Impact

The root level commentable field in the API allows access to all commentable resources within the platform, without any permission checks. All Decidim instances are impacted that have not secured the /api endpoint. The /api endpoint is publicly available with the default configuration.

Patches

Not available

Workarounds

To mitigate the issue, you can limit the scope to only authenticated users by limiting access to the /api endpoint. This would require custom code or installing the 3rd party module Decidim::Apiauth.

With custom code, the /api endpoint can be limited to only authenticated users with the following code (needs to run during application initialization):

# Within your application
# config/initializers/limit_api_access.rb

module LimitApiAccess
  extend ActiveSupport::Concern

  included do
    prepend_before_action do |controller|
      unless controller.send(:user_signed_in?)
        render plain: I18n.t("actions.login_before_access",
          scope: "decidim.core"), status: :unauthorized
      end
    end
  end
end

Rails.application.config.to_prepare do
  Decidim::Api::ApplicationController.include(LimitApiAccess)
end

Please note that this would only disable public access to the API and all authenticated users would be still able to exploit the vulnerability. This may be sufficient for some installations, but not for all.

Another workaround is to limit the availability of the /api endpoint to only trusted ranges of IPs that need to access the API. The following Nginx configuration would help limiting the API access to only specific IPs:

location /api {
  allow 192.168.1.100;
  allow 192.168.1.101;
  deny all;
}

The same configuration can be also used without the allow statements to disable all traffic to the the /api endpoint.

When considering a workaround and the seriousness of the vulnerability, please consider the nature of the platform. If the platform is primarily serving public data, this vulnerability is not serious by its nature. If the platform is protecting some resources, e.g. inside private participation spaces, the vulnerability may expose some data to the attacker that is not meant public.

If you have enabled the organization setting "Force users to authenticate before access organization", the scope of this vulnerability is limited to the users who are allowed to log in to the Decidim platform. This setting was introduced in version 0.19.0 and it was applied to the /api endpoint in version 0.22.0.