TX/threat_model.md

Threat Model — Tx OS

Date: 2026-05-13 Version: 1.0 (initial) Companion documents: .local/security/scan-report.md (automated scanners), .local/security/manual-review.md (manual review findings).

This document captures Tx OS's assets, trust boundaries, threats (STRIDE-style), and the security guarantees the system commits to upholding. It is the design-level companion to the per-finding manual review and is intended to outlive any individual fix task.

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1. System overview

Tx OS is an internal "office OS" for a single-tenant organization. It is deployed as a Replit app (single Express + Postgres + Object Storage stack) with a React/Vite SPA, a small mobile-friendly UI, and a real-time notification layer over Socket.IO. The app is bilingual (Arabic / English), and the workforce uses it to:

• Order internal services (e.g. coffee, printing).
• Manage notes, including shared-folder collaboration with checklist items.
• Schedule and track Executive Meetings, including PDF exports and an alert popup.
• Administer apps, roles, permissions, groups, and user accounts.

The app is single-tenant — there is no multi-tenant isolation requirement; all authenticated users belong to the same organization. The core authorization model is per-user roles + per-user groups, with permissions attached to roles and visibility attached to apps.

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2. Assets

Asset	Sensitivity	Where it lives	Why it matters
User credentials (users.password_hash)	Critical	Postgres users table	Compromise = account takeover for every user.
Session ids	Critical	Postgres user_sessions (connect-pg-simple) + browser cookie	Session hijack = full impersonation.
Password reset tokens (hashed)	High	password_reset_tokens table	One-shot account takeover if intercepted.
Executive meeting content (titles, attendees, notes, location, URL)	High	executive_meetings* tables	Org-confidential. Attendee lists reveal org structure.
Generated PDF archives	High	Object Storage PRIVATE_OBJECT_DIR/uploads/<uuid>	Same content as above, persisted.
Notes content (titles, body, checklist, replies)	High	notes* tables	User-private; some shared via folders.
User directory (name, email, avatar)	Medium	users table	Phishing fuel.
Service order history	Medium	service_orders table	Personal consumption record.
Audit logs (audit_logs, role/permission audit)	Medium	Postgres	Tamper-evidence; integrity matters more than confidentiality.
App / role / permission configuration	Medium	apps, roles, permissions, app_permissions	Mis-edit = privilege change.
Brand logo + uploaded service images / avatars	Low	Object Storage	Public-ish; logo is rendered into PDFs.

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3. Trust boundaries

┌─────────────────────────────────────────────────────────────────────┐
│ Public internet                                                     │
│                                                                     │
│   ┌────────────┐   HTTPS    ┌──────────────────────────────────┐    │
│   │  Browser   │ ─────────▶ │  Replit edge (mTLS-terminated   │    │
│   │  (SPA)     │            │  proxy, deploys.replit.app)     │    │
│   └────────────┘            └──────────────┬──────────────────┘    │
│                                            │                       │
└────────────────────────────────────────────┼───────────────────────┘
                                             │ HTTP (proxied)
                                             ▼
                              ┌──────────────────────────────┐
                              │  Express API server          │
                              │  (artifacts/api-server)      │
                              │  • express-session (Postgres)│
                              │  • Socket.IO                 │
                              │  • Drizzle ORM               │
                              └────────────┬─────────────────┘
                                           │
            ┌──────────────────────────────┼──────────────────────────────┐
            │                              │                              │
            ▼                              ▼                              ▼
    ┌───────────────┐            ┌──────────────────┐            ┌───────────────┐
    │ PostgreSQL    │            │ Replit Object    │            │ Replit sidecar│
    │ (Replit-      │            │ Storage (GCS via │            │ http://127.   │
    │  managed)     │            │  signed URLs)    │            │ 0.0.1:1106    │
    └───────────────┘            └──────────────────┘            └───────────────┘

Trust boundaries (numbered):

1. Browser ↔ Replit edge. TLS-terminated by the platform proxy. The SPA is the only first-party client.
2. Edge ↔ API server. Loopback within the Replit container. Trust is established via the upstream proxy's mTLS. The API server treats X-Forwarded-For as trusted (app.set("trust proxy", 1)).
3. API server ↔ Postgres. Connection over the in-cluster network; credentials in DATABASE_URL. Drizzle ORM is the only query path.
4. API server ↔ Object Storage. Signed-URL pattern via the Replit sidecar at http://127.0.0.1:1106 (loopback HTTP — see Task #522 SAST suppression). All upload URLs are short-lived (15 min) and target randomUUID() paths under PRIVATE_OBJECT_DIR.
5. API server ↔ Socket.IO clients. Same origin as the API server. Auth shared via session cookie; every socket joins exactly its user:${userId} room.

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4. Security guarantees the system must uphold

These are the invariants every change must preserve. A regression on any one of them is, by default, at least High severity.

• G1 — Authentication required. Every /api/* endpoint other than the intentionally-public set requires a valid session. The intentionally-public set today is: /api/auth/login, /api/auth/register, /api/auth/forgot-password, /api/auth/reset-password, /api/auth/reset-password/verify, /api/health, /api/settings (see MR-L5 — recommended to remove from this list), and /api/storage/public-objects/*. Adding any new endpoint to this list must be a deliberate, reviewed change.
• G2 — Server-side authorization. UI hiding is not a permission boundary. Every mutating endpoint and every read of non-public data must enforce its permission server-side, even if the SPA hides the control.
• G3 — Cross-user isolation. A user can never read or modify another user's notes, orders, password, settings, sessions, or notifications unless an explicit share / group / admin permission grants it.
• G4 — Built-in app routes are immutable (Task #517). Slugs in BUILTIN_APP_SLUGS cannot have their route or slug mutated, even by an admin, because the SPA hardcodes those routes.
• G5 — Sensitive HTML is sanitized. Every user-supplied string that enters an HTML or PDF rendering path passes through htmlToSafeHtml/safeHtml (sanitize-html + DOMPurify) with the established allow-list.
• G6 — Secrets never leave the server. No .env value is sent to the browser. SESSION_SECRET is required in production (app.ts:64).
• G7 — Database access via parameterized queries only. Drizzle ORM template tags and the column-name allow-list in executive-meetings.ts:109 are the only sql\`` consumers; no string-spliced SQL is permitted.
• G8 — Loopback-only services stay loopback. The Object Storage sidecar at http://127.0.0.1:1106 is the only intentional plaintext fetch and must never be reachable from outside the container.

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5. STRIDE threats

For each STRIDE category, the table lists the most consequential threats, the relevant assets, current mitigations, and any open gaps. References like (MR-H1) point to entries in .local/security/manual-review.md.

5.1 Spoofing

#	Threat	Mitigation today	Gap
S1	Login as another user via stolen password	Bcrypt password hashing; session cookie httpOnly	No rate limit on /auth/login (MR-H3); bcrypt cost 10 instead of 12 (MR-L2); account enumeration on register (MR-L3)
S2	Hijack victim's session by fixing a session id	connect.sid, httpOnly, secure (prod), sameSite=lax, trust proxy=1	No req.session.regenerate after login (MR-M2)
S3	Spoof another user over the realtime channel	Server-side userId from session; per-user rooms	None — the server defines no client-originated socket events
S4	Spoof the Object Storage sidecar	Hardcoded loopback URL, no DNS lookup	None

5.2 Tampering

#	Threat	Mitigation today	Gap
T1	Cross-site request forces state change as the victim	sameSite=lax cookie, custom Content-Type: application/json for most writes	No CSRF token; lax permits top-level GET/POST navigation (MR-M1)
T2	Mutate another user's note/order/meeting	Per-route permission checks; tenant-scoping via userId/folderId lookups	OK — no mass-assignment issues found
T3	Edit a built-in app's route to break navigation	Task #517 lock with regression tests	OK — apps-builtin-route-lock.test.mjs covers it
T4	Pollute the audit log with fake events	app.opens audit row is recorded for any caller	POST /apps/:id/open doesn't gate on visibility (MR-M5)
T5	Pollute the PDF archive list with attacker-chosen filePath	requireExecutiveAccess only	Any executive role can write arbitrary paths (MR-H2)

5.3 Repudiation

#	Threat	Mitigation today	Gap
R1	Admin denies having changed a permission	permission-audit.ts records role/permission changes	OK
R2	User denies sending a note	notes table has createdAt + sender; reply chain timestamped	OK
R3	App-open audit is unreliable due to T4 above	—	Same fix as T4 (MR-M5)

5.4 Information disclosure

#	Threat	Mitigation today	Gap
I1	Read another user's PDF or avatar by guessing object UUID	Object paths are random UUIDs	/api/storage/objects/* only requires auth, not per-object access (MR-H1)
I2	Enumerate executive meeting dates as an unprivileged user	Per-meeting fetches require executive_access	The realtime executive_meetings_changed and executive_meeting_notifications_changed events are global broadcasts (MR-L1)
I3	Harvest the org's user directory	requireAuth gate	No permission gate; entire directory is visible to every user (MR-M4)
I4	Leak DB schema via verbose error messages	Most routes return coded short strings	executive-meetings.ts sites echo Error.message (MR-M6)
I5	Leak SPA bundle internals via missing CSP	TLS only	No helmet / CSP (MR-M3)
I6	Leak public flags via /api/settings	—	Endpoint is unauthenticated (MR-L5)

5.5 Denial of service

#	Threat	Mitigation today	Gap
D1	Brute-force /auth/login until success	bcrypt slowdown only	No rate limit (MR-H3)
D2	Flood /auth/forgot-password to fill table + send mail	None	No rate limit (MR-H3)
D3	Open thousands of sockets to exhaust file descriptors	sessionMiddleware gate at handshake	No per-user / per-IP cap (MR-L4)
D4	Send giant JSON bodies to slow parse	express.json() default 100 KB	No tighter cap (MR-L7)
D5	Trigger expensive PDF render in a loop	requireExecutiveAccess only	No per-user PDF render rate limit (latent)

5.6 Elevation of privilege

#	Threat	Mitigation today	Gap
E1	Low-priv user reads admin-only meeting PDFs	—	(MR-H1, MR-H2) — currently exploitable via storage download
E2	Low-priv user changes their own role	roles.ts is requireAdmin	OK
E3	User edits another user's password	users.ts password change is requireAdmin or self-only	OK
E4	Bypass built-in route lock by renaming slug then route	Task #517 locks both fields with regression tests	OK
E5	Smuggle XSS into PDF or note rendering	htmlToSafeHtml + DOMPurify allow-list	OK at HEAD, but no CSP backstop (MR-M3)

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6. What is explicitly out of scope

• Multi-tenant isolation. Tx OS is single-tenant. There is no per-tenant data partition.
• Data residency. Production data is stored wherever Replit hosts the project; no regional commitments.
• Penetration testing of the live deployment. This document is design-level only.
• Compliance frameworks (SOC 2 / ISO 27001). Not in scope today; the threat model is structured so the most material gaps would map cleanly into a future compliance effort if one is opened.

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7. Open recommendations (prioritized for follow-up)

The detailed remediation text lives in .local/security/manual-review.md (per-finding) and in .local/security/scan-report.md (dependency batches). The order below is the recommended fix sequence based on risk × effort.

1. MR-H1 + MR-H2 + MR-M7 — wire up object-level authorization for /api/storage/objects/* and constrain the PDF archive write endpoint. (Highest user impact; one focused change to routes/storage.ts plus a small change to pdf-archives write.)
2. MR-H3 + MR-L4 — add express-rate-limit on auth endpoints and a per-user socket cap. (Small library change, big DoS / brute-force win.)
3. MR-M1 + MR-M2 — CSRF posture (move session cookie to sameSite=strict if no third-party deep-links, otherwise add a CSRF token middleware) + req.session.regenerate on login.
4. MR-M3 — add helmet() and a basic CSP for the SPA bundle.
5. MR-M4 + MR-M5 + MR-M6 + MR-L5 — tighten the small set of over-exposed read endpoints (directory, settings) and audit-log write paths.
6. Dependency Batch A (from Task #522) — production-runtime patches for fast-xml-builder, lodash, path-to-regexp, postcss.
7. Dependency Batches B–C (from Task #522) — build-tool and codegen patches.
8. MR-L1 / MR-L2 / MR-L3 / MR-L6 / MR-L7 / MR-L8 — defense-in-depth and hardening.