# CORS Configuration Research

**Date**: 2026-01-02
**Feature**: Configurable CORS for Modbus Relay Control System
**Research Focus**: Production-ready CORS configuration to replace permissive defaults

## Executive Summary

Current CORS implementation uses `Cors::new()` with default settings that allow ALL origins, ALL methods, and ALL headers - acceptable for development but insecure for production. This research documents how to implement configurable CORS following the project's existing patterns for middleware configuration.

## Current State Analysis

### Implementation Location
- **File**: `backend/src/startup.rs:86`
- **Current Code**: `.with(Cors::new())`
- **Status**: Hardcoded permissive CORS with no configuration

### Security Profile (Current)
- ✗ Allows ALL origins (including potentially malicious sites)
- ✗ Allows ALL HTTP methods (GET, POST, DELETE, PATCH, etc.)
- ✗ Allows ALL request headers
- ✓ Does NOT allow credentials (default: false)
- ✓ 24-hour preflight cache (default max_age: 86400)

### Configuration Gap
- `settings.frontend_url` exists in `backend/src/settings.rs:20` but is NOT used for CORS
- No CORS-specific settings struct
- No environment-aware CORS configuration

## Research Findings

### 1. Poem CORS API

**Core Configuration Methods**:
```rust
Cors::new()
    .allow_origin(origin: &str)                    // Single origin
    .allow_origins(origins: Vec<&str>)             // Multiple origins
    .allow_origin_regex(pattern: &str)             // Wildcard patterns
    .allow_method(method: Method)                  // HTTP method
    .allow_methods(methods: Vec<Method>)           // Multiple methods
    .allow_header(header: HeaderName)              // Request header
    .allow_headers(headers: Vec<HeaderName>)       // Multiple headers
    .expose_header(header: HeaderName)             // Response header
    .expose_headers(headers: Vec<HeaderName>)      // Multiple response headers
    .allow_credentials(bool)                       // Cookie/auth support
    .max_age(seconds: i32)                         // Preflight cache duration
```

**Default Behavior**:
- Empty collections → permits ALL values (permissive)
- `max_age`: 86400 seconds (24 hours)
- `allow_credentials`: false

### 2. Production Security Best Practices

#### Origin Configuration
| Approach | Security | Use Case |
|----------|----------|----------|
| Specific origin | High | Production (single frontend domain) |
| Multiple specific origins | High | Multi-environment (dev + prod) |
| Wildcard patterns | Medium | Subdomains (*.example.com) |
| Custom validation function | Medium | Complex rules |
| Default (empty) | Low | Development only |

**Recommendation**: Use specific origins from `frontend_url` setting.

#### Credentials
- **false**: Public APIs without authentication
- **true**: APIs requiring cookies/auth headers (Authelia requires this)

**For STA**: Set to `true` because Traefik uses Authelia authentication.

**Critical**: When credentials=true, wildcard `*` origin is NOT allowed by browsers. Must use specific origins.

#### Methods
- **Minimal**: GET only (read-only endpoints)
- **Standard**: GET, POST (basic REST)
- **Full**: GET, POST, PUT, DELETE, PATCH (complete CRUD)

**For STA**: GET, POST, PUT (relay control requires POST/PUT for state changes)

#### Headers
- **Essential**: Content-Type (for POST/PUT bodies)
- **Authentication**: Authorization (for bearer tokens)
- **Custom**: X-Request-Id, X-Custom-Header (application-specific)

**For STA**: Minimum required: `content-type`, `authorization`

#### Max Age
| Duration | Implications | Security |
|----------|--------------|----------|
| 0 | No caching, preflight every request | High but inefficient |
| 3600 (1 hour) | Balance security & performance | High (recommended) |
| 86400 (24 hours) | Fewer preflights, slower updates | Medium |
| 604800 (7 days) | Very few preflights | Low |

**Recommendation**: 3600 seconds (1 hour) for production.

### 3. Deployment Architecture Context

From `specs/001-modbus-relay-control/decisions.md`:

**Production Setup**:
- **Frontend**: Cloudflare Pages (static hosting with global CDN)
- **Backend**: Raspberry Pi 3B+ (local network, same as Modbus device)
- **Reverse Proxy**: Traefik on Raspberry Pi
  - HTTPS termination (TLS certificates)
  - Authelia middleware (user authentication)
  - Routes HTTPS requests to backend HTTP service

**Communication Flow**:
```
Frontend (Cloudflare Pages)
    ↓ HTTPS
Traefik (Raspberry Pi)
    ↓ Authelia authentication
    ↓ HTTP (local network)
Backend (Raspberry Pi)
    ↓ Modbus TCP
Relay Device (local network)
```

**CORS Implications**:
- Origin must be the Cloudflare Pages URL (HTTPS)
- Credentials must be allowed (Authelia auth tokens)
- Backend sees requests from Traefik, but CORS origin is frontend domain

### 4. Configuration Pattern (from Rate Limiting)

**Reference**: `backend/src/middleware/rate_limit.rs`

**Pattern Structure**:
1. **Configuration struct** with settings
2. **Middleware implementation** using configuration
3. **Conditional instantiation** in `startup.rs` based on settings
4. **YAML configuration** in `settings/*.yaml`
5. **Environment variable overrides** via `APP__` prefix

**Example from Rate Limiting**:
```rust
// Settings struct
pub struct RateLimitSettings {
    pub enabled: bool,
    pub burst_size: u32,
    pub per_seconds: u64,
}

// Conditional instantiation in startup.rs
let rate_limit_config = if value.settings.rate_limit.enabled {
    RateLimitConfig::new(burst_size, per_seconds)
} else {
    RateLimitConfig::new(u32::MAX, 1)  // Effectively disabled
};

let app = value.app
    .with(RateLimit::new(&rate_limit_config))
    .with(Cors::new())  // ← Should follow same pattern
    .data(value.settings);
```

## Recommended Approach

### Option A: Minimal Configuration (MVP)

Add simple CORS settings that cover 80% of use cases:

```rust
#[derive(Debug, serde::Deserialize, Clone)]
pub struct CorsSettings {
    pub allowed_origins: Vec<String>,
    pub allow_credentials: bool,
}

impl Default for CorsSettings {
    fn default() -> Self {
        Self {
            allowed_origins: vec!["*".to_string()],
            allow_credentials: false,
        }
    }
}
```

**YAML Configuration**:
```yaml
# development.yaml
cors:
  allowed_origins:
    - "http://localhost:3000"
    - "http://127.0.0.1:3000"
  allow_credentials: false

# production.yaml
cors:
  allowed_origins:
    - "https://REACTED"  # Cloudflare Pages URL
  allow_credentials: true
```

**Pros**:
- Simple to implement
- Covers most critical security concerns
- Easy to understand and maintain

**Cons**:
- Limited flexibility (no method/header customization)
- Cannot disable CORS entirely
- No max_age configuration

### Option B: Full Configuration (Production-Ready)

Complete CORS configuration matching Poem's capabilities:

```rust
#[derive(Debug, serde::Deserialize, Clone)]
pub struct CorsSettings {
    pub enabled: bool,
    pub allowed_origins: Vec<String>,
    pub allowed_methods: Vec<String>,
    pub allowed_headers: Vec<String>,
    pub expose_headers: Vec<String>,
    pub allow_credentials: bool,
    pub max_age_secs: i32,
}

impl Default for CorsSettings {
    fn default() -> Self {
        Self {
            enabled: true,
            allowed_origins: vec!["*".to_string()],
            allowed_methods: vec!["GET".to_string(), "POST".to_string(), "PUT".to_string()],
            allowed_headers: vec!["content-type".to_string(), "authorization".to_string()],
            expose_headers: vec![],
            allow_credentials: false,
            max_age_secs: 3600,
        }
    }
}
```

**YAML Configuration**:
```yaml
# production.yaml
cors:
  enabled: true
  allowed_origins:
    - "https://REDACTED"
  allowed_methods:
    - "GET"
    - "POST"
    - "PUT"
  allowed_headers:
    - "content-type"
    - "authorization"
  expose_headers:
    - "x-ratelimit-remaining"
  allow_credentials: true
  max_age_secs: 3600
```

**Pros**:
- Complete control over CORS behavior
- Can disable CORS entirely (enabled: false)
- Production-ready security
- Follows Poem API closely

**Cons**:
- More complex configuration
- More YAML to maintain
- Risk of misconfiguration

### Option C: Hybrid Approach (Recommended)

Combine simplicity with essential production features:

```rust
#[derive(Debug, serde::Deserialize, Clone)]
pub struct CorsSettings {
    pub allowed_origins: Vec<String>,
    pub allow_credentials: bool,
    pub max_age_secs: i32,
}

impl Default for CorsSettings {
    fn default() -> Self {
        Self {
            allowed_origins: vec!["*".to_string()],
            allow_credentials: false,
            max_age_secs: 3600,
        }
    }
}
```

**Methods and Headers**: Hardcoded in code based on API requirements (not configurable).

**Rationale**:
- Origins and credentials are deployment-specific (need configuration)
- Methods and headers are API-specific (shouldn't change per environment)
- Simpler than Option B, more secure than Option A
- Reduces configuration surface area

## Implementation Checklist

### Files to Modify

1. **`backend/src/settings.rs`**
   - Add `CorsSettings` struct (after `RateLimitSettings`)
   - Add `cors: CorsSettings` field to `Settings` struct
   - Add `#[serde(default)]` attribute for backward compatibility

2. **`backend/src/startup.rs`**
   - Import CORS-related types from Poem
   - Create helper method `build_cors(settings: &CorsSettings) -> Cors`
   - Replace `.with(Cors::new())` with `.with(Self::build_cors(&value.settings.cors))`

3. **`backend/settings/development.yaml`**
   - Add `cors:` section with permissive development settings
   - Allow localhost:3000 and 127.0.0.1:3000

4. **`backend/settings/production.yaml`**
   - Add `cors:` section with restrictive production settings
   - Use actual Cloudflare Pages URL
   - Set `allow_credentials: true`

5. **`specs/001-modbus-relay-control/plan.md`**
   - Update technical context with CORS configuration
   - Add CORS configuration task to implementation plan

6. **`CLAUDE.md`** (optional)
   - Document CORS configuration in project instructions

### Testing Strategy

1. **Unit Tests**: Verify `build_cors()` creates correct Poem `Cors` instance
2. **Integration Tests**: Verify CORS headers in HTTP responses
3. **Manual Tests**: Test with actual frontend (Vite dev server + Cloudflare Pages)

## Key Files Referenced

| File | Lines | Purpose |
|------|-------|---------|
| `/backend/src/startup.rs` | 9, 23, 83-87 | Current CORS implementation |
| `/backend/src/settings.rs` | 13-28, 75-90 | Settings structure |
| `/backend/src/middleware/rate_limit.rs` | 16-127 | Middleware pattern reference |
| `/backend/settings/development.yaml` | 1-9 | Development configuration |
| `/backend/settings/production.yaml` | 1-9 | Production configuration |
| `/specs/001-modbus-relay-control/decisions.md` | 71-92 | Deployment architecture |

## Security Considerations

### Critical Points

1. **Never use wildcard `*` origin with credentials enabled** - browsers reject this
2. **Specific origins only in production** - wildcards increase attack surface
3. **Minimal methods and headers** - only expose what's needed by API
4. **1-hour max_age** - allows policy updates within reasonable timeframe
5. **Test with actual frontend** - CORS errors only appear in browser, not curl/Postman

### Attack Vectors Mitigated

- **CSRF via foreign domains**: Specific origins prevent malicious sites from making requests
- **Credential theft**: Credentials only sent to whitelisted origins
- **Data exfiltration**: Restrictive CORS prevents unauthorized cross-origin reads

## User Decisions (2026-01-02)

### Q1: Configuration Approach
**Decision**: **Option C (Hybrid)** - Configure origins, credentials, and max_age
**Rationale**: Balance of simplicity and production needs. Methods/headers hardcoded based on API requirements.

### Q2: Production URL
**Decision**: `https://REDACTED`
**Usage**: Set in `production.yaml` as allowed origin

### Q3: Development Port
**Decision**: Port 5173 (Vite's default)
**Note**: Previous `frontend_url: http://localhost:3000` was leftover from Nuxt project. Update to 5173.

### Q4: Exposed Headers
**Decision**: Not needed for v1.0
**Future**: May add in v1.1+ (e.g., `x-ratelimit-remaining`)

### Q5: CORS Disable Option
**Decision**: No `enabled` flag
**Approach**: Development.yaml will have permissive settings (`*` origin, all methods)

### Q6: Default Behavior
**Decision**: Restrictive (fail-safe) when `cors:` section missing
**Note**: Development.yaml explicitly sets permissive settings, so this only affects missing config

### Q7: Multiple Origins Support
**Decision**: Nice to have, include support via `Vec<String>`
**Implementation**: Use array in YAML, trivial to support

### Q8: Specification Update
**Decision**: Add new FR-023 for configurable CORS in production
**Action**: Update `spec.md` with new functional requirement

## Implementation Decisions Summary

**Configuration Structure**:
```rust
#[derive(Debug, serde::Deserialize, Clone)]
pub struct CorsSettings {
    pub allowed_origins: Vec<String>,
    pub allow_credentials: bool,
    pub max_age_secs: i32,
}

impl Default for CorsSettings {
    fn default() -> Self {
        Self {
            // Restrictive default (fail-safe)
            allowed_origins: vec![],
            allow_credentials: false,
            max_age_secs: 3600,
        }
    }
}
```

**Development Configuration** (`development.yaml`):
```yaml
cors:
  allowed_origins:
    - "*"  # Permissive for local development
  allow_credentials: false
  max_age_secs: 3600

frontend_url: http://localhost:5173  # Updated from 3000
```

**Production Configuration** (`production.yaml`):
```yaml
cors:
  allowed_origins:
    - "https://REDACTED"
  allow_credentials: true  # Required for Authelia authentication
  max_age_secs: 3600

frontend_url: "https://REDACTED"
```

**Hardcoded Settings** (in code, not configurable):
- **Methods**: GET, POST, PUT (based on API requirements)
- **Allowed Headers**: content-type, authorization (minimum for API)
- **Exposed Headers**: None for v1.0

## Next Steps

1. ✅ **Clarify ambiguities** - Complete
2. ✅ **Choose approach** - Option C (Hybrid)
3. **Design architecture** with multiple implementation approaches
4. **Get user approval** on preferred approach
5. **Generate implementation plan** with tasks
6. **Review plan** for completeness

## References

- Poem CORS documentation: Inferred from codebase patterns
- CORS specification: MDN Web Docs (Cross-Origin Resource Sharing)
- Project constitution: `specs/constitution.md` v1.1.0
- Deployment decisions: `specs/001-modbus-relay-control/decisions.md`