Man In The Middle in Chi

Man In The Middle (MITM) attacks in Chi applications occur when an attacker intercepts communication between the Chi client and server, potentially modifying requests or responses. This is particularly concerning in Chi-based microservices where API endpoints communicate across networks without proper TLS validation.

// Vulnerable Chi middleware - missing TLS verification
func insecureMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // No certificate pinning or hostname verification
        next.ServeHTTP(w, r)
    })
}

In Chi applications, MITM vulnerabilities often manifest through insecure proxy configurations. When Chi routes handle requests through reverse proxies without validating the proxy's identity, attackers can position themselves between the client and the Chi server.

// Vulnerable Chi proxy configuration
router := chi.NewRouter()
router.Use(chi.ProxyHeaders) // Exposes internal IPs without validation
router.Get("/api/data", func(w http.ResponseWriter, r *http.Request) {
    // Attacker can modify X-Forwarded-For headers
    next.ServeHTTP(w, r)
})

Another common pattern in Chi applications is improper handling of CORS headers, which can enable MITM attacks when combined with misconfigured TLS termination. Attackers exploit the trust relationship between Chi middleware and upstream services.

// Vulnerable CORS configuration in Chi
router := chi.NewRouter()
cors := cors.New(cors.Options{
    AllowedOrigins:   []string{"*"}, // Too permissive
    AllowCredentials: true,
})
router.Use(cors.Handler)

Detecting MITM vulnerabilities in Chi applications requires examining both configuration and runtime behavior. middleBrick's black-box scanning approach is particularly effective for Chi APIs because it tests the actual attack surface without requiring source code access.

When scanning a Chi API endpoint, middleBrick examines TLS configurations, header handling, and proxy setups. The scanner attempts to establish connections with invalid certificates to test if the Chi server properly validates TLS.

# Scanning a Chi API with middleBrick CLI
middlebrick scan https://api.example.com/v1/users

middleBrick's MITM detection includes testing for:

• TLS certificate validation bypass attempts
• Header injection vulnerabilities in Chi middleware
• Proxy configuration weaknesses
• CORS misconfigurations that enable request smuggling

The scanner also checks for Chi-specific patterns like improper use of chi.ProxyHeaders middleware, which can expose internal network details to attackers positioned between the client and server.

Securing Chi applications against MITM attacks requires implementing proper TLS validation and secure middleware configurations. The first step is ensuring all communications use properly validated TLS connections.

// Secure Chi middleware with TLS validation
func secureMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Validate TLS connection
        if r.TLS == nil || !r.TLS.HandshakeComplete {
            http.Error(w, "TLS required", http.StatusUnauthorized)
            return
        }
        
        // Validate certificate hostname
        if err := r.TLS.PeerCertificates[0].VerifyHostname(r.Host); err != nil {
            http.Error(w, "Invalid certificate", http.StatusUnauthorized)
            return
        }
        
        next.ServeHTTP(w, r)
    })
}

For proxy configurations, Chi applications should implement strict header validation and avoid exposing internal network details.

// Secure proxy configuration in Chi
router := chi.NewRouter()
router.Use(func(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Validate X-Forwarded-For headers
        if forwarded := r.Header.Get("X-Forwarded-For"); forwarded != "" {
            // Only allow specific trusted proxies
            allowed := map[string]bool{"192.168.1.1": true}
            if !allowed[forwarded] {
                http.Error(w, "Invalid proxy", http.StatusForbidden)
                return
            }
        }
        next.ServeHTTP(w, r)
    })
})

CORS policies should be explicitly configured rather than using wildcard origins.

// Secure CORS configuration
router := chi.NewRouter()
cors := cors.New(cors.Options{
    AllowedOrigins:   []string{"https://yourdomain.com"},
    AllowCredentials: true,
    AllowedMethods:   []string{"GET", "POST", "PUT", "DELETE"},
})
router.Use(cors.Handler)