Dangling Dns in Buffalo with Hmac Signatures

A dangling DNS reference in a Buffalo application becomes significant when request validation relies on HMAC signatures without additional host verification. In Buffalo, incoming requests often include an HMAC signature in a header (for example, X-API-Signature) computed with a shared secret over selected parts of the request. If the application uses the signature mainly to verify integrity or authenticity but does not independently validate the request’s target host, an attacker can leverage a dangling DNS entry to redirect or cache responses in a way that affects which host the signed request ultimately reaches.

Specifically, this can occur when a Buffalo app performs signature verification and then forwards or proxies the request based on a user-controlled host header or a loosely configured route that resolves to a dangling DNS record. The dangling DNS record may point to an unintended or unclaimed host that can receive proxied requests. Because the signature was computed over the original request path and possibly a trusted hostname, an attacker who can influence the host portion (e.g., via a Host header or a redirect) might cause the signed request to be processed in a context where the server’s expectations about the host differ from reality. This mismatch can expose logic that should have been protected: for example, host-based access controls or origin checks may be bypassed because the signature does not bind tightly to a verified, canonical hostname.

Consider a scenario where a Buffalo API validates an HMAC over the request path and selected headers but does not enforce that the request target matches an expected origin. An attacker can register or control a DNS name that points to a server they manage, then induce a victim’s browser or service to send signed requests to that name. If the Buffalo app follows redirects or uses the Host header for routing without re-validating the host against the signature context, the signed request may be processed as if it originated from the legitimate domain. The dangling DNS entry thus becomes a pivot point where the integrity guarantee of the HMAC does not prevent host confusion, potentially leading to unauthorized actions or information disclosure tied to the presumed origin.

This combination is risky when the signature scope is too narrow (e.g., excludes the hostname or uses a mutable Host header). HMAC signatures are strong for ensuring data integrity, but they must be paired with explicit host validation and strict routing rules. In Buffalo, you should not rely on the signature alone to protect against host-based confusion; you must ensure the request’s target host is verified before using the signature to authorize sensitive operations.

To mitigate dangling DNS issues tied to HMAC signatures in Buffalo, bind the signature verification to a canonical host and avoid relying on mutable request metadata for security decisions. Below are concrete remediation steps and code examples that you can apply in your Buffalo application.

1) Include the hostname in the HMAC scope

When computing and verifying the HMAC, incorporate the request’s canonical hostname so that a signature is only valid for a specific host. This prevents an attacker from replaying a signed request to a different host, including one pointed by a dangling DNS record.

-- Example: computing HMAC over method, path, and host in Go (using the Buffalo helper pattern)
package middleware

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

func ComputeSignature(secret, method, host, path string) string {
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(method + "\n" + host + "\n" + path + "\n"))
    return hex.EncodeToString(mac.Sum(nil))
}

func VerifySignature(secret, method, host, path, receivedSig string) bool {
    expected := ComputeSignature(secret, method, host, path)
    return hmac.Equal([]byte(expected), []byte(receivedSig))
}

// In a Buffalo before action:
func HMACAuth(req *buffalo.Request) error {
    const sharedSecret = "your-256-bit-secret"
    host := strings.ToLower(req.Host)          // canonicalize host
    method := req.Method
    path := req.RequestURI
    sig := req.Header.Get("X-API-Signature")
    if !VerifySignature(sharedSecret, method, host, path, sig) {
        return req.Render(401, r.JSON(map[string]string{"error": "invalid signature"}))
    }
    return nil
}

2) Validate the Host header against an allowlist

Even when a signature covers the host, explicitly validate the Host header against an allowlist of expected origins before processing the request. This prevents processing requests that resolve through dangling DNS entries to unexpected hosts.

-- Example: host validation in a Buffalo before action
func HostValidation(req *buffalo.Request) error {
    allowedHosts := map[string]bool{
        "api.example.com": true,
        "staging.api.example.com": true,
    }
    host := strings.ToLower(req.Host)
    if !allowedHosts[host] {
        return req.Render(400, r.JSON(map[string]string{"error": "host not allowed"}))
    }
    return nil
}

// Combine both checks in the action pipeline:
func Actions() http.Handler {
    return buffalo.New(buffalo.Options{
        // ... other options
        Before: []buffalo.BeforeAction{
            HostValidation,
            HMACAuth,
        },
    })
}

3) Use absolute URLs for redirects and avoid following untrusted redirects

If your Buffalo app issues redirects, always use absolute URLs with a trusted host rather than echoing a user-supplied host. Avoid following redirects or proxying to hosts derived from user input without strict validation.

-- Example: safe redirect in Buffalo
func RedirectToDashboard(c buffalo.Context) error {
    // Do not use c.Param("host") directly to build redirect URLs
    dashboardURL := "https://api.example.com/dashboard"
    return c.Redirect(302, dashboardURL)
}

By combining host-included HMAC scopes, strict host allowlisting, and safe redirect practices, you reduce the risk that a dangling DNS entry can be leveraged to bypass signature-based protections in Buffalo.