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Automated Security Protocol

How to fix Improper Error Handling
in Phoenix

Executive Summary

In the Phoenix framework, improper error handling often manifests as verbose stack traces or raw database exceptions leaked to the client. For an attacker, this is a blueprint of your internals—schema names, library versions, and logic flows. To secure a Phoenix app, you must suppress debug info in production and centralize error mapping using ErrorViews/ErrorJSON.

The Vulnerable Pattern

VULNERABLE CODE
def show(conn, %{"id" => id}) do
  try do
    user = Repo.get!(User, id)
    render(conn, "show.json", user: user)
  rescue
    e ->
      # LEAK: inspect(e) dumps the entire exception struct and stack trace to the response
      conn
      |> put_status(:internal_server_error)
      |> json(%{status: "error", message: inspect(e)})
  end
end

The Secure Implementation

Stop using rescue blocks to return error strings. The secure approach involves three layers: 1. Use 'debug_errors: false' in production config to prevent Phoenix from rendering its internal debug page. 2. Use the 'ErrorJSON' or 'ErrorHTML' modules to define generic, safe error messages for specific status codes (e.g., 404, 500). 3. Avoid 'inspect' or 'raw' on exception data in controllers. Instead, log the full error to a secure backend like Telemetry or Sentry for developers, while returning a sanitized JSON object to the user.

SECURE CODE
def show(conn, %{"id" => id}) do
  # Use pattern matching instead of rescue for expected failures
  case Repo.get(User, id) do
    nil ->
      conn
      |> put_status(:not_found)
      |> put_view(MyAppWeb.ErrorJSON)
      |> render("404.json")
    user ->
      render(conn, :show, user: user)
  end
end

In config/prod.exs


config :my_app, MyAppWeb.Endpoint,
debug_errors: false,
render_errors: [formats: [json: MyAppWeb.ErrorJSON], layout: false]

System Alert • ID: 8055
Target: Phoenix API
Potential Vulnerability

Your Phoenix API might be exposed to Improper Error Handling

74% of Phoenix apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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