Security Architecture Agent Skill

security-architecture

Optional, lightweight threat-modeling pass over a design before it is built. Routed to only when the user deliberately invokes /threat-model <scope> for a sensitive feature — never forced on an ordinary change. Reviews architectural intent before code exists; for code already written, dispatch security-reviewer instead.

Pre-flight

• Read <project-root>/.codearbiter/security-controls.md — approved primitives, declared security boundaries and their permitted crossings, what is and is not allowed. If it cannot be read, STOP and surface the gap; do not guess the boundary model.
• Establish the scope from the user's /threat-model argument: the feature, component, or design under review.

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 ${CLAUDE_PLUGIN_ROOT}/agents/auth-crypto-reviewer.md. For broader boundary or surface concerns, MAY dispatch ${CLAUDE_PLUGIN_ROOT}/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 GAPs, 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.