Identification Failures in Grape with Dynamodb

Identification failures occur when an API fails to properly identify and enforce access controls on resources. In a Grape API backed by DynamoDB, this typically manifests as insecure direct object references (IDOR) or broken object-level authorization (BOLA). The API may expose DynamoDB primary keys or custom identifiers in URLs or response payloads without verifying that the requesting user has permission to access the corresponding item.

DynamoDB itself does not enforce application-level permissions; it only provides data storage and query capabilities. Authorization must be implemented in the application layer. When Grape endpoints directly use user-supplied identifiers to construct DynamoDB queries, the risk of identification failures increases. For example, an endpoint like GET /users/:user_id/profile might query DynamoDB for an item with a matching user_id without confirming that the authenticated subject is authorized to view that profile.

Common patterns that lead to identification failures include using predictable identifiers (sequential integers or UUIDs without ownership checks), missing ownership validation in query conditions, and returning data when an item does not exist. A missing existence check can leak information through timing differences or error messages. Inadequate handling of composite keys also contributes to these failures; for instance, using only a sort key without validating the partition key relationship enables horizontal privilege escalation across user boundaries.

These vulnerabilities map directly to the OWASP API Top 10 (2023) category Broken Object Level Authorization. Attackers can manipulate identifiers in requests to access other users' data, such as changing a numeric user ID or a path parameter to reference different DynamoDB items. Because DynamoDB queries are constructed in application code, any gap in authorization logic before the query is executed results in an identification failure.

In the context of middleBrick scanning, endpoints using DynamoDB are tested for BOLA/IDOR across 12 security checks run in parallel. The scanner validates whether authorization checks are consistently applied, whether object ownership is verified, and whether error handling avoids information disclosure. Findings include severity ratings and remediation guidance tailored to the Grape and DynamoDB implementation patterns observed during the scan.

Remediation focuses on enforcing ownership checks and avoiding direct exposure of DynamoDB keys. Always resolve the authenticated subject's identity on the server side and use it as a partition key component in queries. Do not rely solely on client-supplied identifiers for authorization.

Example 1: Enforcing ownership with partition key

Ensure that every query includes the authenticated user's ID as part of the key condition. This prevents horizontal privilege escalation across users.

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

class ProfileAPI < Grape::API
  resource :profiles do
    desc 'Get current user profile',
         failure_codes: [401, 403, 404]
    params do
      requires :user_id, type: String, desc: 'Authenticated user UUID'
    end
    get ':user_id' do
      authenticated_user_id = current_user.uuid # resolved server-side
      provided_user_id = params[:user_id]

      # Fail early if the authenticated subject does not match the requested user
      halt 403, { error: 'Forbidden' }.to_json unless authenticated_user_id == provided_user_id

      dynamodb = Aws::DynamoDB::Client.new(region: 'us-east-1')
      response = dynamodb.get_item({
        table_name: 'Users',
        key: {
          'user_id' => { s: provided_user_id }
        }
      })

      item = response.item
      halt 404, { error: 'Not found' }.to_json unless item

      { id: item['user_id'], name: item['name'], email: item['email'] }
    end
  end
end

Example 2: Using composite keys safely

When your table uses a partition key and a sort key, always validate both components against the authenticated subject.

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

class MessagesAPI < Grape::API
  resource :conversations do
    desc 'List messages for a specific conversation',
         failure_codes: [401, 403]
    params do
      requires :user_id, type: String
      requires :conversation_id, type: String
    end
    get ':user_id/:conversation_id/messages' do
      user_id = params[:user_id]
      conversation_id = params[:conversation_id]

      # Verify ownership: the authenticated user must own the conversation
      conversation = dynamodb.get_item({
        table_name: 'Conversations',
        key: {
          'conversation_id' => { s: conversation_id },
          'owner_user_id'   => { s: user_id }
        }
      })

      halt 403, { error: 'Forbidden' }.to_json unless conversation.item

      # Query messages belonging to this conversation
      messages_resp = dynamodb.query({
        table_name: 'Messages',
        key_condition_expression: 'conversation_id = :cid',
        expression_attribute_values: {
          ':cid' => { s: conversation_id }
        }
      })

      { messages: messages_resp.items }
    end
  end
end

General best practices

• Never trust path parameters as the sole authorization mechanism.
• Use server-side resolved identifiers for partition keys in all DynamoDB operations.
• Return generic error messages (e.g., not found vs. forbidden) to avoid information leakage, but log detailed reasons internally.
• Validate that referenced resources belong to the requesting subject before returning data.
• Leverage DynamoDB condition expressions for optimistic concurrency when updating resources to prevent tampering.

middleBrick’s continuous monitoring (Pro plan) can track these patterns across your API surface and flag endpoints where identification controls are missing or inconsistent. The GitHub Action can fail builds when scans detect BOLA/IDOR findings, and the MCP Server allows you to run on-demand checks directly from your AI coding assistant during development.