Clickjacking in Buffalo with Hmac Signatures

Clickjacking is a client-side attack where an attacker tricks a user into interacting with a hidden or disguised UI element inside an iframe. In Buffalo, Hmac Signatures are commonly used to protect forms and API requests by ensuring integrity and origin authenticity. However, relying solely on Hmac Signatures without proper framing defenses can expose the application to clickjacking when pages are embedded in hostile frames.

Consider a Buffalo application that uses Hmac Signatures to validate form submissions. If the server embeds its forms inside an <iframe> or fails to set frame-deny headers, an attacker can overlay invisible controls or transparent layers on top of the embedded page. The user may unknowingly click a button or link that triggers a signed request, because the cryptographic signature does not protect against UI redressing — it only validates that the payload was generated by a trusted server-side key. The signature remains valid, but the context of the user interaction is manipulated.

For example, an endpoint /transfer might accept a POST with parameters amount, to, and a signature computed over the concatenated values. An attacker can craft a page that loads /transfer in an invisible iframe and programmatically triggers a click on the submit button via JavaScript. The Hmac Signature is included in the forged request, and if the server does not validate the Referer or Origin headers, it may accept the request as legitimate. This demonstrates that Hmac Signatures alone do not prevent clickjacking; they must be paired with anti-framing controls and origin checks.

Buffalo’s use of Hmac Signatures can also interact dangerously with features like Turbo links or single-page app integrations, where forms are dynamically injected into the DOM. If an attacker can influence any part of the UI that participates in the signing process (e.g., hidden form fields), they may shift the signed context without invalidating the signature. Therefore, the combination of clickjacking vectors and Hmac Signatures requires defense in depth, ensuring that even if a signature is valid, the request context is verified through strict framing policies and same-origin enforcement.

Remediation focuses on preventing the forged UI context from executing trusted actions protected by Hmac Signatures. This involves strict origin validation, anti-CSRF tokens, and frame denial headers. Below are concrete, working examples in Go using the Buffalo framework.

1. Enforce Origin and Referer checks before verifying Hmac Signatures

Always validate that the request originates from your own frontend. Reject requests that do not include a matching Origin or Referer.

func verifyOrigin(r *http.Request) bool {
    origin := r.Header.Get("Origin")
    referer := r.Header.Get("Referer")
    allowed := "https://your-app.com"
    return origin == allowed || referer == allowed
}

func TransactionsController(c buffalo.Context) error {
    if !verifyOrigin(c.Request()) {
        return c.Error(403, errors.New("invalid origin"))
    }
    // proceed to verify Hmac signature
    return nil
}

2. Include a per-request nonce in Hmac computation to bind the signature to a specific render

Generate a nonce server-side, embed it in the form, and include it in the Hmac signature. This prevents attackers from reusing a signed payload in a different UI context.

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "math/rand"
    "time"
)

func generateNonce() string {
    rand.Seed(time.Now().UnixNano())
    return fmt.Sprintf("%d", rand.Int63())
}

func signData(data, key, nonce string) string {
    mac := hmac.New(sha256.New, []byte(key))
    mac.Write([]byte(data + nonce))
    return hex.EncodeToString(mac.Sum(nil))
}

// In your handler that renders the form
func NewPaymentForm(c buffalo.Context) error {
    nonce := generateNonce()
    c.Session().Set("nonce", nonce)
    signature := signData(fmt.Sprintf("amount=%s&to=%s", amount, to), secretKey, nonce)
    return c.Render(200, r.HTML("payments/new.html", map[string]interface{}{
        "Nonce":     nonce,
        "Signature": signature,
    }))
}

// In your submit handler
func CreatePayment(c buffalo.Context) error {
    nonce := c.Session().Get("nonce").(string)
    expected := signData(fmt.Sprintf("amount=%s&to=%s", amount, to), secretKey, nonce)
    if !hmac.Equal([]byte(expected), []byte(c.Param("signature"))) {
        return c.Error(403, errors.New("invalid signature"))
    }
    // process payment
    return nil
}

3. Set Content-Security-Policy and X-Frame-Options headers to prevent embedding

Ensure that your responses explicitly disallow framing, regardless of the strength of your Hmac Signatures.

func App() *buffalo.App {
    app := buffalo.New(buffalo.Options{})
    app.Use(func(next buffalo.Handler) buffalo.Handler {
        return func(c buffalo.Context) error {
            c.Response().Header().Set("X-Frame-Options", "DENY")
            c.Response().Header().Set("Content-Security-Policy", "frame-ancestors 'none'")
            return next(c)
        }
    })
    return app
}

4. Combine Hmac Signatures with anti-CSRF tokens for state-changing requests

Use a separate anti-CSRF token stored in a secure, HttpOnly cookie and required in the form body, independent of the Hmac signature.

func GenerateCSRFToken() string {
    token := make([]byte, 32)
    rand.Read(token)
    return hex.EncodeToString(token)
}

// Set cookie and form field
csrfToken := GenerateCSRFToken()
c.Response().Cookie(&http.Cookie{
    Name:     "csrf_token",
    Value:    csrfToken,
    HttpOnly: true,
    Secure:   true,
    SameSite: http.SameSiteStrictMode,
})
c.Form().Set("csrf_token", csrfToken)

// Verify in handler
if c.Param("csrf_token") != c.Request().Cookie("csrf_token").Value {
    return c.Error(403, errors.New("csrf token mismatch"))
}