Dns Rebinding in Buffalo with Hmac Signatures

Buffalo is a Go web framework that encourages rapid development with sensible defaults, yet it does not automatically protect against certain network-level attacks when Hmac Signatures are used for request authentication. Dns Rebinding is an attack where a malicious page resolves a hostname to an attacker-controlled IP, then later switches the resolution to a different internal IP (such as 127.0.0.1 or a corporate service). When Hmac Signatures are used to validate requests in Buffalo, an attacker can exploit weak or missing source verification to make a signed request appear legitimate while the endpoint resolves to an internal host.

Consider a Buffalo app that uses Hmac Signatures to ensure request integrity between a client and server. The server generates a signature over selected headers and the body using a shared secret, and the endpoint trusts that a valid signature implies a trusted source. If the application does not validate the destination IP or enforce strict hostname-to-IP binding, an attacker can first resolve api.example.com to an external, attacker-controlled server that returns a valid response signed with a leaked or predictable key. Then, via a rebinding technique (e.g., using a CNAME or DNS cache flush), the same hostname resolves to 127.0.0.1 or an internal service. Because the Hmac Signature is tied to the request content and headers, and the server does not verify the actual network destination, the signed request is accepted even though it now targets an internal endpoint.

This becomes critical when sensitive internal endpoints are exposed through the same domain or subdomain namespace, or when CORS and origin checks are based solely on hostname without corroborating IP validation. The Hmac Signature mechanism in Buffalo does not inherently prevent the request from being forwarded to a different IP after the DNS rebinding occurs; the framework validates the signature, but does not inherently validate that the resolved IP matches an expected network location. Attack patterns such as internal service enumeration, SSRF, or authentication bypass via localhost manipulation are feasible when DNS Rebinding intersects with Hmac Signatures that lack IP binding.

In practice, an attacker might host a page that uses JavaScript to perform signed API calls to api.example.com, first pointing to a benign resolver, then switching resolution to 127.0.0.1 to probe internal health checks or administrative interfaces. If the Buffalo application uses Hmac Signatures without additional network context checks, the signed probe can reach internal routes that should never be exposed to external or untrusted network positions. Compounded risks include weak secret management, predictable nonces, or reused timestamps, which can further weaken the Hmac Signature scheme under rebinding conditions.

To mitigate Dns Rebinding when using Hmac Signatures in Buffalo, you must couple signature validation with strict network and hostname verification. Do not rely on signatures alone; enforce that the request destination IP matches an expected set of endpoints, and validate the HTTP Host header against a whitelist of authoritative hostnames.

Example: Secure Hmac Signature validation with hostname and IP checks

package actions

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

	"github.com/gobuffalo/buffalo"
)

// verifyHmacWithNetworkContext validates the Hmac Signature and ensures the request
// targets an expected host and IP range.
func verifyHmacWithNetworkContext(next buffalo.Handler) buffalo.Handler {
	return func(c buffalo.Context) error {
		host := c.Request().Host
		if !isAuthorizedHost(host) {
			return c.Error(http.StatusForbidden, "unauthorized host")
		}

		// Ensure the resolved IP is within expected bounds (example: not localhost unless explicitly allowed)
		ip, _, err := net.SplitHostPort(c.Request().RemoteAddr)
		if err != nil {
			return c.Error(http.StatusBadRequest, "invalid address")
		}
		if isPrivateOrLocal(ip) && !isAllowedPrivateEndpoint(host) {
			return c.Error(http.StatusForbidden, "request resolved to private IP")
		}

		expectedMAC := c.Request().Header.Get("X-Signature")
		if expectedMAC == "" {
			return c.Error(http.StatusBadRequest, "missing signature")
		}

		body := c.Request().Body
		// In practice, you would read and buffer the body carefully to avoid exhaustion.
		// For this example, we assume a helper that retrieves the raw body.
		raw, err := getRawBody(c.Request())
		if err != nil {
			return c.Error(http.StatusBadRequest, "failed to read body")
		}

		secret := []byte("your-secure-shared-secret") // use secure secret management
		mac := hmac.New(sha256.New, secret)
		mac.Write(raw)
		mac.Write([]byte(host))
		expected := hex.EncodeToString(mac.Sum(nil))

		if !hmac.Equal([]byte(expected), []byte(expectedMAC)) {
			return c.Error(http.StatusForbidden, "invalid signature")
		}

		return next(c)
	}
}

func isAuthorizedHost(host string) bool {
	allowed := map[string]bool{
		"api.example.com": true,
		"app.example.com": true,
	}
	return allowed[host]
}

func isPrivateOrLocal(ipStr string) bool {
	ip := net.ParseIP(ipStr)
	if ip == nil {
		return false
	}
	return ip.IsPrivate() || ip.IsLoopback() || ip.IsLinkLocalMulticast() || ip.IsLinkLocalUnicast()
}

func isAllowedPrivateEndpoint(host string) bool {
	// Only permit localhost for specific health check endpoints if explicitly required
	if host == "health.example.com" {
		return true
	}
	return false
}

func getRawBody(r *http.Request) ([]byte, error) {
	// Example: read the body with a reasonable limit to prevent DoS
	const maxBodyBytes = 1048576 // 1 MB
	r.Body = http.MaxBytesReader(nil, r.Body, maxBodyBytes)
	return io.ReadAll(r.Body)
}

In this remediation pattern, the Buffalo handler validates the Hmac Signature over the raw body and selected headers, and also checks that the request’s resolved IP is not a private or loopback address unless explicitly allowed for a specific host. By binding authorization to both cryptographic integrity and network context, you reduce the surface for Dns Rebinding attacks that rely on mismatched expectations between hostname resolution and actual endpoint location.

Additionally, ensure your DNS configuration and hosting provider prevent unwanted CNAME or alias chains that could facilitate rebinding. Use HSTS and appropriate CORS policies to further limit the contexts in which signed requests can be executed. These measures complement Hmac Signatures in Buffalo by ensuring that the network path and destination remain consistent with your security assumptions.