Broken Access Control in Grape with Basic Auth

Broken Access Control occurs when API endpoints do not properly enforce authorization checks, allowing one user to access or modify another user's resources. In Grape, building an API with HTTP Basic Auth can inadvertently create or expose this vulnerability when authentication and authorization are not explicitly separated and validated on every request.

Grape provides a straightforward mechanism to extract and verify Basic Auth credentials. However, if the authenticated identity is resolved once (for example in a before block) and then reused across multiple endpoints without re-checking scope or ownership, the API can allow BOLA/IDOR-style access violations. An authenticated user might be able to manipulate the resource identifier in the request (such as changing an account ID in the URL) and access data that belongs to another user, because the endpoint trusts the identity resolved earlier rather than validating that the resource belongs to the authenticated subject.

Consider an endpoint that retrieves a user's profile using an ID supplied in the URL. If the before block only ensures a user is logged in via Basic Auth but does not confirm that the requested profile ID matches the authenticated user's ID, the API exposes a classic Broken Access Control flaw. The same issue arises in collection endpoints where authorization is inferred from a token or session without verifying that each item in the list is scoped to the authenticated user. This becomes more pronounced when role checks are coarse (e.g., only distinguishing authenticated versus admin) and do not enforce per-resource permissions.

Another subtle risk stems from how Basic Auth credentials are transmitted and cached. If credentials are accepted once and the resulting identity is stored in a global or class-level variable that persists across requests, there is potential for confused deputy–type confusion where one request's identity is mistakenly used for another. Additionally, if the API relies on path parameters or query strings to identify resources without cross-referencing the authenticated subject, the attack surface expands to include enumeration and horizontal privilege escalation.

middleBrick detects these patterns by correlating authentication findings from its Authentication check with BOLA/IDOR and Property Authorization checks. When an endpoint confirms authentication but lacks explicit ownership or scope validation for resources, the scanner highlights the mismatch. The tool also cross-references the OpenAPI specification, resolving $ref chains to ensure that security schemes are consistently applied across paths and that per-operation authorization requirements are not accidentally omitted.

To illustrate a vulnerable pattern, the following Grape snippet shows a before block that sets current_user from Basic Auth but does not enforce ownership checks at the resource level:

class MyEntity < Grape::API
  before do
    auth = Rack::Auth::Basic::Request.new(env)
    @current_user = User.authenticate(auth.credentials) if auth.provided?
    error!('Unauthorized', 401) unless @current_user
  end

  get '/entities/:id' do
    # Vulnerable: does not verify that entity belongs to @current_user
    entity = Entity.find(params[:id])
    present entity
  end
end

In this example, any authenticated user can enumerate entity IDs and access records that do not belong to them, demonstrating Broken Access Control. The fix is to add explicit ownership validation within the endpoint or enforce a scoping rule that ties the resource to the authenticated subject.

Remediation focuses on ensuring that authentication is followed by strict authorization checks that tie each resource request to the authenticated subject. Instead of resolving the user once and reusing a cached identity, validate ownership or scope directly in the endpoint or through a centralized authorization method that references the request parameters.

First, always authenticate on each request and immediately bind the subject to the resource being accessed. The following example demonstrates a secure pattern where the entity is fetched with a scope that includes the authenticated user's ID:

class MyEntity < Grape::API
  helpers do
    def current_user
      @current_user ||= begin
        auth = Rack::Auth::Basic::Request.new(env)
        if auth.provided?
          User.authenticate(auth.credentials)
        else
          nil
        end
      end
    end

    def authorize_entity!(entity)
      error!('Forbidden', 403) unless entity.user_id == current_user.id
    end
  end

  before do
    error!('Unauthorized', 401) unless current_user
  end

  get '/entities/:id' do
    entity = Entity.where(user_id: current_user.id).find(params[:id])
    authorize_entity!(entity)
    present entity
  end
end

This pattern ensures that the database query includes the authenticated user's ID as a filter, so even if an invalid ID is supplied, the record cannot be retrieved unless it belongs to the subject. The authorize_entity! helper adds an additional layer of verification, making the authorization intent explicit and reusable across endpoints.

For endpoints that list collections, apply the same scoping to prevent enumeration of other users' data:

get '/entities' do
  entities = Entity.where(user_id: current_user.id).all
  present entities
end

When using role-based access, keep role checks separate from ownership checks. For example, an admin may bypass ownership constraints, but this exception should be implemented explicitly rather than by relaxing the scoping rule globally:

helpers do
  def admin?
    current_user && current_user.admin
  end

  def authorize_entity!(entity)
    unless admin? || entity.user_id == current_user.id
      error!('Forbidden', 403)
    end
  end
end

middleBrick's CLI can be used to validate that these fixes are reflected in the runtime behavior. Run middlebrick scan <url> to generate a report that maps authentication flows to observed authorization checks. The dashboard and GitHub Action integrations allow you to track these changes over time and fail builds if new endpoints introduce permissive scoping. For larger codebases, the Pro plan's continuous monitoring can alert you when endpoints deviate from expected ownership constraints, helping you maintain consistent access control without manual review of every route.