Auth Bypass in Sinatra with Dynamodb

A classic Auth Bypass involving Sinatra and DynamoDB often stems from incomplete authorization checks and overly permissive IAM policies. Sinatra routes that directly proxy user input into a DynamoDB request can mistakenly treat the presence of any AWS credential context as proof of identity, rather than validating the caller against a proper authentication mechanism. For example, an endpoint like /user/:user_id that calls GetItem on a DynamoDB table may use the caller-supplied user_id to construct the key without first confirming that the authenticated principal is that same user. If route-level authentication is missing or misconfigured, an attacker can change the user_id in the request to access another user’s data.

DynamoDB-specific factors amplify the exposure. When the application uses IAM policies that grant broad read access (e.g., dynamodb:GetItem on an ARN pattern like arn:aws:dynamodb:*:*:table/Users/*) and the Sinatra app relies on unauthenticated API Gateway or a misconfigured Cognito identity pool, the unauthenticated attack surface expands. An attacker can send crafted HTTP requests that reach the Sinatra backend, which then issues DynamoDB calls with the permissions attached to the unauthenticated role. This can lead to BOLA (Broken Object Level Authorization) where valid user identifiers are enumerated, and data exposure occurs because the backend does not enforce a subject-to-data mapping check.

Another vector specific to this stack is the misuse of DynamoDB condition expressions or missing existence checks. If the Sinatra handler constructs an UpdateItem or DeleteItem call using user input without verifying ownership, an attacker who can manipulate the request path or headers may trigger actions on other users’ items. Because DynamoDB does not enforce row-level ownership natively, authorization must be enforced in the application layer. Without explicit checks that the authenticated identity matches the item’s owner attribute, the combination of Sinatra’s flexible routing and DynamoDB’s flat key-based model creates a BOLA/IDOR scenario that aligns with one of the OWASP API Top 10 categories.

Real-world attack patterns include unauthenticated LLM endpoint exposure when AI-related routes share the same broad IAM permissions, increasing the risk of unauthorized model interaction. Additionally, if the Sinatra service publishes an OpenAPI spec that does not accurately reflect the need for authentication on sensitive paths, automated scanners and developers may assume the endpoint is safe. Runtime findings from such misconfigurations often map to compliance frameworks like PCI-DSS and SOC2, highlighting the need for precise authentication and authorization controls.

Remediation centers on enforcing strict ownership checks and scoping DynamoDB requests to the authenticated subject. In Sinatra, ensure that every route that interacts with DynamoDB validates the session or token and maps the authenticated user to the item key. Below is a concrete, working example that uses the official AWS SDK for Ruby to fetch a user item only when the authenticated user ID matches the item’s owner.

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

# Configure DynamoDB client securely; credentials should come from the runtime environment
$dynamodb = Aws::DynamoDB::Client.new(region: 'us-east-1')

helpers do
  def current_user_id
    # Example: extract from session or bearer token; adapt to your auth setup
    session[:user_id] || request.env['HTTP_X_USER_ID']
  end

  def fetch_user_item(user_id)
    resp = $dynamodb.get_item(
      table_name: 'Users',
      key: {
        'user_id' => { s: user_id }
      }
    )
    resp.item ? resp.item : nil
  end
end

# Protected route: ensure the authenticated user can only access their own item
get '/user/:user_id' do
  user_id = params[:user_id]
  auth_id = current_user_id

  # Enforce ownership: do not proceed if IDs do not match
  halt 403, { error: 'forbidden' }.to_json unless auth_id == user_id

  item = fetch_user_item(user_id)
  if item
    content_type :json
    item.to_json
  else
    halt 404, { error: 'not_found' }.to_json
  end
end

# Example update with condition expression to enforce ownership and prevent race conditions
put '/user/:user_id' do
  user_id = params[:user_id]
  auth_id = current_user_id
  halt 403, { error: 'forbidden' }.to_json unless auth_id == user_id

  request_body = JSON.parse(request.body.read)
  email = request_body['email']

  begin
    resp = $dynamodb.update_item(
      table_name: 'Users',
      key: {
        'user_id' => { s: user_id }
      },
      update_expression: 'SET email = :val',
      condition_expression: 'attribute_exists(user_id)',
      expression_attribute_values: {
        ':val' => { s: email }
      }
    )
    status 200
    body({ message: 'updated' }.to_json)
  rescue Aws::DynamoDB::Errors::ConditionalCheckFailedException
    halt 409, { error: 'conflict' }.to_json
  end
end