Replay Attack in Gorilla Mux with Jwt Tokens

A replay attack against a service using Gorilla Mux and JWT tokens occurs when an attacker intercepts a valid request—typically containing a JWT in the Authorization header—and re-sends it to the same endpoint to gain unauthorized access or cause duplicate effects. Even when JWTs are cryptographically signed, they can be replayed unless additional protections are implemented, because the signature validates authenticity and integrity but does not inherently prevent reuse.

Gorilla Mux is a popular HTTP router for Go that enables powerful routing patterns, including variable path parameters and method-based matching. When combined with JWT tokens, a common pattern is to validate the token in middleware and then route the request to a handler. However, if the middleware only verifies the token signature and expiration once, and the same token is allowed for multiple idempotent or sensitive operations (e.g., POST /transfer), the token can be captured and replayed to repeat those operations. This risk is heightened when requests lack a server-side nonce, timestamp, or other replay-resistant context, and when token revocation mechanisms are absent or slow.

The attack surface is often unauthenticated in the sense that an attacker does not need valid credentials to initiate a probe; they only need to observe or guess a valid JWT. In Gorilla Mux, routes with parameters (e.g., /account/{accountID}/transfer) can expose patterns where the same token and payload lead to consistent outcomes, making replay straightforward. Because the router directs the request based on path and method, the backend may treat the repeated request as legitimate if the JWT is still valid and no additional checks are enforced.

Moreover, JWT claims such as jti (JWT ID) are intended to uniquely identify tokens, but if they are not checked server-side against a denylist or a short-lived cache of seen identifiers, replay remains feasible. Without per-request randomness (e.g., a nonce in headers or a one-time use token binding), an intercepted request can be replayed within the token’s validity window. Middleware in Gorilla Mux that only parses and validates the token without enforcing fresh context inadvertently enables replay scenarios, especially over non-TLS channels where interception is easier.

To mitigate replay attacks in Gorilla Mux with JWT tokens, implement server-side token identity tracking and request context validation. Use the jti claim to maintain a short-lived cache of processed tokens, and enforce strict timestamp validation to reject requests with excessive clock skew.

Example JWT validation middleware in Gorilla Mux with replay protection via a token denylist cache:

//go
package main

import (
	"context"
	"net/http"
	"time"

	"github.com/dgrijalva/jwt-go"
)

var seenTokens = make(map[string]bool)

func jwtMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		auth := r.Header.Get("Authorization")
		if auth == "" {
			http.Error(w, "authorization header required", http.StatusUnauthorized)
			return
		}
		// Expect "Bearer "
		const bearerPrefix = "Bearer "
		if len(auth) < len(bearerPrefix) || auth[:len(bearerPrefix)] != bearerPrefix {
			http.Error(w, "invalid authorization format", http.StatusUnauthorized)
			return
		}
		tokenString := auth[len(bearerPrefix):]

		// Parse token without validation first to extract jti and exp
		token, _, err := new(jwt.Parser).ParseUnverified(tokenString, jwt.MapClaims{})
		if err != nil {
			http.Error(w, "invalid token", http.StatusUnauthorized)
			return
		}
		claims, ok := token.Claims.(jwt.MapClaims)
		if !ok || !token.Valid {
			http.Error(w, "invalid claims", http.StatusUnauthorized)
			return
		}

		// Check expiration
		if exp, ok := claims["exp"].(float64); ok && time.Unix(int64(exp), 0).Before(time.Now()) {
			http.Error(w, "token expired", http.StatusUnauthorized)
			return
		}

		// Replay protection: check jti in denylist
		jti, _ := claims["jti"].(string)
		if jti == "" {
			http.Error(w, "missing token identifier", http.StatusUnauthorized)
			return
		}
		if seenTokens[jti] {
			http.Error(w, "replay detected", http.StatusForbidden)
			return
		}
		// Mark token as seen for this short window; in production use a TTL cache
		seenTokens[jti] = true

		// Optional: enforce not-before and clock skew
		if nbf, ok := claims["nbf"].(float64); ok && time.Unix(int64(nbf), 0).After(time.Now()) {
			http.Error(w, "token not active", http.StatusUnauthorized)
			return
		}

		ctx := context.WithValue(r.Context(), "claims", claims)
		next.ServeHTTP(w, r.WithContext(ctx))
	})
}

func transferHandler(w http.ResponseWriter, r *http.Request) {
	// Example safe handler using Gorilla Mux
	claims := r.Context().Value("claims").(jwt.MapClaims)
	accountID := mux.Vars(r)["accountID"]
	// Process transfer ensuring accountID matches claims subject/roles if needed
	w.Write([]byte("transfer processed"))
}

func main() {
	r := mux.NewRouter()
	r.HandleFunc("/account/{accountID}/transfer", transferHandler).Methods("POST")
	server := &http.Server{
		Addr:    ":8080",
		Handler: jwtMiddleware(r),
	}
	server.ListenAndServe()
}

Key points in the example:

• Extract and validate jti to detect repeated tokens within a short timeframe.
• Enforce exp and nbf claims to bound the validity window and prevent use before or after intended periods.
• Use HTTPS to protect tokens in transit; this example focuses on server-side checks.
• For production, replace the in-memory map with a distributed TTL cache to support multiple instances and avoid memory bloat.

Additionally, prefer short token lifetimes and consider binding tokens to a client context (e.g., IP or device fingerprint) where appropriate, while being mindful of usability trade-offs.