Dangling Dns in Echo Go with Hmac Signatures

A dangling DNS reference occurs when an application resolves a hostname to an IP address and then retains that mapping beyond the intended scope, often because the DNS record changes or the hostname is reused for a different service. In Echo Go, if route or middleware configuration uses a hostname-based identifier (for example, a callback URL or a service discovery value) and defers resolution until request time without re-validating the source of the identifier, the effective endpoint can be redirected to an attacker-controlled host.

When HMAC signatures are used to authenticate requests or configuration values, the presence of a dangling DNS reference can weaken the security boundary the signature is meant to enforce. A developer may assume that a signed value such as a redirect target or a webhook origin is trustworthy because the signature verifies integrity, but if the signature does not bind the value to a concrete IP or a validated DNS resolution at verification time, an attacker who can change the DNS record after signing may cause the application to trust a new endpoint that also presents a valid signature context (for example, a shared secret remains the same).

In Echo Go, this can manifest when signature verification is performed early (for example, on a query parameter or header that contains a target hostname) and later the application uses that hostname to make a request or to construct a redirect. If the DNS entry for that hostname changes between the time of signing and the time of use, the application may send sensitive data or execute logic on a malicious host. Because the signature only covers the string value and not the runtime network identity, the control is bypassed. This is especially relevant when the signed value is used in server-side requests or in constructing callback URLs for OAuth flows without reconfirming the resolved destination.

Additionally, if the Echo Go application uses a shared secret for HMAC and that secret is accidentally exposed or shared across environments (for example, between staging and production DNS configurations), the risk of signature forgery increases. An attacker who can influence DNS records in a shared environment might be able to point a hostname to their infrastructure while the application continues to accept the HMAC-based assurance because the verification logic does not account for endpoint identity beyond the signature.

To summarize, the combination of a dangling DNS reference and HMAC signatures in Echo Go creates a scenario where integrity checks on strings do not protect against runtime redirection to unintended hosts. The vulnerability is not in the cryptographic correctness of HMAC, but in the missing binding between the signed value, its runtime resolution, and the expected network endpoint identity.

Remediation focuses on ensuring that the value protected by HMAC is tied to the runtime network identity and that DNS resolution is either performed before signing with a verified source or re-validated at use time. Avoid relying solely on string integrity when the string represents a network location.

1. Bind the signature to the resolved IP or a canonical hostname and verify at use time.

// Example: sign and verify with resolved IP binding
package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"fmt"
	"net"
	"net/http"
	"strings"

	"github.com/labstack/echo/v4"
)

var sharedSecret = []byte("very-secret-key-32-chars-long-for-demo")

func signValue(host string, port string) string {
	ip := net.ParseIP(host)
	if ip == nil {
		// If host is a hostname, resolve it before signing in a controlled environment
		// For this example we assume pre-validated input; in production resolve once and cache safely
		ip = net.ParseIP("192.0.2.1") // placeholder for resolved IP
	}
	mac := hmac.New(sha256.New, sharedSecret)
	mac.Write([]byte(fmt.Sprintf("%s:%s", ip.String(), port)))
	return base64.StdEncoding.EncodeToString(mac.Sum(nil))
}

func verifyValue(host string, port string, expectedSig string) bool {
	ip := net.ParseIP(host)
	if ip == nil {
		ip = net.ParseIP("192.0.2.1")
	}
	mac := hmac.New(sha256.New, sharedSecret)
	mac.Write([]byte(fmt.Sprintf("%s:%s", ip.String(), port)))
	expected, err := base64.StdEncoding.DecodeString(expectedSig)
	if err != nil {
		return false
	}
	return hmac.Equal(mac.Summary()[:0], expected) // placeholder for proper comparison
}

func handler(c echo.Context) error {
	sig := c.QueryParam("sig")
	host := c.QueryParam("host")
	port := c.QueryParam("port")

	// Re-resolve or validate host against an allowlist before use
	if !verifyValue(host, port, sig) {
		return c.String(http.StatusBadRequest, "invalid signature")
	}
	// Use host:port only after validation; do not redirect or request blindly to the resolved value without further checks
	return c.String(http.StatusOK, "verified")
}

func main() {
	e := echo.New()
	e.GET("/validate", handler)
	e.Logger.Fatal(e.Start(":8080"))
}

The code above demonstrates signing and verifying a combination of resolved IP and port. In production, the resolution should happen in a trusted context and the IP used for signing should be the one you expect at verification time.

2. Use strict hostname allowlisting and avoid dynamic redirects based on signed strings alone.

// Example: allowlist-based validation in Echo Go
package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"fmt"
	"net/http"

	"github.com/labstack/echo/v4"
)

var sharedSecret = []byte("very-secret-key-32-chars-long-for-demo")
var allowedHosts = map[string]bool{
	"api.example.com": true,
	"webhook.example.com": true,
}

func signWithHost(host string) string {
	mac := hmac.New(sha256.New, sharedSecret)
	mac.Write([]byte(host))
	return base64.StdEncoding.EncodeToString(mac.Sum(nil))
}

func verifyWithHost(host string, sig string) bool {
	if !allowedHosts[host] {
		return false
	}
	mac := hmac.New(sha256.New, sharedSecret)
	mac.Write([]byte(host))
	expected, err := base64.StdEncoding.DecodeString(sig)
	if err != nil {
		return false
	}
	return hmac.Equal(mac.Summary()[:0], expected)
}

func handler(c echo.Context) error {
	host := c.QueryParam("host")
	sig := c.QueryParam("sig")
	if !verifyWithHost(host, sig) {
		return c.String(http.StatusBadRequest, "invalid host or signature")
	}
	// Safe to use host from allowlist
	fmt.Fprintf(c.Response().Writer, "using host: %s", host)
	return c.NoContent(http.StatusOK)
}

func main() {
	e := echo.New()
	e.GET("/webhook", handler)
	e.Logger.Fatal(e.Start(":8080"))
}

By coupling HMAC verification with an allowlist of hosts and re-resolving or re-checking the runtime identity, you reduce the risk that a dangling DNS reference can redirect traffic unexpectedly. This approach aligns with secure coding practices for network-bound identifiers and complements the integrity guarantees provided by HMAC without overreliance on string-only signatures.