# JWT & Refresh Token Security Audit

## Security Engineer Review

### ✅ **SECURE IMPLEMENTATIONS**

#### 1. Token Storage (Android)
- **Status**: ✅ SECURE
- **Implementation**: Uses `EncryptedSharedPreferences` with AES256_GCM encryption
- **Location**: `TokenManager.kt`
- **Details**:
  - MasterKey with AES256_GCM scheme
  - PrefKeyEncryptionScheme: AES256_SIV
  - PrefValueEncryptionScheme: AES256_GCM
  - Tokens never stored in plain text
  - Tokens cleared on logout

#### 2. Refresh Token Rotation
- **Status**: ✅ SECURE
- **Implementation**: Refresh tokens rotate on each refresh
- **Location**: `tokenService.js` - `rotateRefreshToken()`
- **Details**:
  - Old refresh token is revoked before issuing new one
  - New refresh token has new token_id (JTI)
  - Prevents token reuse attacks
  - Tracks rotation chain via `rotated_from_id`

#### 3. Token Reuse Detection
- **Status**: ✅ SECURE
- **Implementation**: Detects and handles refresh token reuse
- **Location**: `tokenService.js` - `handleReuse()`
- **Details**:
  - If same refresh token used twice, all device tokens revoked
  - `reuse_detected_at` timestamp recorded
  - Prevents replay attacks

#### 4. Token Versioning (Global Logout)
- **Status**: ✅ SECURE
- **Implementation**: Token version incremented on logout-all-devices
- **Location**: `authMiddleware.js` - validates `token_version`
- **Details**:
  - Each user has `token_version` in database
  - Tokens include `token_version` in payload
  - If version mismatch, token rejected
  - Allows global logout functionality

#### 5. Idle Timeout
- **Status**: ✅ SECURE
- **Implementation**: Refresh tokens expire after 3 days of inactivity
- **Location**: `tokenService.js` - `verifyRefreshToken()`
- **Details**:
  - `REFRESH_MAX_IDLE_MINUTES` = 4320 (3 days)
  - Prevents long-lived abandoned sessions
  - Automatic cleanup of stale tokens

#### 6. Device Binding
- **Status**: ✅ SECURE
- **Implementation**: Refresh tokens bound to device_id
- **Location**: `tokenService.js` - `verifyRefreshToken()`
- **Details**:
  - Token must match user_id AND device_id
  - Prevents token theft across devices
  - Device ID sanitized and validated

#### 7. Token Hashing
- **Status**: ✅ SECURE
- **Implementation**: Refresh tokens hashed with bcrypt before storage
- **Location**: `tokenService.js` - `storeRefreshToken()`
- **Details**:
  - Tokens never stored in plain text in database
  - bcrypt.compare() used for verification
  - Even if database compromised, tokens not directly usable

#### 8. JWT Claims Validation
- **Status**: ✅ SECURE
- **Implementation**: Validates JWT claims (iss, aud, exp, iat, nbf)
- **Location**: `authMiddleware.js` - `validateTokenClaims()`
- **Details**:
  - Prevents token manipulation
  - Ensures tokens are within valid time window
  - Validates issuer and audience

#### 9. Access Token Short Lifetime
- **Status**: ✅ SECURE
- **Implementation**: Access tokens expire in 15 minutes
- **Location**: `tokenService.js` - `signAccessToken()`
- **Details**:
  - Limits exposure window if token stolen
  - Forces frequent refresh
  - Reduces impact of token leakage

### ⚠️ **POTENTIAL ISSUES & RECOMMENDATIONS**

#### 1. Auto-Login on App Restart
- **Status**: ⚠️ FIXED
- **Issue**: User wasn't automatically logged in when reopening app
- **Root Cause**: 
  - MainViewModel tried to validate tokens but didn't handle expired access tokens properly
  - Ktor's auto-refresh might not trigger on initial app startup
- **Fix Applied**:
  - Improved `validateTokens()` to try getUserDetails first (uses Ktor auto-refresh)
  - Falls back to manual refresh if auto-refresh doesn't work
  - Better error handling and token validation flow

#### 2. Token Refresh on Startup
- **Status**: ✅ IMPROVED
- **Implementation**: Now properly handles token refresh on app startup
- **Flow**:
  1. Check if tokens exist
  2. Try to fetch user details (triggers Ktor auto-refresh if needed)
  3. If that fails, manually refresh token
  4. If refresh fails, clear tokens and logout

#### 3. Network Error Handling
- **Status**: ✅ GOOD
- **Implementation**: Proper error handling for network failures
- **Details**:
  - Distinguishes between token expiration and network errors
  - Only clears tokens on authentication failures
  - Network errors don't force logout

### 🔒 **SECURITY BEST PRACTICES FOLLOWED**

1. ✅ **Encrypted Storage**: Tokens stored in EncryptedSharedPreferences
2. ✅ **Token Rotation**: Refresh tokens rotate on each use
3. ✅ **Reuse Detection**: Detects and prevents token reuse
4. ✅ **Short Access Token Lifetime**: 15 minutes
5. ✅ **Idle Timeout**: 3 days of inactivity
6. ✅ **Device Binding**: Tokens bound to specific device
7. ✅ **Token Hashing**: Refresh tokens hashed in database
8. ✅ **Global Logout**: Token versioning enables logout-all-devices
9. ✅ **Claims Validation**: JWT claims properly validated
10. ✅ **Secure Transmission**: Tokens sent in Authorization header (HTTPS required in production)

### 📋 **RECOMMENDATIONS FOR PRODUCTION**

1. **HTTPS Enforcement**:
   - Ensure all API calls use HTTPS
   - Implement certificate pinning for additional security

2. **Token Expiration Monitoring**:
   - Log token refresh events for security monitoring
   - Alert on suspicious refresh patterns

3. **Rate Limiting**:
   - Already implemented on backend
   - Monitor for abuse

4. **Biometric Authentication** (Future Enhancement):
   - Consider adding biometric unlock for sensitive operations
   - Store tokens behind biometric lock

5. **Token Refresh Retry Logic**:
   - Current implementation handles retries well
   - Consider exponential backoff for network failures

### 🎯 **SUMMARY**

**Overall Security Rating**: ✅ **SECURE**

The JWT and refresh token implementation follows security best practices:
- Secure storage (encrypted)
- Token rotation
- Reuse detection
- Short access token lifetime
- Device binding
- Global logout capability

**Auto-Login Issue**: ✅ **FIXED**
- Improved token validation flow
- Better handling of expired tokens on app startup
- Fallback mechanisms for token refresh

The implementation is production-ready from a security perspective. The main issue (auto-login) has been resolved.

