Command Injection in Grape with Dynamodb

Command Injection occurs when untrusted input is concatenated into a system command executed by the application. In a Grape API that interacts with DynamoDB, this typically happens when developers build shell commands or subprocess calls using values derived from request parameters, headers, or payloads. For example, using Ruby’s system, ` (backticks), or Open3 to invoke AWS CLI commands with user-controlled values can lead to arbitrary command execution. DynamoDB itself does not execute shell commands, but the integration layer in Grape may inadvertently construct unsafe commands to query or manage DynamoDB resources.

Consider a Grape endpoint that accepts a table_name and a key to retrieve an item from DynamoDB via the AWS CLI. If the input is not strictly validated or sanitized, an attacker can inject additional shell commands. A vulnerable implementation might look like this:

require 'grape'
require 'json'

class VulnerableAPI < Grape::API
  format :json

  get '/item' do
    table = params['table']
    key   = params['key']
    # Dangerous: user input directly interpolated into shell command
    cmd = "aws dynamodb get-item --table-name #{table} --key #{key}"
    result = `#{cmd}`
    { output: result }
  end
end

An attacker could supply table as mytable; cat /etc/passwd, leading to unintended command execution. Even if the primary goal is DynamoDB access, the command injection vector exists at the OS command construction layer, not within DynamoDB operations themselves. This pattern is especially risky when combined with insufficient input validation or when the runtime environment has broad IAM permissions.

Another scenario involves using environment variables or configuration values that include user input when constructing AWS CLI or SDK wrapper commands. If Grape applications rely on external scripts or system utilities that process untrusted data, the risk persists. The DynamoDB service does not introduce command injection, but the surrounding API design in Grape can expose dangerous execution paths.

To prevent Command Injection when integrating Grape with DynamoDB, avoid constructing shell commands with user input entirely. Instead, use the official AWS SDK for Ruby (aws-sdk-dynamodb) which communicates directly with the service over HTTPS, eliminating shell injection risks. The SDK handles signing requests and does not invoke system commands.

Replace unsafe shell-based approaches with the following SDK-based pattern:

require 'grape'
require 'aws-sdk-dynamodb'

class SecureAPI < Grape::API
  format :json

  helpers do
    def dynamodb_client
      @dynamodb_client ||= Aws::DynamoDB::Client.new(region: 'us-east-1')
    end
  end

  get '/item' do
    table = params['table']
    key   = params['key']

    # Validate inputs strictly: allow only alphanumeric and underscores for table name
    unless table&.match?(\A[a-zA-Z0-9_]+\z)
      error!({ error: 'Invalid table name' }, 400)
    end

    # Key should be a structured hash matching DynamoDB key schema
    # Example expected input: { "id": { "S": "123" } }
    unless key.is_a?(Hash) && key.values.all? { |v| v.is_a?(Hash) && (v.key?('S') || v.key?('N')) }
      error!({ error: 'Invalid key format' }, 400)
    end

    begin
      resp = dynamodb_client.get_item(
        table_name: table,
        key: key
      )
      present resp.item ? { item: resp.item } : { error: 'Item not found' }
    rescue Aws::DynamoDB::Errors::ServiceError => e
      error!({ error: e.message }, 500)
    end
  end
end

This approach validates input types and formats, uses parameterized SDK calls, and avoids any shell interaction. For operations requiring AWS CLI features not covered by the SDK, use structured Ruby methods rather than string concatenation. If dynamic query construction is necessary, rely on the SDK’s expression builder APIs, not string interpolation.

Additionally, enforce least-privilege IAM roles for the application so that even if injection were possible, the scope of actions would be limited. Regularly audit dependencies and ensure that no part of the Grape application invokes system commands with user-controlled arguments.