Threat_Model.md

System Threat Model (STRIDE)

Last Updated: 2026-07-15

🎯 Threat Landscape Visualization

flowchart TD
    %% Define Boundaries
    subgraph "Trust Boundary: Public Internet"
        GitHub[GitHub Webhook Service]
    end
    
    subgraph "Trust Boundary: Edge Serverless (Vercel)"
        HMAC[HMAC Verifier Gate]
        App[ArchiCheck Webhook App]
    end
    
    subgraph "Trust Boundary: Secure Private Networks"
        Redis[(Upstash Redis Cache)]
        LLM[Vertex AI / Gemini API]
    end

    %% Flows
    GitHub -->|1. POST Event Payload & Signature HTTPS| HMAC
    HMAC -->|2. Verify Webhook HMAC TimingSafe| App
    App -->|3. GET Diff & PR Collaborator Roles HTTPS| GitHub
    App -->|4. Store & Fetch QuizState REST| Redis
    App -->|5. Evaluate Diff & Validate Justification HTTPS| LLM

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🛡️ STRIDE Assessment

Component / Flow	Spoofing	Tampering	Repudiation	Information Disclosure	Denial of Service	Elevation of Privilege	Mitigation	Date Added / Updated
GitHub Webhook Ingestion	Yes	Yes	No	No	Yes	No	HMAC Verification: Validate x-hub-signature-256 signature using timing-safe comparisons (crypto.timingSafeEqual).	2026-07-06
Justification Validation	No	Yes	Yes	No	No	Yes	PR Author Restricting & Tag Sanitizer: Reject replies from non-authors. Sanitize user inputs (sanitizePromptInput) by escaping XML tag boundaries (</answers>, </diff>) to block template escapes.	2026-07-07
Emergency Bypass Control	Yes	No	Yes	No	No	Yes	Collaborator Role Check: Verify commenter permission level (admin/maintain) before releasing gate locks.	2026-07-07
Secret Sanitizer Engine	No	No	No	Yes	Yes	No	Lookbehind Scrubber & watchdogs: Use ECMAScript lookbehinds to redact values; truncate lines >500 chars and halt regex after 500ms.	2026-07-07
State Caching	Yes	Yes	No	Yes	Yes	No	Upstash TLS: Use secure REST endpoints with a 1,000ms timeout circuit breaker, failing open to prevent build hangs.	2026-07-07
LLM Inference	Yes	No	No	Yes	Yes	No	Defensive System Prompts & Compliance: Inject strict security instructions directing the model to ignore injected commands inside diff/answers blocks. Call GCP Vertex AI endpoints (zero data retention policy) with 15s timeouts.	2026-07-08
Mock LLM Activation	No	Yes	No	No	No	Yes	Production Block Constraint: Zod schema in env.ts explicitly rejects LLM_PROVIDER_TYPE=mock when NODE_ENV === 'production' to prevent mock bypass in production.	2026-07-09
YAML Config Parser	No	Yes	No	No	Yes	No	Size Limiter & Graceful Fallback: Enforce 50KB maximum size check on fetched .archicheck.yml string. Wrap YAML parsing in try/catch and validate schemas via Zod defaults.	2026-07-09
Local Mock LLM Sandbox	No	Yes	No	No	Yes	No	Fast-Fail & Quarantine: Throw fatal exceptions on malformed config JSON to prevent phantom bugs; keep loading logic strictly quarantined from production runs.	2026-07-09
Local Playground Route Escape	Yes	No	No	Yes	Yes	Yes	Edge Middleware Blocking & Page Guards: Impenetrable Next.js Middleware edge path blocking combined with component-level notFound() checks when process.env.NODE_ENV === 'production'.	2026-07-10
Shadow Mode Command Leakage	No	Yes	No	No	No	Yes	Bypass Disabling: Hardcode bypass slash command parser execution to immediately abort if ARCHICHECK_MODE === 'shadow'.	2026-07-10