Cross Site Request Forgery in Gorilla Mux with Api Keys

Cross Site Request Forgery (CSRF) against a Gorilla Mux router can manifest in unexpected ways when API keys are used as bearer tokens. Gorilla Mux is a powerful HTTP router that supports route variables, strict trailing-slash policies, and method-based matchers. While it does not enforce authentication or authorization itself, it provides the routing surface where CSRF risks can be introduced if endpoints rely solely on API keys exposed to browser contexts.

Consider a scenario where an API key is issued to a web application and then stored in JavaScript or embedded in frontend code. If the API key is sent via an Authorization header on state-changing routes (POST, PUT, DELETE), and the frontend includes forms or links that trigger those routes, an attacker can craft a malicious site that causes a victim’s browser to submit requests with the victim’s API key. Because Gorilla Mux matches routes by path and method, the router will direct the request to the intended handler, and the backend may process it as legitimate, assuming the API key is valid. This violates the same-origin policy expectation for privileged actions and maps directly to the OWASP API Top 10 A05:2023 — Security Misconfiguration and A01:2019 — Broken Access Control.

Key conditions that escalate risk in this combination:

• API keys are transmitted in headers from browser-executed JavaScript.
• Routes protected only by API key lack additional anti-CSRF measures such as custom headers (e.g., X-Requested-With), same-site cookie policies for session-based flows, or CSRF tokens.
• Gorilla Mux routes accept state-changing methods on paths that are reachable from web origins without proper CORS and anti-CSRF alignment.

An example risk pattern: a route /api/v1/transfer matched by Gorilla Mux with methods: []string{http.MethodPost} expects an Authorization: Bearer <key> header. If a malicious site includes an image or form submission pointing to that endpoint, and the user’s browser still holds the API key (e.g., in a header-injected context or legacy pattern), the request may be executed with the user’s permissions. Note that this is not a Gorilla Mux bug — it is a consequence of routing behavior combined with insecure key usage. The scanner detects this as a potential CSRF vector when unauthenticated attack surface testing reveals state-changing methods exposed without anti-CSRF controls.

To validate exposure, a security scan can attempt a simulated CSRF probe (without side effects) to confirm whether routes relying solely on API keys in headers are reachable from an untrusted origin. This aligns with the LLM/AI Security checks in middleBrick, which include active prompt injection and system prompt leakage detection, though CSRF is an API-specific control rather than an LLM exploit. The findings will highlight missing idempotency-safe protections and suggest compensating controls.

Remediation focuses on ensuring API keys are never treated as user proof in browser contexts and that Gorilla Mux routes enforce strict expectations for requests. Below are concrete code examples that demonstrate secure patterns.

1) Avoid exposing API keys to browsers

Do not embed API keys in JavaScript that runs in the browser. Instead, use a backend proxy that holds the key and forwards requests to the protected endpoint.

// Insecure: serving an API key to frontend JavaScript (do not do this)
http.HandleFunc("/api/v1/action", func(w http.ResponseWriter, r *http.Request) {
    key := os.Getenv("SERVICE_API_KEY")
    // Never inject this into client-side code
    fmt.Fprintf(w, `<script>const apiKey = "%s";</script>`, key)
})

2) Require custom headers for state-changing routes

Ensure Gorilla Mux routes that perform mutations require a custom header (e.g., X-API-Key) in addition to or instead of Authorization, and validate the header server-side before processing. This makes it less likely for a browser to send the key inadvertently via an image tag or simple link.

func transferHandler(w http.ResponseWriter, r *http.Request) {
    const expected = "top-secret-x-key"
    if r.Header.Get("X-API-Key") != expected {
        http.Error(w, "forbidden", http.StatusForbidden)
        return
    }
    // proceed with business logic
    w.WriteHeader(http.StatusOK)
    w.Write([]byte("transfer initiated"))
}

func main() {
    r := mux.NewRouter()
    r.HandleFunc("/api/v1/transfer", transferHandler).Methods("POST")
    http.ListenAndServe(":8080", r)
}

3) Use SameSite cookies and CSRF tokens for session flows

If your API key is tied to a session cookie, set SameSite=Strict or Lax and consider anti-CSRF tokens for any state-changing operation. Gorilla Mux does not manage cookies, but you can enforce these policies in your handler or middleware.

func withCSRFToken(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        token := r.Header.Get("X-CSRF-Token")
        if token == "" {
            http.Error(w, "missing csrf token", http.StatusBadRequest)
            return
        }
        // validate token against session store or signed value
        next.ServeHTTP(w, r)
    })
}

func safePostHandler(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("safe action"))
}

func main() {
    r := mux.NewRouter()
    r.Handle("/api/v1/action", withCSRFToken(http.HandlerFunc(safePostHandler))).Methods("POST")
    http.ListenAndServe(":8080", r)
}

4) Enforce CORS strictly and limit origins

Configure CORS to allow only trusted origins and avoid wildcard origins for routes that rely on API keys. This reduces the attack surface for CSRF-like cross-origin requests.

func corsMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Access-Control-Allow-Origin", "https://trusted.example.com")
        w.Header().Set("Access-Control-Allow-Methods", "POST, GET, OPTIONS")
        w.Header().Set("Access-Control-Allow-Headers", "Content-Type, X-API-Key")
        if r.Method == "OPTIONS" {
            w.WriteHeader(http.StatusOK)
            return
        }
        next.ServeHTTP(w, r)
    })
}

5) Prefer short-lived tokens or rotating keys

Where feasible, use short-lived API keys or rotate keys frequently to limit the impact of accidental exposure. Gorilla Mux does not manage key rotation, but your service can enforce key validation against a dynamic lookup on each request.

These steps ensure that Gorilla Mux routes using API keys remain resilient against CSRF by removing reliance on browser-sendable secrets and adding explicit validation layers. middleBrick scans can verify whether state-changing routes are exposed without required headers or custom anti-CSRF controls.