Mass Assignment in Grape with Dynamodb

Mass Assignment in a Grape API that stores data in DynamoDB occurs when a client-supplied JSON payload is mapped directly to a DynamoDB PutItem or UpdateItem request without explicit allowlisting. Because DynamoDB is schemaless at the API layer, a Ruby model or mapper that uses DynamoDB::Document or low-level aws-sdk-dynamodb calls can inadvertently persist attacker-controlled fields, such as admin, role, or permissions, if these keys are present in the params hash.

Grape endpoints often use params declarations for validation, but if the declaration is incomplete or relies on generic hash coercion, a developer might pass the full params hash to DynamoDB conditionally or via a service object. For example, accepting user input for a Profile resource and forwarding attributes like bio and email is expected, but failing to reject is_admin or dynamodb_condition_expression enables privilege escalation or unintended data manipulation.

Because DynamoDB supports nested maps and lists, mass assignment can lead to deeply nested attribute injection that bypasses simple type checks. If the application uses conditional writes with client-supplied ConditionExpression or Expected, an attacker can manipulate attribute existence or versioning to bypass intended constraints. This pattern maps to the BOLA/IDOR and BFLA/Privilege Escalation checks in middleBrick, where unauthenticated or insufficiently scoped tokens allow modification of other users’ resources or elevation of permissions.

Real-world parallels include misconfigured IAM policies that grant dynamodb:PutItem on a broad table, but the vulnerability here is at the API modeling layer. The scanner’s 12 checks, including Property Authorization and Input Validation, detect when attributes outside the intended schema are accepted from unauthenticated or low-privilege contexts. For instance, an endpoint that exposes POST /profiles and forwards all JSON keys to DynamoDB without filtering may be flagged for BOLA if the request lacks proper ownership checks, even when authentication is absent.

In a middleBrick scan, such an endpoint would show a high severity finding under BFLA/Privilege Escalation and Property Authorization, with remediation guidance to implement strict allowlisting and server-side validation. The scan does not fix the issue but provides prioritized guidance to help developers secure the data path between Grape and DynamoDB.

To remediate mass assignment in Grape when working with DynamoDB, explicitly define permitted attributes and avoid passing raw params to DynamoDB operations. Use a dedicated representer or strong parameters method to whitelist fields, and validate nested structures before constructing DynamoDB expressions.

Example: Safe Create Endpoint with Explicit Mapping

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

class ProfileResource < Grape::Entity
  expose :id
  expose :email
  expose :bio
end

class ProfilesAPI & Grape::API
  resource :profiles do
    desc 'Create a profile with safe attribute mapping'
    params do
      requires :email, type: String, desc: 'User email'
      requires :bio, type: String, desc: 'Short biography'
      # Do not accept arbitrary keys
    end
    post do
      client = Aws::DynamoDB::Client.new(region: 'us-east-1')
      table_name = 'Profiles'

      # Explicitly map only allowed fields
      item = {
        id: SecureRandom.uuid,
        email: params[:email],
        bio: params[:bio],
        created_at: Time.now.iso8601
      }

      # Use condition expression to prevent overwrite if needed
      begin
        client.put_item({
          table_name: table_name,
          item: item,
          condition_expression: 'attribute_not_exists(id)'
        })
        present item, with: ProfileResource
      rescue Aws::DynamoDB::Errors::ConditionalCheckFailedException
        error!('Conflict: resource already exists', 409)
      end
    end
  end
end

Example: Update with Allowlisted Fields

class ProfilesUpdateAPI & Grape::API
  resource :profiles do
    desc 'Update only permitted fields'
    params do
      requires :id, type: String
      optional :email, type: String
      optional :bio, type: String
    end
    put ':id' do
      client = Aws::DynamoDB::Client.new(region: 'us-east-1')
      table_name = 'Profiles'
      update_expression_parts = []
      expression_attribute_values = {}

      if params[:email]
        update_expression_parts.push('email = :email')
        expression_attribute_values[':email'] = params[:email]
      end

      if params[:bio]
        update_expression_parts.push('bio = :bio')
        expression_attribute_values[':bio'] = params[:bio]
      endn
      # Ensure at least one attribute to update
      halt 400, { error: 'No fields to update' }.to_json if update_expression_parts.empty?

      update_expr = 'SET ' + update_expression_parts.join(', ')

      begin
        client.update_item({
          table_name: table_name,
          key: { id: { s: params[:id] } },
          update_expression: update_expr,
          expression_attribute_values: expression_attribute_values
        })
        { message: 'Profile updated' }
      rescue Aws::DynamoDB::Errors::ResourceNotFoundException
        error!('Profile not found', 404)
      end
    end
  end
end

General Practices

• Never forward params directly to put_item or update_item without filtering.
• Use strong parameter patterns or representers to define the exact schema accepted from clients.
• Validate nested attributes (e.g., arrays or maps) individually if they are allowed.
• Prefer condition expressions like attribute_not_exists for creation to avoid overwrites.
• Ensure IAM policies align with least privilege, but remember that API-layer validation remains essential even with proper permissions.

These mesures réduisent le risque d’assignation de masse tout en conservant une interaction sécurisée avec DynamoDB via Grape.