/ca-threat-model

name: ca-threat-model
description: "Threat-model a sensitive design with STRIDE on request. Read-only analysis of threats, controls, and implementation constraints."
argument-hint: "<scope description>"

security-architecture

For every security policy reference below, follow [includes/knowledge.md](../../includes/knowledge.md) ("Read security policy before a security decision"): select the repository's context format, read the complete policy from exact `CTX-SECURITY-CONTROLS` records in HTML mode or the full Markdown controls in Markdown mode, and read operative scope blocks when applicable. Required policy that is missing or unreadable blocks a security decision; scope blocks alone are not full policy.

Optional, lightweight threat-modeling pass over a design before it is built. Selected when the user deliberately requests threat modeling of a sensitive design, with or without `$ca-threat-model`. Never forced on an ordinary change. Reviews architectural intent before code exists; for code already written, dispatch `security-reviewer` instead.

Entry boundaries

Read-only: modify no project file, decision record, control or implementation. Optional reviewers inherit this read-only analysis scope. An explanation-only security question does not start the pass; mentioning authentication in an ordinary change does not opt in. The user need not repeat a slash command to request it. Missing required security-controls input is a prerequisite failure, separate from the severity of a finding. A lesser gap is a constraint, not blanket denied clearance.

Pre-flight

• Read the complete format-selected `security-controls` policy — approved primitives, declared security boundaries and their permitted crossings, what is and is not allowed. If the required policy cannot be read, STOP and surface the gap; do not guess the boundary model.
• Establish the scope from the user's request: the feature, component, or design under review.
• Read relevant security ADRs under `<project-root>/.codearbiter/decisions/`; do not invent decisions or interpret their absence as approval.

Phase 1 — Attack surface · gate: BLOCK

Map what the change exposes. Name, for the scope:

• New or changed entry points — routes, endpoints, message consumers, CLI surfaces.
• New egress — outbound calls, external dependencies, new data sinks.
• Security boundaries crossed, as declared in `security-controls.md` (trust transitions, privilege changes, data leaving a controlled zone).
• The data handled and its sensitivity.

A boundary crossing not described in `security-controls.md` is a finding, not a silent pass. A finding outside this scope gets one line with an inline `[NEEDS-TRIAGE]` marker.

Gate: the attack surface and every boundary crossing in scope are enumerated.

Phase 2 — STRIDE pass · gate: STOP

Walk the surface from Phase 1 through STRIDE. For each relevant category, name the concrete threat and the control expected to mitigate it:

• **Spoofing** — identity/authentication of the actor at the boundary.
• **Tampering** — integrity of data in transit and at rest.
• **Repudiation** — whether actions are attributable.
• **Information disclosure** — exposure of sensitive data, including in logs and errors.
• **Denial of service** — exhaustion or availability impact.
• **Elevation of privilege** — privilege gained across the boundary.

Mark each threat's mitigation `PRESENT`, `PLANNED`, or `GAP`. Skip a category only with a one-line reason it does not apply.

If the threat depends on auth, crypto, key handling, or secrets, MAY dispatch [agents/auth-crypto-reviewer.md](../../agents/auth-crypto-reviewer.md). For broader boundary or surface concerns, MAY dispatch [agents/security-reviewer.md](../../agents/security-reviewer.md). Both govern by `security-controls.md`.

Gate: STOP only on a genuinely critical unmitigated threat — a `GAP` that is exploitable now with high impact. Lesser gaps are surfaced as constraints, not stops.

Phase 3 — Report · gate: BLOCK

Produce a terse report:

• **Surface** — entry points, egress, boundaries crossed.
• **Threats** — the STRIDE findings; lead with `GAP`s, then `PLANNED`, summarize `PRESENT` in one line.
• **Verdict** — `PROCEED` (no open gap), `PROCEED-WITH-CONSTRAINTS` (gaps listed with owner), or `STOP` (a critical unmitigated threat is present; name it).

A decision-worthy gap is escalated to the user or to `<project-root>/.codearbiter/decisions/` via `/adr` — never authored as an ADR by this skill.

Gate: report delivered with a stated verdict; every STOP-level threat from Phase 2 is reflected in it.

Hard rules

• MUST NOT force this pass on an ordinary change. It is invoked deliberately for sensitive features only.
• MUST NOT guess the boundary model — read `security-controls.md` or STOP.
• MUST NOT silently pass an undeclared boundary crossing — surface it.
• MUST NOT STOP on anything short of a critical, exploitable, unmitigated threat; lesser gaps are constraints.
• MUST NOT author an ADR — escalate decision-worthy gaps to the user or `/adr`.