Broken Access Control in Gorilla Mux with Hmac Signatures

Broken Access Control occurs when authorization checks are missing or bypassed, allowing attackers to access resources or perform actions they should not. Using Hmac Signatures in Gorilla Mux can introduce this vulnerability when the signature verification is incomplete, applied inconsistently across routes, or implemented in a way that does not protect sensitive operations.

Gorilla Mux is a powerful URL router and dispatcher for Go. When you add Hmac Signatures to secure endpoints, you typically compute a signature on the request payload and headers using a shared secret, then compare it to a value provided by the client (often in a custom header such as x-signature). If you only apply the Hmac verification middleware to a subset of routes—such as read-only endpoints—and omit it from write or administrative routes, an authenticated but unauthorized user can perform actions like modifying or deleting resources.

A common pattern mistake is computing the Hmac over only the request body while ignoring specific headers that influence routing or authorization (e.g., x-user-id, x-role). Because the signature does not cover those headers, an attacker can change the header values to escalate privileges or access another user’s data, even though the body signature appears valid. In Gorilla Mux, this often happens when routes are defined with variable path parameters (e.g., /users/{userID}/resources/{resourceID}) and the signature does not include the path variables or the HTTP method used to reach the handler.

Another scenario is signature replay without nonce or timestamp validation. If your Hmac verification in Gorilla Mux checks only the Hmac correctness but does not ensure freshness, an attacker can capture a signed request and replay it to perform an unauthorized action. This is a Broken Access Control issue because the system fails to guarantee that the request is unique and time-bound, allowing repeated use of a valid signature to manipulate resources.

Additionally, if the signature verification logic is implemented as a per-route handler rather than a consistent middleware applied to all sensitive routes, some endpoints may be left unprotected. In Gorilla Mux, you might attach the Hmac check to a subrouter for a particular API version or group but forget to include newly added routes, creating an access control gap. Because Hmac signatures can provide integrity and authenticity, relying on them incorrectly can give a false sense of security while leaving critical operations exposed.

To address Broken Access Control when using Hmac Signatures in Gorilla Mux, ensure that signature verification is applied consistently, covers all relevant request components, and includes replay protections. The following code examples demonstrate a robust approach.

1. Compute Hmac over method, path, headers, and body

Include the HTTP method, the request path (with variables), selected headers, and the body in the Hmac input so that tampering with any of these elements invalidates the signature.

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
    "strings"
)

func computeHmac(method, path, headerString, body, secret string) string {
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(method))
    mac.Write([]byte("|"))
    mac.Write([]byte(path))
    mac.Write([]byte("|"))
    mac.Write([]byte(headerString))
    mac.Write([]byte("|"))
    mac.Write([]byte(body))
    return hex.EncodeToString(mac.Sum(nil))
}

2. Middleware that verifies Hmac for all routes

Attach a middleware to your Gorilla Mux router that validates the signature on every request to protected routes. Use it on the main router or on a subrouter that covers all sensitive endpoints.

func HmacMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        provided := r.Header.Get("X-Signature")
        if provided == "" {
            http.Error(w, "missing signature", http.StatusUnauthorized)
            return
        }
        secret := "your-256-bit-secret"
        headerString := r.Header.Get("X-User-Id") + "|" + r.Header.Get("X-Role")
        expected := computeHmac(r.Method, r.RequestURI, headerString, "", secret)
        if !hmac.Equal([]byte(expected), []byte(provided)) {
            http.Error(w, "invalid signature", http.StatusForbidden)
            return
        }
        next.ServeHTTP(w, r)
    })
}

3. Include path variables and guard against replay

To prevent Broken Access Control via replay or parameter substitution, incorporate a timestamp and a nonce (or request ID) into the signed data, and maintain short validity windows.

import (
    "time"
)

func computeHmacWithTimestamp(method, path, headerString, body, nonce, timestamp, secret string) string {
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(method))
    mac.Write([]byte("|"))
    mac.Write([]byte(path))
    mac.Write([]byte("|"))
    mac.Write([]byte(headerString))
    mac.Write([]byte("|"))
    mac.Write([]byte(body))
    mac.Write([]byte("|"))
    mac.Write([]byte(nonce))
    mac.Write([]byte("|"))
    mac.Write([]byte(timestamp))
    return hex.EncodeToString(mac.Sum(nil))
}

// In middleware, reject requests where timestamp is older than 2 minutes
func HmacWithReplayProtection(next http.Handler) http.Handler {
    seenNonces := make(map[string]bool)
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        provided := r.Header.Get("X-Signature")
        timestamp := r.Header.Get("X-Timestamp")
        nonce := r.Header.Get("X-Nonce")
        if provided == "" || timestamp == "" || nonce == "" {
            http.Error(w, "missing security headers", http.StatusUnauthorized)
            return
        }
        // reject replayed nonces (in production use a bounded cache or storage)
        if seenNonces[nonce] {
            http.Error(w, "replay detected", http.StatusForbidden)
            return
        }
        // basic freshness check
        t, _ := time.Parse(time.RFC3339, timestamp)
        if time.Since(t) > 2*time.Minute {
            http.Error(w, "stale request", http.StatusForbidden)
            return
        }
        secret := "your-256-bit-secret"
        headerString := r.Header.Get("X-User-Id") + "|" + r.Header.Get("X-Role")
        expected := computeHmacWithTimestamp(r.Method, r.RequestURI, headerString, "", nonce, timestamp, secret)
        if !hmac.Equal([]byte(expected), []byte(provided)) {
            http.Error(w, "invalid signature", http.StatusForbidden)
            return
        }
        seenNonces[nonce] = true
        next.ServeHTTP(w, r)
    })
}

By applying these patterns in Gorilla Mux, you ensure that Hmac Signatures enforce authorization consistently, cover all mutable request dimensions, and resist replay attacks, reducing the risk of Broken Access Control.