Dns Cache Poisoning in Gorilla Mux with Hmac Signatures

DNS cache poisoning targets the resolution path that routes incoming requests to the underlying handler in Gorilla Mux. When you use Hmac Signatures primarily for request authentication between services, the signature is typically verified after the router has already selected a route. If an attacker can manipulate DNS responses so that a victim client or service resolves a legitimate domain to a malicious IP, traffic is redirected to an attacker-controlled server. That server can present a valid TLS certificate for the domain and still pass the Hmac signature verification if the shared secret is compromised or if the client mistakenly validates the signature on a response that originated from an untrusted endpoint.

Gorilla Mux does not validate DNS provenance; it relies on the operating system’s resolver and the client’s trust in the resolved IP. An attacker who successfully poisons a cache can serve a malicious API endpoint that mimics the legitimate service. If the client uses Hmac Signatures for request authentication but does not also bind the signature to the resolved endpoint’s identity (for example, by pinning the certificate or including the hostname in the signed payload), the signature check may pass even though the request is being sent to an attacker. This scenario commonly occurs when the signature covers only HTTP method, path, and headers, but not the hostname or the TLS certificate fingerprint.

Consider a microservice architecture where Service A calls Service B using Hmac Signatures. If Service B’s DNS record is poisoned, Service A may unknowingly send signed requests to a rogue Service B. Because the Hmac signature is often computed from a canonical representation of the request that excludes the hostname, the signature remains valid. The combination of Gorilla Mux’s flexible routing (which can host multiple domains on overlapping path prefixes) and Hmac Signatures that do not incorporate hostname verification increases the impact of DNS cache poisoning by making it harder to detect the redirection.

To observe this in a controlled test, you can simulate a poisoned resolver by modifying your local hosts file or using a custom DNS resolver that returns a malicious IP. Then, send a request with a valid Hmac Signature to the legitimate domain. If the server at the malicious IP echoes the same validation logic and shares the secret (or a weak implementation allows signature forgery), the request may be accepted despite the redirection. This highlights that Hmac Signatures protect integrity and origin only when the signing process includes endpoint identity and the client validates that identity independently of DNS.

In summary, DNS cache poisoning does not break Hmac Signatures directly, but it undermines the trust assumptions that make those signatures meaningful. Gorilla Mux’s routing layer does not mitigate DNS-level attacks, so you must ensure that your Hmac implementation binds signatures to verifiable endpoint identity, such as hostname and certificate information, and that clients validate this binding before trusting the response.

Remediation focuses on ensuring that Hmac Signatures cover the hostname and, where applicable, the TLS certificate fingerprint, and that clients validate these elements before processing the response. Below is a complete, realistic example of a server that signs requests with Hmac-SHA256 and includes the hostname and a timestamp to prevent replay and mitigate DNS-based redirection.

// server.go
package main

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

const sharedSecret = "super-secure-secret"

func generateSignature(method, path, hostname string, ts int64, body string) string {
    message := fmt.Sprintf("%s|%s|%s|%d|%s", method, path, hostname, ts, body)
    mac := hmac.New(sha256.New, []byte(sharedSecret))
    mac.Write([]byte(message))
    return hex.EncodeToString(mac.Sum(nil))
}

func handler(w http.ResponseWriter, r *http.Request) {
    // In production, verify hostname matches expected service
    expectedHost := "api.example.com"
    if r.Host != expectedHost {
        http.Error(w, "invalid host header", http.StatusBadRequest)
        return
    }

    tsStr := r.Header.Get("X-Request-Timestamp")
    sig := r.Header.Get("X-Request-Signature")
    if tsStr == "" || sig == "" {
        http.Error(w, "missing signature or timestamp", http.StatusUnauthorized)
        return
    }

    ts, err := strconv.ParseInt(tsStr, 10, 64)
    if err != nil || time.Since(time.Unix(ts, 0)) > 5*time.Minute {
        http.Error(w, "invalid timestamp", http.StatusUnauthorized)
        return
    }

    bodyBytes := make([]byte, r.ContentLength)
    r.Body.Read(bodyBytes)
    body := string(bodyBytes)
    computedSig := generateSignature(r.Method, r.URL.Path, r.Host, ts, body)

    if !hmac.Equal([]byte(computedSig), []byte(sig)) {
        http.Error(w, "invalid signature", http.StatusUnauthorized)
        return
    }

    fmt.Fprintf(w, "OK")
}

func main() {
    r := http.NewServeMux()
    r.HandleFunc("/v1/resource", handler)
    http.ListenAndServe(":8080", r)
}

The client must include the same hostname and timestamp in its signature calculation and must validate the server’s hostname against a pinned value before trusting the response. This prevents an attacker who successfully poisoned DNS from passing signature checks when the client enforces hostname binding.

// client.go
package main

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

const sharedSecret = "super-secure-secret"

func generateSignature(method, path, hostname string, ts int64, body string) string {
    message := fmt.Sprintf("%s|%s|%s|%d|%s", method, path, hostname, ts, body)
    mac := hmac.New(sha256.New, []byte(sharedSecret))
    mac.Write([]byte(message))
    return hex.EncodeToString(mac.Sum(nil))
}

func main() {
    client := &http.Client{}
    ts := time.Now().Unix()
    body := `{"action":"read"}`
    sig := generateSignature("POST", "/v1/resource", "api.example.com", ts, body)

    req, _ := http.NewRequest("POST", "https://api.example.com/v1/resource", strings.NewReader(body))
    req.Header.Set("X-Request-Timestamp", strconv.FormatInt(ts, 10))
    req.Header.Set("X-Request-Signature", sig)
    req.Header.Set("Host", "api.example.com")

    // Pin hostname and optionally certificate in production
    resp, err := client.Do(req)
    if err != nil {
        panic(err)
    }
    defer resp.Body.Close()

    if resp.Host != "api.example.com" {
        panic("hostname mismatch")
    }
    fmt.Println("Request succeeded")
}

For Gorilla Mux, ensure routes are defined with strict host matching to reduce ambiguity. Combine Hmac Signatures with transport-layer hostname validation to make DNS cache poisoning less impactful. If you use the middlebard CLI (middlebrick scan <url>) or the GitHub Action to add API security checks to your CI/CD pipeline, you can detect missing hostname binding and other misconfigurations before deployment.