Broken Access Control in Grape with Dynamodb

Broken Access Control in a Grape API backed by DynamoDB typically arises when authorization checks are missing, incomplete, or bypassed before data access. Grape allows you to define resource routes and helpers, but it does not enforce data-level permissions by default. If a developer writes an endpoint that uses a DynamoDB query key (such as a user ID from a token) to fetch items without validating that the requesting user is entitled to that specific item, an authenticated user can manipulate parameters to access other users' records.

Consider an endpoint like /users/:user_id/profile. If the route uses the user_id from the URL to build a DynamoDB KeyConditionExpression (for example, querying a table where the partition key is user_id) but does not verify that the user_id in the path matches the authenticated subject, an attacker can change the ID to enumerate or modify other profiles. Because DynamoDB queries are fast and can return large datasets if filters are weak, missing ownership checks can lead to mass data exposure. This becomes more dangerous when combined with features like DynamoDB Streams or global secondary indexes that may expose additional attributes unintentionally.

Insecure default configurations in DynamoDB can amplify the issue. For example, if a table relies only on partition key queries without sort key scoping, a developer might retrieve an item using a key that should be constrained to the requester’s scope. A missing equality filter on the owner attribute in the query means the API could return items belonging to other users. In a Grape API, this often manifests as helper methods that build request parameters without strict binding between the authenticated identity and the key expression. The result is an authorization bypass that aligns with OWASP API Top 10 A01:2023 — Broken Access Control, and frameworks like middleBrick highlight such findings by correlating runtime behavior with OpenAPI paths and DynamoDB key patterns to detect missing or weak authorization.

Additional risk appears when endpoints accept filters or sorting options that are not validated against the subject. An endpoint that accepts query parameters to refine a DynamoDB scan or query can inadvertently allow privilege escalation if those parameters are not scoped to the requester. For instance, an attacker might supply filter[role]=admin to gain access to administrative records, assuming the backend does not enforce RBAC at the data-access layer. Because DynamoDB does not enforce row-level permissions natively, the API must implement scoping explicitly in the request logic, and any omission is detectable through behavioral analysis, such as testing different identifiers and inspecting returned data sets.

Remediation centers on enforcing ownership and scoping on every DynamoDB request and validating inputs before constructing queries. In Grape, this means tying the authenticated subject to the key expression and rejecting any request where identifiers do not match.

Example: secure profile endpoint with ownership check.

require 'aws-sdk-dynamodb'

class ProfileResource < Grape::API
  resource :profiles do
    helpers do
      def current_user
        # Your auth logic, e.g., from token
        @current_user ||= User.find_by(sub: env['jwt_payload']['sub'])
      end

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

      def profile_params
        declared(params, include_missing: false, type: Hash)
      end
    end

    get ':user_id' do
      user_id = params[:user_id]
      raise Grape::Exceptions::Forbidden unless user_id == current_user.id

      resp = dynamodb.get_item(
        table_name: 'UserProfiles',
        key: {
          'user_id' => { s: user_id },
          'profile_type' => { s: 'public' }
        }
      )
      present resp.item ? resp.item.to_h : { error: 'not_found' }
    end
  end
end

Example: query with explicit partition key and sort key scoping, avoiding filters that bypass ownership.

class DataResource < Grape::API
  resource :data do
    helpers do
      def current_user
        @current_user ||= User.find_by(sub: env['jwt_payload']['sub'])
      end

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

    get '/' do
      user_id = current_user.id
      status_filter = params[:status]

      query_input = {
        table_name: 'UserData',
        key_condition_expression: 'user_id = :uid AND status = :status',
        expression_attribute_values: {
          ':uid' => { s: user_id },
          ':status' => { s: status_filter || 'active' }
        }
      }

      # Optional: add sort key constraints to limit scope further
      # query_input[:limit] = 50

      result = dynamodb.query(query_input)
      present result.items.map(&:to_h)
    end
  end
end

Additional practices: always prefer get_item or query with explicit key expressions rather than scans; validate and whitelist filter/sort parameters; use DynamoDB condition expressions for updates to prevent race conditions; and log authorization failures without exposing sensitive data. Tools like middleBrick can complement these fixes by scanning your OpenAPI spec and runtime behavior to detect missing authorization patterns across endpoints that interact with DynamoDB.