Broken Access Control in Sinatra with Dynamodb

Broken Access Control is a Top 10 API risk (OWASP API Top 10 #1) and commonly manifests when authorization checks are missing or bypassed at API endpoints. Using Sinatra with Amazon DynamoDB can unintentionally amplify this risk when application logic relies on client-supplied identifiers (e.g., user_id, resource_id) without verifying that the authenticated subject has permission to access or modify the targeted DynamoDB item.

Consider a Sinatra route that retrieves a user profile by ID from DynamoDB using a path parameter :user_id. If the route uses the parameter directly to form a DynamoDB query—such as a GetItem or Query key condition—without confirming the requesting user is allowed to view that specific user_id, an IDOR (Insecure Direct Object Reference) / BOLA (Broken Level Authorization) vulnerability occurs. For example, an attacker can change /profile/123 to /profile/124 and, if the endpoint only validates authentication but not authorization, access another user’s data.

DynamoDB’s permissions model (IAM policies) is often misapplied in this context. IAM can restrict who can call dynamodb:GetItem or dynamodb:Query, but it typically cannot enforce row-level constraints such as "user can only access items where user_id = :token_user_id." If the Sinatra app does not enforce that constraint in application code, an attacker with valid credentials can still traverse other users’ data. Common insecure patterns include constructing a KeyConditionExpression that includes a client-supplied ID without cross-checking it against the caller’s identity, or using a FilterExpression alone (which does not reduce read capacity cost and does not prevent access if the key is known).

Moreover, DynamoDB’s flexible schema and lack of native referential integrity can encourage designs where related resources (e.g., tenant, organization, or group IDs) are stored as attributes. If Sinatra endpoints expose operations like update or delete using an identifier without validating that the caller belongs to the same tenant or group, BFLA (Business Function Level Authorization) issues arise. For instance, an HR role might be allowed to update any employee record, but a regular user should not; if the Sinatra route does not enforce role-based checks against DynamoDB item attributes, privilege escalation occurs.

Because middleBrick scans test unauthenticated attack surfaces and authenticated scenarios where applicable, it can surface these authorization gaps by detecting missing ownership checks and overly permissive IAM or application logic. The scanner’s BOLA/IDOR and BFLA/Privilege Escalation checks, combined with Property Authorization and Input Validation, highlight cases where endpoints accept user-controlled identifiers that directly map to DynamoDB keys without proper authorization.

To mitigate Broken Access Control when using Sinatra and DynamoDB, enforce authorization at the data-access layer by combining the authenticated subject’s identity with DynamoDB key conditions and item-level checks. Below are concrete, secure patterns.

1. Enforce ownership using the authenticated subject’s identity

Never rely solely on a client-supplied ID. Derive the allowed key from the authenticated user (e.g., from a session or token) and use it as the partition key (or part of the key condition).

require 'aws-sdk-dynamodb'
require 'sinatra'

# Assume current_user is set after authentication, e.g., via session or token payload
def current_user
  @current_user ||= { id: 'u-123', role: 'user' } # simplified example
end

# Secure: use authenticated subject as the partition key
get '/profile' do
  client = Aws::DynamoDB::Client.new(region: 'us-east-1')
  response = client.get_item(
    table_name: 'users',
    key: {
      'user_id' => { s: current_user[:id] }
    }
  )
  item = response.item
  item ? item.to_h : { error: 'not found' }.to_json
end

2. Use KeyConditionExpression with owner partition key

For queries, include the owner identifier as the partition key in the condition. Avoid allowing the client to specify the partition key directly.

get '/messages' do
  client = Aws::DynamoDB::Client.new(region: 'us-east-1')
  owner_id = current_user[:id]
  # client_supplied_sort_key should be validated (e.g., format, range) but not used as the partition key
  response = client.query(
    table_name: 'messages',
    key_condition_expression: 'owner_id = :owner_id AND timestamp >= :start',
    expression_attribute_values: {
      ':owner_id' => { s: owner_id },
      ':start' => { n: params[:start_timestamp] || '0' }
    }
  )
  { messages: response.items }.to_json
end

3. Apply role-based checks for sensitive operations

For endpoints that act on resources with shared access (e.g., admin or cross-team actions), enforce role/attribute checks against DynamoDB item attributes after retrieving the item.

put '/records/:record_id' do
  record_id = params[:record_id]
  client = Aws::DynamoDB::Client.new(region: 'us-east-1')
  # First, fetch the item to inspect attributes
  fetched = client.get_item(
    table_name: 'records',
    key: { 'record_id' => { s: record_id } }
  ).item

  unless fetched
    halt 404, { error: 'not found' }.to_json
  end

  # Enforce ownership or role-based rules
  if current_user[:id] != fetched['owner_id'] && current_user[:role] != 'admin'
    halt 403, { error: 'forbidden' }.to_json
  end

  # Proceed with update using the confirmed key
  client.update_item(
    table_name: 'records',
    key: { 'record_id' => { s: record_id } },
    update_expression: 'set #status = :status',
    expression_attribute_names: { '#status' => 'status' },
    expression_attribute_values: { ':status' => { s: params[:status] } }
  )
  { updated: true }.to_json
end

4. Avoid FilterExpression for authorization

FilterExpression is applied after the query returns results and does not reduce read capacity or prevent access at the key level. Always use KeyConditionExpression with the owner’s partition key instead.

5. Validate and scope identifiers

Ensure that any user-controlled input used in DynamoDB requests is validated (type, length, format) and scoped to the requesting subject. For tenant-aware designs, include the tenant_id as part of the key (partition or sort) and enforce it in every request.

# Example tenant-aware key design
# Partition key: tenant_id#user_id
get '/tenant-items' do
  tenant_prefix = "#{current_user[:tenant_id]}#"
  response = client.query(
    table_name: 'tenant_items',
    key_condition_expression: 'pk = :pk AND begins_with(sk, :sort_prefix)',
    expression_attribute_values: {
      ':pk' => { s: tenant_prefix + current_user[:id] },
      ':sort_prefix' => { s: 'item#' }
    }
  )
  { items: response.items }.to_json
end