ln-773-cors-configurator

Type: L3 Worker Category: 7XX Project Bootstrap

Configures Cross-Origin Resource Sharing (CORS) policy with security-first approach.

Overview

Phase 1: Receive Context

Accept Context Store from coordinator.

Required Context:

• STACK

  : .NET or Python

• PROJECT_ROOT

  : Project directory path

• ENVIRONMENT

  : Development or Production

Idempotency Check:

• .NET: Grep for

  AddCors

  or

  UseCors

• Python: Grep for

  CORSMiddleware

• If found: Return

  { "status": "skipped" }

Phase 2: Analyze Project Structure

Determine frontend configuration.

Detection Steps:

1. Check for frontend in same repository (

   /frontend

   ,

   /client

   ,

   /web

   )
2. Read

   .env

   or

   appsettings.json

   for CORS_ORIGINS
3. Identify common frontend ports (3000, 5173, 4200)

Detected Frontend Origins:

Phase 3: Decision Points

Q1: Allowed Origins

Security Warning: Never use

*

Q2: Allowed Methods

Q3: Credentials Support

Warning: AllowCredentials = true prohibits

*

Q4: Preflight Cache Duration

Phase 4: Generate Configuration

.NET Output Files

Extensions/CorsExtensions.cs

appsettings.json

appsettings.Development.json

Generation Process:

1. Use MCP ref for current ASP.NET Core CORS API
2. Generate CorsExtensions with:
   • Development policy (permissive)
   • Production policy (restrictive)
   • Environment-based policy selection
3. Update appsettings with CORS:Origins

Registration Code:

builder.Services.AddCorsPolicy(builder.Configuration);
// ...
app.UseCors(builder.Environment.IsDevelopment() ? "Development" : "Production");

Python Output Files

middleware/cors_config.py

.env

Generation Process:

1. Use MCP ref for FastAPI CORSMiddleware
2. Generate cors_config.py with:
   • Origin parsing from environment
   • Method and header configuration
   • Credentials handling
3. Update .env with CORS_ORIGINS

Registration Code:

from middleware.cors_config import configure_cors
configure_cors(app)

Phase 5: Validate

Validation Steps:

1. Syntax check:

   • .NET:

     dotnet build --no-restore

   • Python:

     python -m py_compile middleware/cors_config.py

2. CORS test:

   # Test preflight request
   curl -X OPTIONS http://localhost:5000/api/test \
     -H "Origin: http://localhost:3000" \
     -H "Access-Control-Request-Method: POST" \
     -v
   

3. Verify headers:

   • Access-Control-Allow-Origin

     : Should match request origin

   • Access-Control-Allow-Methods

     : Should list allowed methods

   • Access-Control-Allow-Credentials

     : true (if enabled)

   • Access-Control-Max-Age

     : Cache duration

Security Checklist

Before completing, verify:

• No wildcard

  *

  origin in production
• Explicit allowed methods (not

  AllowAnyMethod

  in prod)
• Credentials only if needed
• Origins from environment variables in production
• Preflight caching enabled in production

Return to Coordinator

{
  "status": "success",
  "files_created": [
    "Extensions/CorsExtensions.cs"
  ],
  "packages_added": [],
  "registration_code": "builder.Services.AddCorsPolicy(configuration);",
  "message": "Configured CORS with Development and Production policies"
}

Reference Links

• ASP.NET Core CORS
• FastAPI CORS
• MDN CORS

Critical Rules

• Never use wildcard

  *

  origin with credentials — security violation per CORS spec
• Production origins from environment variables only — no hardcoded URLs in code
• Separate Development and Production policies — permissive locally, restrictive in production
• Idempotent — if

  AddCors

  /

  UseCors

  or

  CORSMiddleware

  exists, return

  status: "skipped"

• Enable preflight caching in Production — MaxAge 86400 (24h) to reduce OPTIONS requests

Definition of Done

• Context Store received (stack, project root, environment)
• Frontend origins detected (port/framework auto-detection)
• User decisions collected (origins, methods, credentials, cache duration)
• CORS configuration generated with environment-specific policies
• Security checklist verified (no wildcard + credentials, explicit methods, env-based origins)
• Syntax validated (

  dotnet build

  or

  py_compile

  )
• Structured JSON response returned to ln-770 coordinator

Version: 2.0.0 Last Updated: 2026-01-10