Replay Attack in Gorilla Mux

Replay attacks in Gorilla Mux applications occur when an attacker intercepts a legitimate request and retransmits it to achieve unauthorized actions. This vulnerability is particularly insidious in Go APIs using Gorilla Mux because the router itself does not enforce request integrity checks—it merely directs traffic. Without explicit safeguards, stateless endpoints become susceptible.

The attack typically manifests in two Gorilla Mux-specific patterns:

• Absent Nonce/Timestamp Validation: A common vulnerable pattern is a route handler that processes sensitive operations (e.g., fund transfers, password changes) without validating a one-time token or timestamp. For example, consider this vulnerable endpoint:

r.HandleFunc("/api/transfer", func(w http.ResponseWriter, r *http.Request) {
    // VULNERABLE: No replay protection
    from := r.FormValue("from_account")
    to := r.FormValue("to_account")
    amount := r.FormValue("amount")
    // ... execute transfer
}).Methods("POST")

An attacker who captures this POST request (e.g., via a proxy or network sniffing) can replay it repeatedly, duplicating the transaction. Gorilla Mux's routing does not interfere, as it matches the same URL and method each time.

• State-Changing GET Requests: While less common, Gorilla Mux routes that use HTTP GET for state-changing operations (violating REST principles) are trivially replayable via URL sharing or browser history. Example:

r.HandleFunc("/api/delete-user/{id}", deleteUserHandler).Methods("GET")

An attacker who obtains this URL (e.g., from logs or referrers) can trigger deletion repeatedly by simply reloading the page or using curl. Gorilla Mux's path variable extraction ({id}) does not prevent replay.

Detecting replay vulnerabilities in a Gorilla Mux application requires testing for missing request uniqueness guarantees. Since middleBrick performs black-box scanning without source code access, it identifies these issues through behavioral analysis:

• Duplicate Request Submission: The scanner sends identical requests (same method, URL, headers, body) to an endpoint in quick succession. If both requests succeed and produce identical effects (e.g., two resources created with the same ID, or two financial transactions processed), it indicates no replay protection. This is flagged under middleBrick's BOLA/IDOR and Authentication checks, as replay often bypasses intent verification.
• Missing Anti-Replay Headers: middleBrick inspects responses for the absence of standard anti-replay mechanisms like Cache-Control: no-store, Pragma: no-cache, or custom nonce/timestamp headers in sensitive responses. While not definitive, their absence in state-changing operations raises the risk score.
• Predictable Request Patterns: For endpoints that include timestamps or nonces in the request body/query, the scanner attempts to reuse an old valid request after its expected expiry. If the server still accepts it, the vulnerability is confirmed.

Running middlebrick scan https://your-gorilla-api.com will surface replay findings in the report, typically categorized as High severity under BOLA/IDOR or Authentication, with evidence showing duplicated successful requests. The OpenAPI/Swagger spec analysis also cross-references: if the spec defines a parameter like X-Request-ID but the runtime accepts duplicates without validation, middleBrick highlights the discrepancy.

Remediation in Gorilla Mux centers on implementing middleware that enforces request uniqueness. Go's standard library and Gorilla Mux's middleware chaining make this straightforward. Two robust patterns are:

• Nonce-Based Validation: Generate a cryptographically random nonce per sensitive operation, store it server-side (e.g., in a cache with short TTL), and require its inclusion in the request. The middleware checks and consumes the nonce to prevent reuse.

// Nonce middleware for Gorilla Mux
func NonceMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Only protect state-changing methods
        if r.Method != "GET" && r.Method != "HEAD" {
            nonce := r.Header.Get("X-Request-Nonce")
            if nonce == "" {
                http.Error(w, "Missing nonce", http.StatusBadRequest)
                return
            }
            // Check if nonce was already used (example using simple map; use Redis/DB in production)
            if _, used := usedNonces[nonce]; used {
                http.Error(w, "Replay detected", http.StatusUnauthorized)
                return
            }
            usedNonces[nonce] = true
            // Set expiry (e.g., 5 minutes) in real implementation
        }
        next.ServeHTTP(w, r)
    })
}

// Apply to specific routes
r.Handle("/api/transfer", NonceMiddleware(http.HandlerFunc(transferHandler))).Methods("POST")

• Idempotency-Key Pattern: For operations that should be idempotent (like payments), require an Idempotency-Key header. The server stores the result of the first request with that key and returns it for subsequent replays. This is safer than nonce for financial APIs.

func IdempotencyMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        key := r.Header.Get("Idempotency-Key")
        if key == "" {
            next.ServeHTTP(w, r)
            return
        }
        // Check cache for existing response
        if cached, found := idempotencyCache.Get(key); found {
            w.Write(cached.([]byte))
            return
        }
        // Capture response for caching
        rr := &responseRecorder{w, 200, bytes.Buffer{}}
        next.ServeHTTP(rr, r)
        idempotencyCache.SetWithTTL(key, rr.body.Bytes(), 1, 24*time.Hour)
        w.Write(rr.body.Bytes())
    })
}

// Wrap sensitive routes
r.Use(IdempotencyMiddleware)

Both patterns should be applied selectively to state-changing endpoints (POST/PUT/DELETE). Additionally, enforce Cache-Control: no-store on such responses via Gorilla Mux middleware to prevent browser caching. Always combine with TLS to protect nonces/keys in transit.