Cors Wildcard in Buffalo

CORS wildcard misconfigurations in Buffalo applications typically appear through overly permissive cross-origin resource sharing settings that allow any domain to access API endpoints. In Buffalo, this often manifests when developers use wildcard origins (*) in their CORS middleware configuration, either explicitly or through default settings.

A common Buffalo-specific pattern involves the cors middleware being configured with wildcard origins in the actions/app.go file. Developers might add middleware like this:

import cors "github.com/rs/cors"

corsOpts := cors.Options{
    AllowedOrigins: []string{"*"},
    AllowedMethods:   []string{"GET", "POST", "PUT", "DELETE"},
    AllowedHeaders:   []string{"*"},
}

This configuration allows any website to make requests to your Buffalo API, potentially exposing sensitive data or enabling unauthorized actions. The issue becomes more severe when combined with authentication bypass vulnerabilities or when the API exposes administrative endpoints.

In Buffalo's pop/soda ORM context, CORS wildcard issues can lead to data exposure scenarios. For example, an endpoint like /api/users that returns user data without proper authentication checks becomes accessible to any origin when wildcard CORS is enabled. An attacker could host a malicious site that makes requests to your Buffalo API and harvests data from legitimate users.

Buffalo's middleware stack makes it easy to accidentally introduce wildcard CORS. The app.Use(middleware.CORS()) helper might be added without understanding its default configuration, or developers might copy-paste CORS configurations from documentation without considering security implications.

Detecting CORS wildcard issues in Buffalo applications requires examining both the configuration and runtime behavior. Start by checking your actions/app.go file for CORS middleware configuration. Look for patterns like:

app.Use(middleware.CORS())

or:

corsOpts := cors.Options{
    AllowedOrigins: []string{"*"},
    // ... other permissive settings
}

middleBrick's API security scanner can automatically detect CORS wildcard misconfigurations in Buffalo applications. The scanner tests CORS policies by making cross-origin requests from different domains and analyzing the response headers. It specifically looks for Access-Control-Allow-Origin: * headers and permissive CORS configurations.

To manually test, use curl to check CORS headers:

curl -I https://your-buffalo-app.com/api/endpoint

Look for Access-Control-Allow-Origin headers. If you see * or a wildcard pattern, this is a security concern. Also check for overly permissive Access-Control-Allow-Methods and Access-Control-Allow-Headers headers.

middleBrick's scanner goes further by testing the actual behavior of CORS policies. It attempts to make authenticated requests from different origins to see if the wildcard CORS allows unauthorized access to protected resources. The scanner also checks if sensitive endpoints like admin panels or user data APIs are exposed to cross-origin requests.

For Buffalo applications using pop/soda for database operations, middleBrick can correlate CORS findings with data exposure risks, showing you exactly which endpoints might leak user data due to permissive CORS combined with other vulnerabilities.

Remediating CORS wildcard issues in Buffalo requires a security-first approach to cross-origin resource sharing. The first step is to replace wildcard origins with a specific list of trusted domains. In your actions/app.go file, modify the CORS configuration:

corsOpts := cors.Options{
    AllowedOrigins: []string{
        "https://your-trusted-app.com",
        "https://admin.your-company.com",
    },
    AllowedMethods:   []string{"GET", "POST"},
    AllowedHeaders:   []string{"Content-Type", "Authorization"},
    AllowCredentials: true,
}

Notice the key changes: specific origins instead of *, restricted methods to only what's necessary, limited headers to those actually used, and AllowCredentials: true for authenticated requests. When AllowCredentials is true, the wildcard origin is forbidden by the CORS specification, so you must list specific origins.

For Buffalo applications with multiple environments, consider using environment variables:

allowedOrigins := strings.Split(os.Getenv("CORS_ALLOWED_ORIGINS"), ","))
corsOpts := cors.Options{
    AllowedOrigins: allowedOrigins,
    // ... other settings
}

This allows different origin lists for development, staging, and production without changing code.

When using Buffalo's middleware helpers, be explicit about the configuration:

app.Use(middleware.CORSWithConfig(cors.Options{
    AllowedOrigins: []string{"https://app.example.com"},
    AllowedMethods:   []string{"GET", "POST"},
    AllowedHeaders:   []string{"Content-Type", "Authorization"},
}))

For APIs that need to support multiple trusted clients, create a middleware that validates the origin against a whitelist:

func corsMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        origin := c.Request().Header.Get("Origin")
        allowedOrigins := map[string]bool{
            "https://client1.com": true,
            "https://client2.com": true,
        }
        
        if allowedOrigins[origin] {
            c.Response().Header().Set("Access-Control-Allow-Origin", origin)
            c.Response().Header().Set("Access-Control-Allow-Credentials", "true")
        }
        
        return next(c)
    }
}

middleBrick's CLI tool can help verify your remediation:

middlebrick scan https://your-buffalo-app.com --output json

This will show you if CORS wildcard issues persist and provide specific findings about which endpoints are affected.