GitHub Security Advisory: GHSA-7mp4-25j8-hp5q
Package: sylius/sylius (COMPOSER)
Severity: HIGH (CVSS 8.2)
CWE: CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition'), CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition
Vulnerable versions: <= 1.9.11
Patched in: 1.9.12

Summary: Sylius has a Promotion Usage Limit Bypass via Race Condition

Details:
### Impact
A Time-of-Check To Time-of-Use (TOCTOU) race condition was discovered in the promotion usage limit enforcement. The same class of vulnerability affects three independent limits:

1. **Promotion usage limit** - the global `used` counter on `Promotion` entities
2. **Coupon usage limit** - the global `used` counter on `PromotionCoupon` entities
3. **Coupon per-customer usage limit** - the per-customer redemption count on `PromotionCoupon` entities

In all three cases, the eligibility check reads the `used` counter (or order count) from an in-memory Doctrine entity during validation, while the actual usage increment in `OrderPromotionsUsageModifier` happens later during order completion — with no database-level locking or atomic operations between the two phases.

Because Doctrine flushes an absolute value (`SET used = 1`) rather than an atomic increment (`SET used = used + 1`), and because the affected entities lack optimistic locking, concurrent requests all read the same stale usage counts and pass the eligibility checks simultaneously.

An attacker can exploit this by preparing multiple carts with the same limited-use promotion or coupon and firing simultaneous `PATCH /api/v2/shop/orders/{token}/complete` requests. All requests pass the usage limit checks and complete successfully, allowing a single-use promotion or coupon to be redeemed an arbitrary number of times. The per-customer limit can be bypassed in the same way by a single customer completing multiple orders concurrently. No authentication is required to exploit this vulnerability.

This may lead to direct financial loss through unlimited redemption of limited-use promotions and discount coupons.

### Patches
The issue is fixed in versions: 1.9.12, 1.10.16, 1.11.17, 1.12.23, 1.13.15, 1.14.18, 2.0.16, 2.1.12, 2.2.3 and above.

### Workarounds

Decoration of the `OrderPromotionsUsageModifier` service to use atomic operations based on actual database-synchronized values.

The decorated service id in Sylius >=2.0 is `sylius.modifier.promotion.order_usage`, while <2.0 it's `sylius.promotion_usage_modifier`; The following instruction uses the latter, but it needs to be changed depending on the Sylius version.

#### Step 1. Create the decorator service

`src/Modifier/AtomicOrderPromotionsUsageModifier.php`:

```php
<?php

declare(strict_types=1);

namespace App\Modifier;

use Doctrine\DBAL\Connection;
use Doctrine\ORM\OptimisticLockException;
use Sylius\Component\Core\Model\OrderInterface;
use Sylius\Component\Core\Model\PromotionCouponInterface;
use Sylius\Component\Core\Promotion\Modifier\OrderPromotionsUsageModifierInterface;
use Sylius\Component\Promotion\Model\PromotionInterface;
// use Symfony\Component\DependencyInjection\Attribute\AsDecorator;

// #[AsDecorator(decorates: 'sylius.promotion_usage_modifier')]
final class AtomicOrderPromotionsUsageModifier implements OrderPromotionsUsageModifierInterface
{
    /** @var Connection */
    private $connection;

    public function