Broken Access Control in Rails with Dynamodb

Broken Access Control occurs when application logic fails to enforce proper authorization checks, allowing one user to access or modify data belonging to another. In a Ruby on Rails application using Amazon DynamoDB as the persistence layer, this risk is amplified by the mismatch between ActiveRecord-style expectations and DynamoDB’s schema-less, low-level access patterns.

Rails encourages developer convenience with features like find, where, and model-level scopes that implicitly assume a relational model and consistent tenant isolation. When DynamoDB is used directly (e.g., via the AWS SDK or an ODM like aws-record), it becomes easy to construct queries that read or write items without validating tenant or ownership context. For example, a developer might write a controller that uses a user-supplied id to fetch an item and then operate on it, assuming the item belongs to the current user because the route or URL seems safe.

DynamoDB does not enforce row-level security natively; it enforces permissions at the IAM policy level. If an IAM policy is broad (for convenience during development) or if application-level checks are incomplete, an authenticated but malicious user can manipulate IDs to reference other users’ data. This is a classic BOLA (Broken Object Level Authorization) / IDOR pattern. Because DynamoDB queries are explicit key-value lookups, an attacker can iterate through identifiers, looking for accessible items, especially when partition keys are predictable (e.g., composite keys like USER#123 for user data and ORDER#abc for order data).

Additionally, Rails parameter handling and mass assignment protections do not automatically apply to low-level DynamoDB operations. If a developer builds item updates by merging params directly into an update expression without validating ownership, they may unintentionally allow privilege escalation across users or roles. This is especially risky when combined with DynamoDB’s flexible schema, where items may contain sensitive attributes that should not be readable or modifiable by all authenticated users.

In practice, a vulnerable endpoint might look like a standard REST route:

get '/users/:user_id/orders/:id', to: 'orders#show'

If the controller does not re-validate that the order’s partition key includes the current user’s ID, an attacker can change :user_id to another user’s ID and retrieve or modify data. DynamoDB will return the item if the IAM role permits the dynamodb:GetItem action, and Rails will not automatically block the request because the route matched and the developer omitted an explicit ownership check.

To mitigate Broken Access Control when using DynamoDB in Rails, enforce ownership and tenant checks at the data-access layer, never rely on route or parameter assumptions alone. Always include the user identifier in the key condition and validate it before performing any read or write operation.

Below are concrete, safe patterns for DynamoDB in Rails. These examples use the official AWS SDK for Ruby (v3) and assume you manage the current user via a helper like current_user.

Safe read with composite key validation

Ensure that any query includes the user’s identifier as part of the key. Avoid fetching by a client-supplied ID without verifying ownership.

require 'aws-sdk-dynamodb'

class OrderRepository
  def initialize(table_name = 'Orders')
    @client = Aws::DynamoDB::Client.new
    @table = table_name
  end

  def find_by_user_and_order_id(user_id, order_id)
    resp = @client.get_item(
      table_name: @table,
      key: {
        pk: { s: "USER##{user_id}" },
        sk: { s: "ORDER##{order_id}" }
      }
    )
    resp.item ? parse_item(resp.item) : nil
  end

  private

  def parse_item(item)
    {
      id: item['sk']['s'].split('##').last,
      user_id: item['pk']['s'].split('##').last,
      amount: item['amount']['n'],
      status: item['status']['s']
    }
  end
end

Safe update with ownership guard

When updating an item, recompute the keys from the current user context and avoid merging raw input into update expressions.

class OrderUpdater
  def initialize(repo = OrderRepository.new)
    @repo = repo
  end

  def perform(user_id, order_id, update)
    item = @repo.find_by_user_and_order_id(user_id, order_id)
    raise 'Not found or unauthorized' unless item

    update_expr = []
    update_expr << 'set status = :status' if update[:status]
    # Add more guarded updates as needed

    return unless update_expr.any?

    update_expr = update_expr.join(', ')
    @client.update_item(
      table_name: 'Orders',
      key: {
        pk: { s: "USER##{user_id}" },
        sk: { s: "ORDER##{order_id}" }
      },
      update_expression: update_expr,
      expression_attribute_values: {
        ':status' => { s: update[:status] }
      }
    )
  end
end

Policy-driven access with a simple service object

Encapsulate authorization logic in a service object that is called before any DynamoDB operation, ensuring consistent checks across controllers and background jobs.

class OrderPolicy
  def initialize(user, order_id, repo = OrderRepository.new)
    @user = user
    @order_id = order_id
    @repo = repo
  end

  def readable?
    item = @repo.find_by_user_and_order_id(@user.id, @order_id)
    item.present?
  end
end

# In a controller
order_policy = OrderPolicy.new(current_user, params[:id])
if order_policy.readable?
  render json: order_policy.instance_variable_get(:@repo).find_by_user_and_order_id(current_user.id, params[:id])
else
  head :forbidden
end

By coupling DynamoDB key design with explicit ownership checks in Rails code, you reduce the attack surface for Broken Access Control. Avoid global scans or broad IAM permissions for user-specific operations, and prefer narrow queries that include the user identifier in the primary key.