Mass Assignment in Grape with Bearer Tokens

Mass Assignment in a Grape API becomes high risk when endpoints accept Bearer Tokens for authentication but do not enforce strong parameter whitelisting. Grape allows developers to declare params that are permitted for mass assignment using params declarations, but when authorization is only enforced via a Bearer Token and resource attributes are mapped directly from user input, attackers can inject unexpected fields to modify sensitive properties.

Consider an endpoint that updates a user profile and relies solely on a Bearer Token for authentication. If the request body includes fields such as role, admin, or permissions and Grape does not explicitly restrict which keys are allowed, an authenticated request can change ownership or privilege attributes. This is a classic BOLA/IDOR + Mass Assignment scenario: the token proves identity, but the server applies no authorization checks on individual attributes, enabling privilege escalation.

In a real-world example, a Grape API might define a resource like Project and use ProjectParams to permit attributes. If the permitted list is incomplete, an attacker can add fields such as is_billed or billing_role to the JSON payload. Because the request includes a valid Bearer Token belonging to a regular user, the authentication check passes, but the mass assignment allows the attacker to escalate to a billing role or manipulate billing state.

OpenAPI specifications can inadvertently worsen the issue if the schema defines many optional properties and the server maps them naively. During an unauthenticated scan, middleBrick tests inputs against the declared spec and runtime behavior, checking whether the Grape endpoint enforces attribute-level authorization. Findings often highlight missing strong parameter controls and missing authorization on sensitive fields, even when Bearer Tokens are required for access.

Because Grape does not automatically protect against over-permissive params, developers must explicitly declare permitted keys and validate ownership of sensitive attributes. Relying only on Bearer Token authentication without tight parameter whitelisting exposes the API to Mass Assignment that can lead to unauthorized data changes or privilege escalation.

To secure Grape endpoints using Bearer Tokens, combine token-based authentication with strict parameter whitelisting and ownership checks. Never rely on authentication alone to prevent Mass Assignment.

First, ensure your authentication logic validates the Bearer Token and loads the current user. Then, explicitly declare permitted parameters for each action and filter incoming attributes accordingly.

# config/initializers/grape.rb (example setup)
class API > Grape::API
  format :json

  helpers do
    def authenticate!
      token = request.headers['Authorization']&.to_s.split(' ').last
      error!('Unauthorized', 401) unless token
      @current_user = User.find_by(api_token: token) # simplified lookup
      error!('Forbidden', 403) unless @current_user
    end

    def permitted_project_params
      # Explicitly whitelist only safe, expected fields
      declared(params, include_missing: false).each do |p|
        p[:allowed_field] = p[:allowed_field] if Project.permitted_attributes.include?(p[:allowed_field])
      end
      params.permit(:name, :description, :status, :owner_id)
    end
  end

  before { authenticate! }

  desc 'Update a project'
  params do
    requires :id, type: Integer
    optional :name, type: String
    optional :description, type: String
    optional :status, type: String
    # Do NOT permit :role, :admin, or sensitive attributes here
  end
  put '/projects/:id' do
    project = Project.find(params[:id])
    error!('Forbidden', 403) unless project.user_id == @current_user.id
    project.update(permitted_project_params)
    { success: true, project: project }
  end
end

In this example, authentication uses a Bearer Token to identify the user, but the endpoint still enforces ownership and only permits explicitly declared attributes. The params.permit list is intentionally narrow, excluding sensitive fields such as role or admin. This pattern prevents Mass Assignment even when a valid token is presented.

For more complex cases, use a dedicated policy or service object to validate attribute-level permissions. The key takeaway is that Bearer Token authentication must be paired with server-side parameter filtering and ownership checks; middleBrick scans can surface missing parameter controls by comparing declared params and runtime behavior, helping you tighten the whitelist and avoid over-posting vulnerabilities.