Cross Site Request Forgery in Gorilla Mux with Hmac Signatures

Cross Site Request Forgery (CSRF) occurs when an attacker tricks a victim’s browser into making an unwanted authenticated request to a site the victim is logged into. When using Gorilla Mux with Hmac Signatures for request authentication, developers may assume that HMACs inherently prevent CSRF. This assumption can create a false sense of security if the HMAC is only validated on the server for non-GET requests but the application still relies on cookies for session identity and does not enforce same-site cookie attributes or anti-CSRF tokens for state-changing methods.

In this setup, the client typically includes an HMAC—computed from the request method, path, headers, and body—within a custom header (e.g., X-API-Signature). The server recomputes the HMAC using a shared secret and compares it to the provided signature. This mechanism helps ensure integrity and origin authenticity for API requests, especially for non-browser clients. However, browsers automatically include cookies for same-site requests, which means a forged HTML form or JavaScript fetch can still send valid session cookies alongside a malicious request that carries a valid HMAC if the endpoint accepts both cookie-based identity and Hmac-based validation without additional CSRF-specific checks.

Gorilla Mux does not enforce any same-site semantics or CSRF tokens by itself; it is a routing library that matches incoming requests to handlers based on patterns and methods. If your handler validates Hmac Signatures but does not verify the absence of a CSRF token or the Origin/Referer header for sensitive operations, an attacker can craft a request from another domain that passes HMAC verification (e.g., using an exposed public API key or a leaked shared secret) and still execute unwanted actions on behalf of an authenticated user. Common vulnerable patterns include accepting POST requests with an HMAC in a header while relying on session cookies for authentication and failing to bind the HMAC to a per-user or per-session nonce.

Real-world attack patterns mirror OWASP API Top 10 #7:2023 — Identification and Authentication Failures, and the CVE-2021-28020 class of issues where missing CSRF protections in admin interfaces allowed unauthorized commands. For example, an endpoint like /api/v1/transfers that checks X-API-Signature but uses a JSON body with user-controlled parameters can be exploited via a malicious site that submits a forged form or uses fetch with credentials if CORS is misconfigured. Even with Hmac Signatures, if the signature does not cover the entire request context (including the requestor’s identity or a CSRF token), the attack surface remains open.

To understand the interaction, consider that Hmac Signatures verify that the request content has not been altered and that the sender knows the shared secret, but they do not automatically prevent the browser from initiating the request. CSRF protections must be layered on top: use same-site cookies, anti-CSRF tokens, or custom headers that are not automatically included by browsers (e.g., X-Requested-With). For APIs consumed by browsers, ensure that Hmac validation is coupled with explicit CSRF defenses and that CORS policies restrict origins and methods strictly.

To remediate CSRF risks while using Hmac Signatures in Gorilla Mux, you must ensure that signature validation covers all inputs that an attacker cannot control (including headers that browsers may send automatically) and that your handlers do not rely solely on cookies for state-changing operations. Below are concrete code examples showing how to structure Hmac validation and CSRF-safe request handling.

• Example Hmac signature generation and verification in a Gorilla Mux handler (client and server):

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

// Client: compute HMAC for a request
func computeHmac(method, path, body, secret string) string {
  h := hmac.New(sha256.New, []byte(secret))
  h.Write([]byte(method + path + body))
  return hex.EncodeToString(h.Sum(nil))
}

// Server: verify HMAC in middleware
func verifyHmac(next http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    signature := r.Header.Get("X-API-Signature")
    if signature == "" {
      http.Error(w, "missing signature", http.StatusUnauthorized)
      return
    }
    body := ""
    if r.Body != nil {
      // read and restore body for further use
      // omitted for brevity; ensure deterministic body reading
    }
    expected := computeHmac(r.Method, r.URL.Path, body, "shared-secret")
    if !hmac.Equal([]byte(expected), []byte(signature)) {
      http.Error(w, "invalid signature", http.StatusUnauthorized)
      return
    }
    next.ServeHTTP(w, r)
  })
}
CSRF-safe handler design: require a custom header that browsers won’t send automatically, and avoid relying on cookies alone:
func transferHandler(w http.ResponseWriter, r *http.Request) {
  // Ensure the request includes a non-browser-automated header
  if r.Header.Get("X-Requested-With") != "XMLHttpRequest" && r.Header.Get("X-CSRF-Token") == "" {
    http.Error(w, "forbidden: missing CSRF protection header", http.StatusForbidden)
    return
  }
  // Proceed with business logic only after Hmac verification passed
Use same-site cookies and anti-CSRF tokens for browser-originated requests:
// When setting session cookie, restrict scope and same-site attribute
http.SetCookie(w, &http.Cookie{
  Name:     "session_id",
  Value:    sessionToken,
  HttpOnly: true,
  Secure:   true,
  SameSite: http.SameSiteStrictMode,
  Path:     "/",
})

// Generate and validate per-request CSRF token (e.g., stored in session)
func generateCSRFToken(sessionID string) string {
  // use a secure random or HMAC-based token
}
// In handler, compare token from header/cookie/form with session-bound token
Ensure Hmac covers user context to prevent token reuse across users:
// Include user/session identifier in HMAC computation
func computeHmacUser(method, path, body, userID, secret string) string {
  h := hmac.New(sha256.New, []byte(secret))
  h.Write([]byte(method + path + body + userID))
  return hex.EncodeToString(h.Sum(nil))
}

// Server-side: retrieve user identity after authentication, include in verification
func verifyHmacUser(next http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    userID := r.Context().Value("userID") // authenticated identity
    signature := r.Header.Get("X-API-Signature")
    // recompute with userID
  })}
In summary, Hmac Signatures in Gorilla Mux should be combined with explicit CSRF defenses: validate signatures over a canonical representation of method, path, body, and user context; enforce same-site and Secure cookie attributes; require anti-CSRF headers or tokens for state-changing operations; and avoid treating HMAC alone as a complete CSRF mitigation.