Arp Spoofing in Phoenix with Bearer Tokens

Arp Spoofing is a Layer 2 attack where an attacker sends falsified ARP messages to associate their MAC address with the IP address of another host, typically the default gateway. In Phoenix, this becomes especially risky when Bearer Tokens are used for API authentication because traffic between clients and services is often transmitted in clear text at the application layer, relying on HTTPS for transport security. If an attacker successfully spoofs ARP entries on the local network, they can intercept HTTP requests that include Bearer Tokens in the Authorization header, enabling session hijacking even when TLS is in use.

Phoenix applications commonly rely on stateless token-based authentication. When a Bearer Token is included in the Authorization header, it is only protected by the transport layer. If an attacker compromises the local network segment through ARP spoofing, they can capture these headers and reuse the tokens. This is especially dangerous in environments where network segmentation is weak or where clients connect to public Wi‑Fi in co‑working spaces common in Phoenix. The attacker does not need to break the token itself; they simply insert themselves into the communication path.

middleBrick scans for risks related to Data Exposure and insecure transport configurations. While it does not test local network attacks like ARP spoofing directly, it can identify endpoints that transmit sensitive authorization data without sufficient additional validation. For example, if an API response contains tokens or secrets in logs or error messages, this amplifies the impact of a successful ARP spoofing incident. The scanner also checks for missing or weak input validation and excessive agency patterns in LLM integrations, which could be exploited to automate token exfiltration once network position is gained.

An attacker leveraging ARP spoofing in combination with captured Bearer Tokens might perform request tampering, escalating from read‑only endpoints to administrative ones if the token includes broad scopes. This aligns with BOLA/IDOR and BFLA/Privilege Escalation checks in middleBrick, where token usage is analyzed for improper authorization assumptions. Because Phoenix services often integrate with external LLM endpoints, output scanning becomes relevant to ensure that intercepted or manipulated responses do not leak credentials or private data.

Real world impact can be illustrated with a scenario where a developer uses a Bearer Token stored in an environment variable within a Phoenix script. If the local network is compromised, the token can be extracted from HTTP requests using packet inspection tools. The following example shows a vulnerable HTTP request generated by a Phoenix client:

GET /api/v1/users/me HTTP/1.1
Host: api.example.com
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6InphIiwiYWRtaW4iOnRydWV9.TJVA95OrM7E2cBab30RMHrHDcEfxjoYZgeFONFh7HgQ
Accept: application/json

An attacker conducting ARP spoofing can capture this request and reuse the token. middleBrick’s checks for Data Exposure and Unsafe Consumption help identify whether such tokens appear in logs or error payloads, providing remediation guidance to reduce the attack surface.

Remediation focuses on ensuring Bearer Tokens are never exposed over insecure channels and are handled in a way that limits the impact of network layer attacks. In Phoenix, developers should enforce strict transport security and minimize token lifetime and scope. The following examples demonstrate secure practices using the HTTPoison and Tesla libraries, which are commonly used in Phoenix projects.

First, always use HTTPS and pin certificates where possible. Never construct URLs or headers with tokens in plain text logs or error messages. The following example shows a secure request using Tesla with forced HTTPS and explicit timeout settings:

defmodule MyApp.SecureClient do
  use Tesla

  plug Tesla.Middleware.BaseUrl, "https://api.example.com"
  plug Tesla.Middleware.Headers, [{"Accept", "application/json"}]
  plug Tesla.Middleware.BearerAuth, token: System.get_env("BEARER_TOKEN")
  plug Tesla.Middleware.RequestId
  plug Tesla.Middleware.Logger, log_level: :warn

  def get_user_profile do
    case get("/api/v1/users/me") do
      {:ok, %Tesla.Env{status: 200, body: body}} -> {:ok, body}
      {:ok, %Tesla.Env{status: status}} -> {:error, status}
      {:error, reason} -> {:error, reason}
    end
  end
end

Second, rotate tokens frequently and avoid embedding them in source code or configuration files that might be exposed in version control. Use runtime environment variables and consider integrating with a secrets manager. The following snippet shows how to fetch a token securely at runtime and apply it only for the duration of the request:

defmodule MyApp.SecureRequest do
  def fetch_protected_resource do
    token = fetch_token_securely()
    headers = [{"Authorization", "Bearer #{token}"}]

    :httpc.request(:get, {"https://api.example.com/api/v1/users/me", headers}, [], [])

Third, implement additional application-level checks such as validating token scope and origin. Even if a token is intercepted via ARP spoofing, restricting its usage to specific endpoints and short time windows reduces the attack surface. The following example demonstrates scope validation within a Phoenix controller:

defmodule MyAppWeb.UserController do
  use MyAppWeb, :controller

  def me(conn, _params) do
    with "Bearer " <<token::binary>> <- conn.req_headers["authorization"],
         {:ok, claims} <- MyApp.Token.verify(token, "api.example.com"),
         true <- claims["scope"] in ["read:profile", "read:email"] do
      render(conn, "show.json", user: claims["sub"])
    else
      _ -> send_resp(conn, 401, "Unauthorized")
    end
  end
end

These practices align with middleBrick’s findings related to Authentication, Authorization, and Data Exposure. By combining transport security with strict token lifecycle management, the risk of ARP spoofing leading to token compromise is significantly reduced.