Insecure Design in Grape with Dynamodb

Insecure Design in a Grape API backed by DynamoDB often arises from how authorization and data access are modeled before any code is written. When resources are modeled only as DynamoDB items without explicit ownership fields and access control decisions pushed into the application layer, endpoints can inadvertently expose one user’s data to another. This typically manifests as Insecure Direct Object References (IDOR) or Broken Object Level Authorization (BOLA), where a predictable identifier (e.g., a numeric primary key) allows an authenticated user to iterate or modify items they should not access.

Consider a Grape endpoint that retrieves an order by an order_id path parameter and queries DynamoDB with a simple GetItem using the provided ID. If the request is authenticated only at the API gateway or via a token that identifies the user, but the query does not enforce that the item’s user_id matches the authenticated subject, an attacker can substitute any known ID and read or act on other users’ orders. This is a classic IDOR: the object reference (the ID) is not coupled with authorization checks, and the design assumes the client will only use references it is allowed to access. The vulnerability is baked into the API design when developers do not enforce ownership or scope constraints in the query itself.

DynamoDB’s schema flexibility can exacerbate insecure design when access patterns are not aligned with authorization requirements. For example, storing tenant identifiers as an attribute is necessary for multi-tenant isolation, but if the design omits this attribute in key condition expressions or does not enforce it programmatically, a malicious actor who knows or guesses a partition key might cross tenant boundaries. Similarly, secondary indexes that expose additional query paths must also incorporate scope attributes; otherwise, an index can become an unintended access channel. The insecure pattern is designing the data model and endpoints without embedding authorization into each access pattern, leading to runtime checks that are easy to miss or forget.

Another insecure design pattern is over-permissive write paths. A Grape endpoint that accepts a full item representation and uses PutItem or UpdateItem without filtering or validating attributes can allow a user to modify reserved fields such as admin flags, payment statuses, or version counters. Because DynamoDB does not enforce field-level permissions at the database layer, the onus is on the API design to strip or reject unauthorized fields. If the design relies on client-supplied data without server-side allowlists, attackers can escalate privileges or tamper with business logic. Effective design instead models commands explicitly, validates each attribute against the user’s role, and ensures conditional expressions prevent overwriting critical attributes.

To detect these issues, middleBrick scans the unauthenticated and authenticated attack surface of Grape APIs that interact with DynamoDB. It correlates endpoint definitions, parameter usage, and DynamoDB access patterns with authorization logic to highlight missing ownership checks, underspecified key schemas, and missing attribute-level constraints. The scanner maps findings to the OWASP API Top 10 and provides prioritized remediation guidance, helping developers align their API and data model design with secure access patterns before deployment.

Remediation centers on embedding authorization into every DynamoDB access pattern and validating inputs at the API boundary. In Grape, this means scoping queries by the authenticated user and using conditional expressions to enforce ownership and constraints. Below are concrete, realistic code examples that demonstrate secure design for common scenarios.

Secure GET with ownership scope

Ensure every read includes a partition key attribute that ties the item to the requesting user. Use a composite key like PK = USER#<user_id> and SK = ORDER#<order_id>, and require the endpoint to derive the user from the token rather than trusting a client-supplied user identifier.

require 'aws-sdk-dynamodb'

class OrderResource < Grape::Entity
  expose :id, :product, :quantity
end

class OrdersEndpoint < Grape::API
  resource :users do
    desc 'Get a specific order for the authenticated user'
    params do
      requires :user_id, type: String, desc: 'User identifier from auth'
      requires :order_id, type: String, desc: 'Order identifier'
    end
    get ':user_id/orders/:order_id' do
      user_id = params[:user_id]
      order_id = params[:order_id]

      client = Aws::DynamoDB::Client.new(region: 'us-east-1')
      response = client.get_item(
        table_name: 'orders',
        key: {
          pk: { s: "USER##{user_id}" },
          sk: { s: "ORDER##{order_id}" }
        }
      )
      item = response.item
      if item
        OrderResource.represent(item)
      else
        error!('Not found', 404)
      end
    end
  end
end

Write with attribute filtering and conditional expression

When creating or updating items, use explicit parameter whitelisting and conditional expressions to prevent privilege escalation via reserved attributes. For example, never allow a client to set admin or balance through the request body.

require 'aws-sdk-dynamodb'

class CreateOrderEndpoint < Grape::API
  resource :orders do
    desc 'Create an order for the authenticated user'
    params do
      requires :user_id, type: String
      requires :product, type: String
      requires :quantity, type: Integer
    end
    post do
      user_id = params[:user_id]
      product = params[:product]
      quantity = params[:quantity]

      client = Aws::DynamoDB::Client.new(region: 'us-east-1')
      # Use a condition to ensure the item does not already exist unexpectedly
      begin
        client.put_item(
          table_name: 'orders',
          item: {
            pk: { s: "USER##{user_id}" },
            sk: { s: "ORDER##{SecureRandom.uuid}" },
            product: { s: product },
            quantity: { n: quantity.to_s },
            created_at: { n: Time.now.to_i.to_s }
          },
          condition_expression: 'attribute_not_exists(pk) AND attribute_not_exists(sk)'
        )
        status 201
      rescue Aws::DynamoDB::Errors::ConditionalCheckFailedException
        error!('Conflict', 409)
      end
    end
  end
end

Query with tenant or user scope on secondary indexes

If using Global Secondary Indexes (GSI), include the scope attribute in both the index design and the query. This prevents an attacker from leveraging the index to enumerate items across partitions.

client = Aws::DynamoDB::Client.new(region: 'us-east-1')
# Query a GSI that includes user_id as partition key
response = client.query(
  table_name: 'orders',
  index_name: 'user_created_at-index',
  key_condition_expression: 'user_id = :uid AND created_at BETWEEN :start AND :end',
  expression_attribute_values: {
    ':uid' => { s: "USER##{current_user.id}" },
    ':start' => { n: (Time.now - 30*24*60*60).to_i.to_s },
    ':end' => { n: Time.now.to_i.to_s }
  }
)

Batch operations and validation

When using BatchGetItem or TransactGetItems, validate that all requested IDs belong to the same user or tenant. Do not simply forward arrays of IDs from the client without scoping.

ids_to_fetch = params[:ids].select { |id| valid_order_id_for_user?(id, current_user.id) }
request_items = {
  'orders' => {
    keys: ids_to_fetch.map { |id| { pk: { s: "USER##{current_user.id}" }, sk: { s: "ORDER##{id}" } } },
    attributes_to_project: %w[id product quantity]
  }
}
response = client.transact_get_items(request_items)

By designing DynamoDB access patterns with ownership scoping, input validation, and conditional expressions, Grape APIs reduce the risk of IDOR and privilege escalation. middleBrick’s scans can verify that these controls are present in runtime behavior and OpenAPI specifications, providing findings tied to the specific checks for authentication, BOLA/IDOR, and Privilege Escalation.