload, verified before parsing", not "add integrity check".
• Flag any mitigation requiring a schema change or new dependency - those need separate review.

Step 6: Produce the threat table

One row per threat: STRIDE category, affected component, attack scenario in one sentence, severity with the two scores, recommended control, owner team. The whole table must be reviewable in under ten minutes by a non-security engineer.

Worked example: expiring share links for uploaded invoices

Feature: users upload invoice PDFs; the app generates share links that expire after 7 days; recipients need no account. Trust boundaries: browser/API, API/object storage, anonymous internet/share endpoint.

Deliverable

Produce a threat model containing: the prose data-flow summary with named trust boundaries; the threat table with STRIDE category, component, one-sentence scenario, severity with both scores, concrete control, and owner per row; and a flagged list of mitigations that require schema changes or new dependencies for separate review.

Do NOT

• Do not enumerate threats before trust boundaries are explicit - every vague threat traces back to a vague boundary.
• Do not recommend a category instead of a control; a finding nobody can turn into a ticket does not get fixed.
• Do not custom-build what a platform primitive already provides.
• Do not pad the table with Low findings; include a Low only when it chains into a higher-severity path.
• Do not treat a STRIDE pass on a design as clearance for the implementation - pair with secure-code-review once code exists, because STRIDE cannot see injection bugs or missing ownership checks.

Quality bar

• Every trust boundary is named before any threat is listed.
• All six STRIDE categories are evaluated for every boundary-crossing component; absences are stated, not skipped.
• Every Critical and High row names one control specific enough to write a ticket from.
• Severity comes from the exploitability and impact bands, not gut feel.
• The full table reads in under ten minutes for a non-security engineer.

Escalation

Route implementation review of written code to secure-code-review, prioritization of already-discovered vulnerabilities to vulnerability-triage, and any credential exposure found during modeling to secrets-hygiene. For regulated data (PCI, HIPAA), bring in a qualified security engineer or assessor - this skill structures the analysis; it does not replace a formal assessment.