Cors Wildcard in Buffalo with Jwt Tokens

A CORS wildcard (Access-Control-Allow-Origin: *) combined with JWT-based authentication creates a critical misconfiguration in Buffalo applications. When a route is configured to accept any origin while also requiring a JWT in the Authorization header, browsers enforce CORS preflight checks for non-simple requests (e.g., those with custom headers like Authorization). The wildcard allows any domain to pass the CORS preflight, enabling malicious websites to make authenticated requests on behalf of a victim if the JWT is stored in browser-accessible storage (such as localStorage). This scenario is relevant to the BOLA/IDOR and Property Authorization checks in middleBrick, which flag overly permissive CORS rules that coexist with bearer-token authentication.

In Buffalo, CORS is typically managed by the middleware package. If a developer applies cors.DefaultHandler with a wildcard while also enabling JWT middleware, the application inadvertently permits cross-origin authenticated calls. For example, an attacker hosting a malicious page can execute JavaScript that calls your Buffalo API with a stolen JWT. Because the API sees a valid token and a permitted origin, it processes the request, potentially exposing sensitive endpoints enumerated by middleBrick under the Inventory Management and Unsafe Consumption checks.

Real-world impact aligns with OWASP API Top 10 A07:2021 — Identification and Authentication Failures. A leaked JWT combined with a wildcard CORS policy can lead to unauthorized data access or privilege escalation, a scenario also covered by middleBrick’s BFLA/Privilege Escalation and Data Exposure scans. The scanner detects mismatches between declared CORS policy and authentication requirements, providing prioritized findings with severity and remediation guidance.

To secure a Buffalo API that uses JWT tokens, align CORS configuration with strict origin policies and ensure token validation occurs before any route handling. Below is a concrete, working example demonstrating secure setup.

Secure CORS + JWT Middleware Setup

Configure CORS to allow only trusted origins and enforce JWT validation centrally. This prevents wildcard origins from interacting with authenticated routes.

// app.js
const { app, middleware } = buffalo.Default()

// Define allowed origins explicitly
const allowedOrigins = []string{
    "https://your-frontend.com",
    "https://app.yourcompany.com",
}

// Configure CORS with specific origins and exposed headers
corsMiddleware := middleware.CORS({
    AllowedOrigins:   allowedOrigins,
    AllowedMethods:   []string{"GET", "POST", "PUT", "DELETE", "OPTIONS"},
    AllowedHeaders:   []string{"Content-Type", "Authorization"},
    ExposedHeaders:   []string{"X-Total-Count"},
    AllowCredentials: pointer.To(true),
    MaxAge:           300,
})

// Apply CORS middleware globally
app.Use(corsMiddleware)

// JWT validation middleware
jwtMiddleware := middleware.JWT([]byte(os.Getenv("JWT_SECRET")))
app.Use(jwtMiddleware)

// Ensure routes are protected
app.Get("/api/profile", profileHandler)
app.Post("/api/data", dataHandler)

buffalo.Run(app)

JWT Verification in a Handler

Even with middleware, explicitly validate token claims in handlers that require strict authorization. This complements middleBrick’s Property Authorization checks.

// handlers/profile.go
package handlers

import (
    "net/http"
    "os"

    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/packr/v2"
    "github.com/golang-jwt/jwt/v5"
)

func profileHandler(c buffalo.Context) error {
    tokenString := c.Request().Header.Get("Authorization")
    if tokenString == "" {
        return c.Error(http.StatusUnauthorized, errors.New("unauthorized"))
    }

    // Remove "Bearer " prefix if present
    if strings.HasPrefix(tokenString, "Bearer ") {
        tokenString = strings.TrimPrefix(tokenString, "Bearer ")
    }

    token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
        if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
            return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
        }
        return []byte(os.Getenv("JWT_SECRET")), nil
    })

    if err != nil || !token.Valid {
        return c.Error(http.StatusUnauthorized, errors.New("invalid token"))
    }

    // Optional: check roles/claims
    if claims, ok := token.Claims.(jwt.MapClaims); ok {
        if role, ok := claims["role"].(string); ok && role != "admin" {
            return c.Error(http.StatusForbidden, errors.New("insufficient permissions"))
        }
    }

    // Proceed with safe logic
    c.Response().WriteHeader(http.StatusOK)
    return c.Render(200, r.JSON(map[string]string{"profile": "secure"}))
}

These examples ensure that CORS does not bypass authentication and that JWT validation is explicit where needed. middleBrick’s scans can verify that no wildcard origins exist alongside authenticated routes and that security headers are properly enforced.