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Security Architecture

Security Architecture Document

Project: Bilko — Balkan Accounting SaaS Version: 1.0 Date: 2026-02-23 Author: Compliance Architect Status: Draft Reviewers: CTO, DPO, Engineering Lead Classification: Confidential

Document History

Version Date Author Changes
0.1 2026-02-23 Compliance Architect Initial draft — Bilko security architecture

1. Security Architecture Overview

Security Owner: Compliance Architect ([email protected]) Last Security Review: 2026-02-23 Next Scheduled Review: 2026-08-23 Compliance Targets: GDPR | Zakon o zaštiti podataka o ličnosti RS (ZZPL) | Zakon o zaštiti ličnih podataka BiH (ZZLP) | GDPR via AZOP (HR) | Zakon o računovodstvu RS/HR/BA | Zakon o PDV RS/BA/HR

Architecture Model: Bilko is a multi-tenant cloud accounting SaaS. Processes invoices, expenses, VAT returns, and financial reports for organizations in Serbia, Bosnia & Herzegovina, and Croatia. Each organization's data strictly isolated by organizationId at the database layer.

Defense-in-Depth Overview

graph TD
    CLIENT["Client Browser / PWA"]

    subgraph NETWORK["Network Layer"]
        CF["Cloudflare WAF\nDDoS Protection\nTLS 1.3 termination\nHSTS"]
    end

    subgraph APP_LAYER["Application Layer"]
        HELMET["Helmet.js\nCSP + X-Frame + HSTS\nno X-Powered-By"]
        CORS["CORS Whitelist\nbilko.io only\nno wildcard *"]
        RATE["Rate Limiter\nexpress-rate-limit\n5 req/15min auth\n100 req/15min general"]
        AUTH_MW["Auth Middleware\nJWT verify (15min access)\norg-scope injection"]
        RBAC_MW["RBAC Middleware\nowner / admin / accountant / viewer"]
        ZOD["Zod Validation\nall request bodies\ntype-safe parsing"]
    end

    subgraph DATA_LAYER["Data Layer"]
        PRISMA_ORM["Prisma ORM\nparameterized queries\nno raw SQL for user input\norg-scoped WHERE clauses"]
        PG_ENC["PostgreSQL (Railway EU West)\nAES-256 disk encryption\nbackup encryption"]
    end

    subgraph AUDIT["Audit Layer"]
        LOG["LoggedAction table\nAPPEND-ONLY\nIP + user + timestamp\nold/new values (changedFields)"]
    end

    CLIENT --> CF --> HELMET --> CORS --> RATE --> AUTH_MW --> RBAC_MW --> ZOD --> PRISMA_ORM --> PG_ENC
    PRISMA_ORM --> LOG

2. Authentication

2.1 Strategy: JWT (JSON Web Tokens)

Stateless JWT, scales horizontally on Railway. Access tokens (15 min, memory-only) + refresh tokens (7 days, httpOnly cookie). Rotation on every refresh. Revocation via hashed token storage in DB.

2.2 JWT Auth Flow

sequenceDiagram
    actor User
    participant FE as Frontend (bilko.io — Vercel)
    participant API as Express API (api.bilko.io — Railway EU)
    participant DB as PostgreSQL (Railway EU West)

    User->>FE: Enter email + password
    FE->>API: POST /api/v1/auth/login
    API->>DB: SELECT user WHERE email = ? (parameterized)
    DB-->>API: User record (passwordHash)
    API->>API: bcrypt.compare(password, hash) — 12 rounds
    alt Password valid
        API->>API: jwt.sign({sub, org, role}, JWT_SECRET, 15m)
        API->>DB: INSERT refreshToken (hashed, expiresAt)
        API-->>FE: 200 { accessToken } + Set-Cookie: refreshToken (httpOnly, secure, sameSite=strict)
        FE->>FE: Store accessToken in memory only
    else Password invalid
        API-->>FE: 401 Unauthorized (generic — no user enumeration)
    end

    Note over FE,API: 15 minutes later — access token expires
    FE->>API: POST /api/v1/auth/refresh (Cookie: refreshToken)
    API->>API: Rotate: delete old, issue new
    API-->>FE: 200 { newAccessToken } + Set-Cookie: newRefreshToken

    Note over User,DB: Logout
    FE->>API: POST /api/v1/auth/logout
    API->>DB: DELETE refreshToken WHERE userId = ?
    API-->>FE: 204 No Content

2.3 Two-Factor Authentication (2FA)

Method: TOTP (RFC 6238) — Google Authenticator, Authy, 1Password

  • Setup: POST /api/v1/auth/2fa/setup → QR code + base32 secret
  • Verify: POST /api/v1/auth/2fa/verify { code }
  • Login: returns { requires2FA: true, tempToken } → POST /api/v1/auth/2fa/login
  • Backup: 10 single-use codes, bcrypt-hashed

3. Authorization (RBAC)

3.1 Role Permission Matrix

Action owner admin accountant viewer
Create invoice ✅ ✅ ❌ ❌
Edit invoice ✅ ✅ ❌ ❌
Delete invoice ✅ ❌ ❌ ❌
View invoice ✅ ✅ ✅ ✅
Approve expense ✅ ✅ ❌ ❌
Generate report ✅ ✅ ✅ ❌
Invite user ✅ ❌ ❌ ❌
Edit org settings ✅ ❌ ❌ ❌

3.2 Organization Scoping (IDOR Prevention)

// Injected by auth middleware on all /api/v1/* routes
app.use('/api/v1/*', (req, res, next) => {
  req.prismaWhere = { organizationId: req.user.organizationId };
  next();
});

// Applied to every Prisma query
await prisma.invoice.findMany({ where: { ...req.prismaWhere } });

UUID primary keys throughout — no sequential ID enumeration possible.

4. Encryption

4.1 In Transit: TLS 1.3

All traffic HTTPS. Cloudflare TLS 1.3 at edge, re-encrypted to Railway. HSTS: max-age=63072000; includeSubDomains; preload.

4.2 At Rest: AES-256

Store Method Location
PostgreSQL AES-256 TDE (Railway) Railway EU West (Frankfurt/Paris)
PostgreSQL backups AES-256 auto-backup Railway EU West — 30 days
Tax IDs (PIB/JMBG/OIB/JIB), IBAN AES-256-GCM field encryption Application layer — Railway env secret
Cloudflare R2 (receipts, PDFs) AES-256 server-side Cloudflare EU region

4.3 Password Security

bcrypt, 12 salt rounds. Min 8 chars. Block top 10K common passwords. Last 5 hashes retained.

4.4 Financial Data Precision

All monetary amounts: NUMERIC(19,4) — never float. Exchange rates locked at transaction date.

5. OWASP Top 10 Mitigations

OWASP Risk Mitigation Status
A01: Broken Access Control RBAC + org-scoped WHERE + UUID PKs Designed
A02: Cryptographic Failures TLS 1.3 + AES-256 + bcrypt(12) + no PII in JWT Designed
A03: Injection Prisma ORM parameterized queries exclusively Designed
A04: Insecure Design Multi-tenant org isolation at DB layer, immutable audit Designed
A05: Security Misconfiguration Helmet.js, CORS whitelist (no *), sanitized errors Designed
A06: Vulnerable Components Dependabot + weekly npm audit + lock file Planned
A07: Auth Failures Rate limiting + JWT rotation + 2FA + bcrypt(12) Designed
A08: Software Integrity Signed commits + CI/CD + Dependabot Planned
A09: Logging Failures Immutable LoggedAction table + Railway logs + Sentry Designed
A10: SSRF Zod validation + allowlist for SEF/HR-FISK/FINA API Designed

6. Rate Limiting

Endpoint Limit Window
POST /api/v1/auth/login 5 req 15 min
POST /api/v1/auth/register 3 req 60 min
POST /api/v1/auth/refresh 10 req 15 min
GET /api/v1/reports/* 10 req 15 min
All other /api/v1/* 100 req 15 min

7. Input Validation (Zod)

const createInvoiceSchema = z.object({
  customerId: z.string().uuid(),
  invoiceDate: z.string().regex(/^\d{4}-\d{2}-\d{2}$/),
  dueDate: z.string().regex(/^\d{4}-\d{2}-\d{2}$/),
  currencyCode: z.enum(['EUR', 'RSD', 'BAM']),
  items: z.array(z.object({
    description: z.string().min(1).max(500),
    quantity: z.number().positive(),
    unitPrice: z.number().nonnegative(),
    taxRate: z.number().min(0).max(100),
  })),
});

8. File Upload Security

Allowed: JPG, PNG, PDF. Max 10 MB. MIME + extension validation. Stored in Cloudflare R2 EU. Phase 2: ClamAV scanning.

9. Audit Trail — LoggedAction (APPEND-ONLY)

Field Description
eventId Auto-incrementing
tableName Mutated table
action INSERT / UPDATE / DELETE
userId Actor
actionTimestamp UTC
rowData Full row before mutation
changedFields { field: { old: X, new: Y } }
clientIp Requester IP

On GDPR erasure: userId → "deleted-user". Financial entries retained 11 years (law). LoggedAction never deleted.

10. Security Headers (Helmet.js)

Header Value
Strict-Transport-Security max-age=63072000; includeSubDomains; preload
Content-Security-Policy default-src 'self'; script-src 'self' 'unsafe-inline'
X-Content-Type-Options nosniff
X-Frame-Options DENY
X-Powered-By Removed

11. Pre-Launch Security Checklist

  • JWT_SECRET generated (32+ chars, CSPRNG) — Railway env secret
  • JWT_REFRESH_SECRET separate key (32+ chars)
  • FIELD_ENCRYPTION_KEY generated (32 bytes hex) — for PIB/JMBG/OIB/JIB + IBAN
  • HTTPS enforced
  • CORS: bilko.io only
  • Rate limiting tested
  • Helmet.js headers verified
  • bcrypt rounds = 12
  • All Prisma queries use org-scoped WHERE
  • Zod validation on all endpoints
  • LoggedAction trigger active on all tables
  • Error responses sanitized
  • Dependabot alerts enabled
  • Railway region = EU West confirmed
  • DPAs signed (Railway, Vercel, Cloudflare, SendGrid)
  • Data deletion workflow tested

Approval

Role Name Date Signature
Author Compliance Architect 2026-02-23
CTO
DPO
Engineering Lead

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