Arp Spoofing in Phoenix with Mutual Tls

Arp Spoofing in a Phoenix application protected with Mutual TLS (mTLS) can still occur at the network level even though transport-layer identity verification is enforced. In mTLS, both client and server present certificates during the TLS handshake, and once the session is established, higher-layer identity is trusted. However, ARP operates below IP and TLS, so an attacker on the same local network can send falsified ARP replies that map their MAC address to the IP address of a legitimate service or client. Because Phoenix applications typically run behind a reverse proxy or within a private network (e.g., Kubernetes pods or VMs), an attacker who successfully spoofs ARP can intercept, modify, or drop traffic before it reaches the TLS endpoint. This does not break the TLS cipher suite or certificate validation, but it does redirect traffic to the attacker’s host, enabling interception of requests and responses that appear valid to the server and client. In a Phoenix deployment using mTLS, the framework verifies peer certificates, but it does not validate link-layer reachability or prevent ARP manipulation. Therefore, an attacker can position themselves as a man-in-the-middle without invalidating the mTLS session, potentially observing unencrypted request payloads before TLS encryption or injecting malicious requests that the server accepts as legitimate. This is especially relevant in shared or cloud environments where multiple tenants share network segments. The risk is not in the cryptographic strength of mTLS but in the lack of layer-2 protections and network segmentation. Phoenix applications that rely on mTLS for client authentication should be deployed in isolated subnets or with additional network controls to mitigate ARP Spoofing.

To reduce exposure to ARP Spoofing in a Phoenix application using Mutual TLS, focus on network-level hardening and strict endpoint verification. While mTLS ensures that only clients with valid certificates can establish secure sessions, it does not prevent ARP manipulation. Compensating controls include placing services in isolated VPCs or security groups, using static ARP entries in controlled environments, and enabling host-based firewall rules to limit unnecessary ARP responses. In Phoenix, you can integrate mTLS directly in the endpoint pipeline using plug-based connection verification. Below is a concrete example of configuring a Phoenix endpoint to require and verify client certificates.

defmodule MyAppWeb.Endpoint do
  use Phoenix.Endpoint, otp_app: :my_app

  # Enable SSL with client certificate verification
  socket "/socket", MyAppWeb.UserSocket,
    websocket: true,
    verify_ssl: true,
    ssl_opts: [
      certfile: "priv/cert/server.crt",
      keyfile: "priv/cert/server.key",
      cacertfile: "priv/cert/ca.crt",
      verify: :verify_peer,
      depth: 2,
      customize_hostname_check: [match_fun: :public_key.pkix_verify_hostname_match_fun(:https)]
    ]

  # Optional: reject connections without valid client certs
  before_start do
    # Ensures only clients with trusted certificates are allowed
    :ssl.set_session_cb(MyApp.SslSessionValidator)
  end
end

On the client side, ensure that requests include the correct client certificate and that the server’s CA is trusted. Below is an example using Finch to make mTLS requests to a Phoenix endpoint.

client = Finch.build(:get, "https://api.myapp.com/secure", [
  {"accept", "application/json"}
])

ssl_opts = [
  certfile: "priv/cert/client.crt",
  keyfile: "priv/cert/client.key",
  cacertfile: "priv/cert/ca.crt"
]

case Finch.request(client, ssl: ssl_opts) do
  {:ok, %Finch.Response{status: 200, body: body}} ->
    IO.puts("Secure response: #{body}")

  {:error, reason} ->
    Logger.error("MTLS request failed: #{inspect(reason)}")
end

These configurations ensure that both server and client mutually authenticate via certificates, reducing the risk of unauthorized clients connecting even if ARP spoofing redirects traffic. Network-level defenses such as disabling gratuitous ARP replies and using static ARP tables in controlled environments further complement these measures. In production, combine mTLS with network segmentation and continuous monitoring to detect anomalies in ARP tables.