Arp Spoofing in Phoenix with Dynamodb

Arp spoofing is a link-layer 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 a Phoenix application that uses Amazon DynamoDB as a backend data store, the risk is not that Arp spoofing directly breaks DynamoDB’s service, but that it undermines the confidentiality and integrity of the traffic between your Phoenix nodes and DynamoDB endpoints.

When a Phoenix server runs in an environment shared with other tenants (e.g., a public cloud VPC or a collocated network), an attacker on the same broadcast segment can use tools like arpspoof to intercept traffic intended for the DynamoDB IP address. Although DynamoDB traffic is usually encrypted in transit via TLS, an active Arp spoofing attack can redirect packets to the attacker’s host, enabling passive observation or injection if other weaknesses exist (for example, weak proxy configurations or leaked credentials in logs).

Consider a Phoenix endpoint that constructs URLs for the AWS SDK without enforcing strict host verification. An attacker who successfully spoofs the gateway may intercept metadata service or credential requests, or exploit misconfigured HTTP clients to redirect DynamoDB API calls to a malicious endpoint. This becomes particularly relevant if custom HTTP clients are used to talk to DynamoDB via a proxy or a non-standard endpoint, bypassing standard AWS SDK protections.

To illustrate, a vulnerable HTTP client setup in Phoenix might look like this:

defmodule MyApp.DynamoProxy do
  def build_url(path) do
    "http://dynamodb.#{System.get_env("DOMAIN")}/" <> path
  end

  def get_item(table, key) do
    url = build_url("item")
    # If an attacker spoofs DNS or ARP for "dynamodb.example.com",
    # this request can be intercepted.
    HTTPoison.get(url, [], hackney: [pool: :default])
    |> handle_response()
  end
end

If the domain resolves to a spoofable address or if the network allows ARP manipulation, the integrity of the data sent to DynamoDB can be compromised. The AWS SDK for Elixir, when used directly with proper region and credential configuration, avoids such proxy pitfalls, but custom networking layers in Phoenix can reintroduce exposure.

Additionally, if credentials are inadvertently exposed through logs or error messages during intercepted sessions, an attacker can combine Arp spoofing with credential harvesting to gain further access. This highlights the importance of ensuring that DynamoDB interactions in Phoenix remain within trusted network boundaries and that client configurations are hardened against manipulation.

Remediation focuses on ensuring that DynamoDB traffic from Phoenix is authenticated, encrypted, and routed through trusted AWS SDKs rather than custom HTTP proxies that can be undermined by Arp spoofing. Below are concrete, safe patterns for integrating DynamoDB in Phoenix applications.

1. Use the official AWS SDK for Elixir (aws-elixir) instead of raw HTTP calls. The SDK handles signing requests with SigV4, uses TLS by default, and respects AWS credential chains, reducing the surface for interception.

defmodule MyApp.DynamoClient do
  alias ExAws.DynamoDB, as: Dynamo
  alias ExAws.Request

  @table System.get_env("DYNAMO_TABLE") || "default_table"

  def get_item(key) do
    Dynamo.get_item(@table, key)
    |> ExAws.request()
  end

  def put_item(item) do
    Dynamo.put_item(@table, item)
    |> ExAws.request()
  end
end

2. Enforce TLS and avoid custom HTTP schemes. Never construct http:// URLs to DynamoDB. If you must use HTTP clients directly, ensure the scheme is https:// and certificate verification is enabled.

defmodule MyApp.SecureHttpClient do
  def fetch_dynamodb(path) do
    url = "https://dynamodb.#{System.get_env("AWS_REGION")}.amazonaws.com/" <> path
    headers = [{"Authorization", "AWS4-HMAC-SHA256 ..."}]
    # Always verify SSL; do not use hackney without TLS verification.
    HTTPoison.get(url, headers, hackney: [ssl: [verify: :verify_peer]])
  end
end

3. Isolate network paths by using VPC endpoints for DynamoDB. In cloud deployments, configure your Phoenix nodes to access DynamoDB via Gateway VPC endpoints, which keep traffic off the public internet and mitigate the risk of interception via Arp spoofing on shared subnets.

# Example configuration in config/runtime.exs
config :my_app, MyApp.Repo,
  database: System.get_env("DB_NAME"),
  hostname: System.get_env("DB_HOST"),
  pool_size: 10

# For DynamoDB, prefer SDK-based clients that resolve to AWS private endpoints
config :ex_aws, 
  access_key_id: [{:system, "AWS_ACCESS_KEY_ID"}, :instance_role],
  secret_access_key: [{:system, "AWS_SECRET_ACCESS_KEY"}, :instance_role],
  region: System.get_env("AWS_REGION") || "us-east-1"

4. Validate server certificates and avoid insecure defaults. If you interact with DynamoDB via a custom client, explicitly set TLS options to reject self-signed or mismatched certificates.

ssl_opts = [
  cacertfile: "/path/to/ca-bundle.crt",
  verify: :verify_peer,
  depth: 2,
  customize_hostname_check: [match_fun: :public_key.pkix_verify_hostname_match_fun(:https)]
]

# Use these opts in your HTTP client or AWS SDK configuration.

These measures ensure that even if an attacker attempts Arp spoofing on the network, the integrity and authentication of DynamoDB requests from Phoenix remain intact, leveraging AWS’s built-in protections and avoiding fragile custom networking code.