Man In The Middle in Buffalo with Jwt Tokens

A Man In The Middle (MitM) scenario in Buffalo when JWT tokens are used occurs because the token is often transmitted between client and server without adequate transport protection or strict validation. Buffalo is a web framework for Go that encourages rapid development, and JWTs are commonly used for stateless authentication. The vulnerability arises when a JWT is sent over an unencrypted channel or when token validation is incomplete, allowing an attacker positioned on the network path to intercept, read, or tamper with the token.

In a typical Buffalo app, a JWT might be issued after a user logs in and then sent in the Authorization header as a Bearer token. If the server does not enforce HTTPS (TLS) consistently, an attacker on the same network can observe the cleartext token. Even when HTTPS is used, additional risks exist: if the application does not validate the token signature, issuer, audience, or expiration rigorously, an attacker who obtains the token can reuse it (replay) or manipulate claims before passing it to the backend. This is especially problematic when tokens contain elevated permissions or when the application relies solely on the presence of a token rather than its content.

Buffalo’s middleware stack makes it straightforward to add authentication, but developers must explicitly enforce secure transport and strict JWT verification. Without explicit checks, an attacker performing MitM can capture tokens from logs, error messages, or client-side JavaScript if Content Security Policy is weak. Moreover, if the application uses JWTs for authorization without re-checking permissions on each request, a captured token can be leveraged for privilege escalation across endpoints that lack per-route authorization.

Real-world attack patterns resemble the OWASP API Top 10 2023 broken object level authorization (BOLA) and injection risks, where token interception leads to unauthorized access. For example, a token issued for a low-privilege user might be reused to access admin routes if the server does not validate scope or role claims. The mitigations therefore focus on preventing token exposure in transit and ensuring the server validates every token rigorously.

Remediation centers on enforcing transport security, validating JWT claims, and avoiding token leakage. Below are concrete code examples for a Buffalo application that demonstrate secure handling of JWTs.

1. Enforce HTTPS and secure cookies

Ensure all requests are served over TLS and that cookies containing tokens or session data are marked secure and httpOnly.

// In apps/app.go or an environment-specific initializer
func app() *buffalo.App {
  app := buffalo.New(buffalo.Options{
    Env:         ENV,
    SessionStore: &sessions.SecureCookieStore{
      Secure: true, // only send over HTTPS
    },
  })
  // Force HTTPS in production
  if ENV == buffalo.ProductionEnv {
    app.Use(forceSSL)
  }
  return app
}

func forceSSL(next buffalo.Handler) buffalo.Handler {
  return func(c buffalo.Context) error {
    if !c.Request().TLS.HandshakeComplete {
      c.Response().WriteHeader(http.StatusMovedPermanently)
      c.Response().Header().Set("Location", "https://"+c.Request().Host+c.Request().RequestURI)
      return nil
    }
    return next(c)
  }
}

2. Validate JWT on each authenticated request

Use a middleware that parses and validates the JWT signature, issuer, audience, and expiration before allowing access to protected handlers.

// jwt_middleware.go
package middleware

import (
  "context"
  "net/http"
  "github.com/golang-jwt/jwt/v5"
  "github.com/gobuffalo/buffalo"
)

func JWTValidator(next buffalo.Handler) buffalo.Handler {
  return func(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    const bearerPrefix = "Bearer "
    if len(auth) < len(bearerPrefix) || auth[:len(bearerPrefix)] != bearerPrefix {
      c.Response().WriteHeader(http.StatusUnauthorized)
      return c.Render(http.StatusUnauthorized, r.JSON(map[string]string{"error": "missing or malformed bearer token"}))
    }
    tokenString := auth[len(bearerPrefix):]

    token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
      if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
        return nil, http.ErrAbortHandler
      }
      return []byte("your-secret-key"), nil
    })
    if err != nil || !token.Valid {
      c.Response().WriteHeader(http.StatusUnauthorized)
      return c.Render(http.StatusUnauthorized, r.JSON(map[string]string{"error": "invalid token"}))
    }

    if claims, ok := token.Claims.(jwt.MapClaims); ok {
      // Validate standard claims
      if iss, ok := claims["iss"].(string); !ok || iss != "your-issuer" {
        c.Response().WriteHeader(http.StatusUnauthorized)
        return c.Render(http.StatusUnauthorized, r.JSON(map[string]string{"error": "invalid issuer"}))
      }
      if aud, ok := claims["aud"].(string); !ok || aud != "your-audience" {
        c.Response().WriteHeader(http.StatusUnauthorized)
        return c.Render(http.StatusUnauthorized, r.JSON(map[string]string{"error": "invalid audience"}))
      }
      if exp, ok := claims["exp"].(float64); !ok || int64(exp) < time.Now().Unix() {
        c.Response().WriteHeader(http.StatusUnauthorized)
        return c.Render(http.StatusUnauthorized, r.JSON(map[string]string{"error": "token expired"}))
      }
      // Attach claims to context for downstream handlers
      c.Set("claims", claims)
    }

    return next(c)
  }
}

3. Avoid logging or exposing tokens

Ensure tokens are not written to logs or exposed in error messages. Configure log formatting to exclude the Authorization header.

// Example middleware to scrub sensitive headers from logs
func scrubHeaders(next buffalo.Handler) buffalo.Handler {
  return func(c buffalo.Context) error {
    // Remove Authorization from request clone if logging would expose it
    req := c.Request().Clone(c.Request().Context())
    req.Header.Del("Authorization")
    c.Set("request", req)
    return next(c)
  }
}

4. Use short-lived tokens and refresh rotation

Issue short-lived access tokens and use refresh tokens with strict rotation and revocation checks to limit the impact of a captured token.

// Example token creation with short expiry
func issueTokens(userID string) (string, string, error) {
  accessClaims := jwt.MapClaims{
    "sub": userID,
    "iss": "your-issuer",
    "aud": "your-audience",
    "exp": time.Now().Add(15 * time.Minute).Unix(),
    "nbf": time.Now().Unix(),
    "scope": "read write",
  }
  accessToken := jwt.NewWithClaims(jwt.SigningMethodHS256, accessClaims)
  signedAccess, err := accessToken.SignedString([]byte("your-secret-key"))
  if err != nil {
    return "", "", err
  }

  refreshClaims := jwt.MapClaims{
    "sub": userID,
    "iss": "your-issuer",
    "aud": "your-audience",
    "exp": time.Now().Add(7 * 24 * time.Hour).Unix(),
    "jti": uuid.New().String(), // unique refresh token ID
  }
  refreshToken := jwt.NewWithClaims(jwt.SigningMethodHS256, refreshClaims)
  signedRefresh, err := refreshToken.SignedString([]byte("your-refresh-secret"))
  return signedAccess, signedRefresh, err
}

5. Apply per-route authorization checks

Even with a valid JWT, enforce role/scope checks at the handler or resource level to prevent BOLA even if a token is intercepted.

// Example authorization check in a handler
func AdminHandler(c buffalo.Context) error {
  claims, ok := c.Get("claims").(jwt.MapClaims)
  if !ok {
    c.Response().WriteHeader(http.StatusUnauthorized)
    return c.Render(http.StatusUnauthorized, r.JSON(map[string]string{"error": "claims missing"}))
  }
  scope, ok := claims["scope"].(string)
  if !ok || !strings.Contains(scope, "admin") {
    c.Response().WriteHeader(http.StatusForbidden)
    return c.Render(http.StatusForbidden, r.JSON(map[string]string{"error": "insufficient scope"}))
  }
  return c.Render(http.StatusOK, r.JSON(map[string]string{"message": "admin access"}))
}