Auth Bypass in Grape with Dynamodb

Grape is a Ruby REST-like API micro-framework often used to build JSON APIs. When authentication is omitted or misapplied in Grape endpoints, and data is stored in Amazon DynamoDB, an Auth Bypass can occur. This happens when authorization checks are not consistently enforced for each request, allowing an attacker to access or modify resources that should be isolated by user context.

DynamoDB itself does not enforce application-level permissions; it only enforces IAM policies at the AWS account level. If a Grape service uses DynamoDB directly with a shared credential (for example, an IAM key with broad read/write access), any request that bypasses authentication can issue arbitrary queries against the table. This is especially risky when endpoints rely on client-supplied identifiers (such as an id path parameter) to retrieve items without verifying that the item belongs to the requesting user.

In practice, an Auth Bypass manifests when a developer writes a route like /users/:id/profile and performs a GetItem using the :id directly, without confirming that the authenticated subject matches that id. Because the request is unauthenticated (or the authentication step is skipped), the attacker can enumerate or modify other users’ data simply by changing the :id. MiddleBrick’s checks for Authentication, BOLA/IDOR, and Unsafe Consumption are designed to surface these missing controls in Grape services that rely on DynamoDB.

OpenAPI/Swagger analysis helps highlight the risk when paths that include user identifiers do not require security schemes or when security schemes are defined but not applied consistently across operations. When combined with DynamoDB’s schema-less nature, missing validation and authorization can lead to excessive data exposure, allowing attackers to read or overwrite items they should not access.

Remediation focuses on enforcing authentication, scoping queries to the authenticated subject, and validating input before constructing DynamoDB requests. Below are concrete patterns for a Grape API that safely uses DynamoDB.

1. Require authentication and scope queries to the current user

Ensure every protected endpoint validates a token or session and uses the authenticated user’s identity to constrain DynamoDB queries. Use a before block to load the current user and pass their user ID into the query key expression.

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

class MyEntityResource < Grape::API
  format :json

  before do
    # Example: authenticate via an Authorization header
    auth_header = request.env['HTTP_AUTHORIZATION']
    error!('Unauthorized', 401) unless auth_header&.start_with?('Bearer ')
    token = auth_header.split(' ').last
    @current_user = verify_token(token) # implement your own verification
    error!('Forbidden', 403) unless @current_user
  end

  helpers do
    def verify_token(token)
      # Replace with actual token validation logic (e.g., JWT, session lookup)
      { id: 'user-123' } if token == 'valid-token' # simplified example
    end

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

  resource :users do
    desc 'Get profile for the authenticated user'
    get ':id/profile' do
      user_id = params[:id]
      # Enforce scoping: ensure requested id matches authenticated user id
      error!('Forbidden', 403) unless user_id == @current_user[:id]

      resp = dynamodb.get_item({
        table_name: 'Users',
        key: {
          'user_id' => { s: user_id }
        }
      })
      error!('Not found', 404) unless resp.item
      resp.item.transform_keys(&:to_sym)
    end
  end
end

2. Validate and parameterize all input used in DynamoDB expressions

Never concatenate or interpolate user input into query expressions. Use ExpressionAttributeNames and ExpressionAttributeValues to avoid injection and ensure safe queries.

  desc 'Update user settings safely'
  params do
    requires :user_id, type: String, desc: 'User ID from token, not from path'
    requires :email, type: String
  end
  put ':id/settings' do
    # Use the authenticated subject, not the path param
    user_id = @current_user[:id]

    dynamodb.update_item({
      table_name: 'Users',
      key: {
        'user_id' => { s: user_id }
      },
      update_expression: 'SET #email = :email',
      expression_attribute_names: {
        '#email' => 'email'
      },
      expression_attribute_values: {
        ':email' => { s: params[:email] }
      },
      return_values: 'UPDATED_NEW'
    })
    { message: 'Settings updated' }
  end

3. Enforce IAM policies least-privilege for the application role

Ensure the AWS credentials used by the Grape service restrict DynamoDB actions to the minimum required. For example, allow dynamodb:GetItem and dynamodb:UpdateItem only on table resources scoped by a user prefix (using condition keys such as dynamodb:LeadingKeys). This prevents a compromised credential from affecting other users’ data even if application logic is bypassed.

4. Use MiddleBrick to validate your controls

Run MiddleBrick scans against your Grape endpoints to confirm that authentication is required, that user-supplied identifiers are properly scoped, and that no unsafe consumption patterns appear. The scanner cross-references your OpenAPI spec with runtime checks and surfaces findings tied to the OWASP API Top 10 and relevant compliance mappings.