Command Injection in Phoenix with Dynamodb

Command Injection occurs when an attacker can inject and execute arbitrary system commands through an application. In a Phoenix application that interacts with DynamoDB, this risk can arise when user-controlled input is passed to OS commands or external program invocations, often while processing or transforming data retrieved from DynamoDB. Although DynamoDB itself is a managed NoSQL service and does not execute shell commands, the surrounding application logic can introduce command injection vulnerabilities.

For example, a Phoenix endpoint might query DynamoDB for a record containing a filename or path, then pass that value to a shell command for further processing (e.g., calling an external tool or script). If the input is not strictly validated and sanitized, an attacker can supply input such as valid-id; cat /etc/passwd, causing the application to execute unintended commands. This scenario is particularly relevant when Phoenix services use AWS credentials with broad permissions and expose administrative endpoints that interact with DynamoDB and the host environment.

In the context of DynamoDB, command injection typically does not stem from the database layer but from unsafe handling of data retrieved from or sent to DynamoDB combined with system command construction. An attacker might exploit insufficient input validation on parameters used in queries or in constructing shell commands, especially if the application uses functions like System.cmd/3 in Elixir without proper sanitization. Common vulnerable patterns include concatenating user input into command strings or using untrusted data as arguments to external executables.

The risk is compounded when the Phoenix application runs in an environment where AWS credentials are available, as compromised commands could leverage those credentials to call AWS APIs, potentially leading to further lateral movement or data access. While DynamoDB does not directly execute shell commands, the integration point between DynamoDB data and system command construction is where the vulnerability resides.

middleBrick detects such patterns by analyzing the unauthenticated attack surface of your Phoenix endpoints, identifying places where user input may reach system command execution. It flags insecure usage such as missing input validation, unsafe use of dynamic command construction, and overprivileged AWS configurations that could amplify the impact of a successful injection.

To prevent command injection when working with DynamoDB in Phoenix, ensure that all user-controlled data is treated as untrusted and never directly interpolated into system commands. Use parameterized queries for DynamoDB operations and avoid constructing shell commands with external data. Validate and sanitize inputs rigorously, and apply the principle of least privilege to AWS credentials.

Below are concrete code examples demonstrating secure practices in a Phoenix application using the AWS SDK for DynamoDB via the ex_aws_dynamo library.

Secure DynamoDB Query with Input Validation

Always validate and sanitize inputs before using them in DynamoDB queries. Use pattern matching and allowlists to restrict acceptable values.

defmodule MyApp.UserController do
  import Ecto.Query, warn: false
  alias MyApp.Repo
  alias MyApp.User

  def get_user_by_id(user_id) when is_binary(user_id) and byte_size(user_id) <= 36 do
    # Assume user_id is a UUID; reject any unexpected formats
    query = "SELECT * FROM users WHERE user_id = :uid"
    input = %{":uid" => user_id}
    case ExAws.DynamoDB.query("MyTable", query, input) do
      {:ok, result} -> result
      {:error, reason} -> {:error, reason}
    end
  end
end

Avoiding Shell Command Construction with DynamoDB Data

Never pass data from DynamoDB responses directly to system commands. If you must invoke external tools, use strict allowlists and avoid string interpolation.

defmodule MyApp.ReportGenerator do
  @allowed_formats ["pdf", "csv", "json"]

  def generate_report(format) when format in @allowed_formats do
    # Fetch template metadata from DynamoDB
    case ExAws.DynamoDB.get_item("ReportTemplates", %{"id" => {"S", "report-123"}}) do
      {:ok, %{"Item" => item}} ->
        template = item["template"]["S"]
        # Use a safe, internal function to render; do not shell out
        render_template(template, format)
      {:error, reason} ->
        {:error, reason}
    end
  end

  defp render_template(template, format) do
    # Internal rendering logic here
    :ok
  end
end

If external commands are unavoidable, use parameterized arguments and avoid shell expansion:

defmodule MyApp.SafeExecutor do
  def export_data(format) when format in ["csv", "json"] do
    # Direct execution with list arguments to avoid shell injection
    System.cmd("aws", ["dynamodb", "export", "--format", format])
  end
end

Additionally, apply middleware to sanitize and log incoming requests, and configure AWS credentials with minimal permissions to limit the blast radius of any potential injection.