Pii Leakage in Grape with Dynamodb

Grape is a Ruby API micro-framework commonly used to build RESTful endpoints, and AWS DynamoDB is a widely adopted NoSQL data store. When a Grape-based service exposes DynamoDB data without first assessing what is returned, PII can be unintentionally surfaced. A typical vulnerability pattern occurs when an endpoint performs a GetItem or Query on DynamoDB and returns the full item to the client. If the item contains fields such as email, ssn, phone, or internal metadata like created_by_ip, and those fields are not explicitly filtered, the API leaks PII.

DynamoDB’s schema-less design exacerbates this risk: developers may store nested structures (e.g., a map containing address details) and inadvertently expose the entire nested object. In a Grape endpoint, returning the raw DynamoDB Item map means any attribute—PII or not—is included unless the code prunes it. The interaction between Grape’s flexible serialization and DynamoDB’s permissive attribute set means sensitive data can be exposed even when the query intent appears limited.

Another scenario involves using DynamoDB Streams or exported backups where PII is retained. If a Grape API relies on a DynamoDB table that contains PII and does not enforce field-level redaction before serialization, any consumer of the endpoint (including frontends or third-party integrations) can receive sensitive information. Additionally, insufficient authorization checks combined with broad DynamoDB scan operations can return many records, each containing PII, multiplying the exposure impact.

The risk is typically identified in a middleBrick scan as PII appearing in responses where it should not exist. For example, an endpoint intended to return a user profile might include fields like password_hash or payment_card_last4 because the code did not limit which attributes are returned. Because DynamoDB does not enforce a schema, there is no compile-time guarantee that a response shape is safe; the onus is on the API code to filter and transform data before it reaches the Grape response body.

To prevent PII leakage, treat DynamoDB items as raw data and explicitly project only required attributes. Below are concrete, working examples for a Grape API that safely interacts with DynamoDB using the AWS SDK for Ruby (v3).

Example 1: Selective attribute retrieval on GetItem

Instead of returning the entire DynamoDB item, specify projection attributes and filter out PII before serialization.

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

class UserResource < Grape::API
  resource :users do
    desc 'Get a user profile without PII'
    params do
      requires :user_id, type: String, desc: 'User identifier'
    end
    get ':user_id' do
      client = Aws::DynamoDB::Client.new(region: 'us-east-1')
      resp = client.get_item(
        table_name: 'users',
        key: { user_id: { s: params[:user_id] } },
        projection_expression: 'user_id,display_name,email,created_at' # explicit whitelist
      )
      item = resp.item
      # Further redaction: remove PII like email if not intended for this consumer
      item.except('email', 'phone', 'ssn', 'password_hash').transform_values { |v| v.is_a?(Hash) ? v[:s] : v }
    end
  end
end

Example 2: Query with attribute projection and nested map handling

When querying, limit returned attributes and recursively strip sensitive nested fields.

class SearchResource < Grape::API
  resource :search do
    desc 'Search users with safe attribute selection'
    params do
      requires :query, type: String, desc: 'Search term'
    end
    get do
      client = Aws::DynamoDB::Client.new(region: 'us-east-1')
      resp = client.query(
        table_name: 'users',
        index_name: 'email-index',
        key_condition_expression: 'email = :val',
        expression_attribute_values: { ':val' => { s: params[:query] } },
        projection_expression: 'user_id,display_name,address' # address may be a nested map
      )
      safe_data = resp.items.map do |item|
        # Remove or mask nested PII inside address
        address = item.dig('address', 'street') || ''
        { user_id: item['user_id'], display_name: item['display_name'], address: address }
      end
      safe_data
    end
  end
end

Example 3: BatchGetItem with consistent filtering

When retrieving multiple items, apply the same filtering to each response item.

class MultiResource < Grape::API
  resource :multi do
    desc 'Retrieve multiple user records safely'
    params do
      requires 'user_ids', type: Array, desc: 'List of user IDs'
    end
    post do
      client = Aws::DynamoDB::Client.new(region: 'us-east-1')
      ids = params[:user_ids].first(100) # limit batch size
      resp = client.batch_get_item(
        request_items: {
          'users' => {
            keys: ids.map { |id| { user_id: { s: id } } },
            projection_expression: 'user_id,display_name,department'
          }
        }
      )
      items = resp.responses['users'] || []
      items.map { |item| item.except('password_hash', 'ssn', 'payment_details') }
    end
  end
end

General remediation practices

• Use ProjectionExpression to limit DynamoDB returned attributes to only those required by the endpoint.
• Avoid Scan in production APIs; prefer indexed queries with explicit key conditions.
• Apply a allow-list filter on the Ruby object before serialization (e.g., item.except('ssn', 'password_hash')).
• Treat nested maps as potentially containing PII; access only the scalar fields you need.
• Use environment-specific table names or attribute-level encryption for highly sensitive fields, acknowledging that client-side handling remains necessary.

middleBrick can validate these patterns by scanning your endpoints and confirming that responses do not contain unexpected PII, ensuring your DynamoDB-backed Grape API adheres to data minimization principles.