CVE-2026-31824: Sylius Race Condition Vulnerability

CVE-2026-31824 Overview

CVE-2026-31824 is a Time-of-Check To Time-of-Use (TOCTOU) race condition vulnerability discovered in Sylius, an Open Source eCommerce Framework built on Symfony. This vulnerability affects the promotion usage limit enforcement mechanism, allowing attackers to bypass coupon and promotion usage restrictions through concurrent request exploitation.

The vulnerability impacts three distinct areas of the Sylius platform: the global promotion usage limit (the used counter on Promotion entities), the coupon usage limit (the global used counter on PromotionCoupon entities), and the per-customer coupon usage limit (the per-customer redemption count on PromotionCoupon entities). In all cases, the eligibility validation reads usage counters from in-memory Doctrine entities while the actual usage increment occurs later during order completion—with no database-level locking or atomic operations protecting the gap between these two phases.

Critical Impact

Attackers can exploit this race condition to redeem single-use promotions and coupons an arbitrary number of times, leading to direct financial loss through unlimited fraudulent discounts. No authentication is required to exploit this vulnerability.

Affected Products

• Sylius versions prior to 1.9.12
• Sylius versions 1.10.x prior to 1.10.16
• Sylius versions 1.11.x prior to 1.11.17
• Sylius versions 1.12.x prior to 1.12.23
• Sylius versions 1.13.x prior to 1.13.15
• Sylius versions 1.14.x prior to 1.14.18
• Sylius versions 2.0.x prior to 2.0.16
• Sylius versions 2.1.x prior to 2.1.12
• Sylius versions 2.2.x prior to 2.2.3

Discovery Timeline

• 2026-03-10 - CVE-2026-31824 published to NVD
• 2026-03-11 - Last updated in NVD database

Technical Details for CVE-2026-31824

Vulnerability Analysis

This vulnerability represents a classic TOCTOU (Time-of-Check To Time-of-Use) race condition, classified under CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) and CWE-367 (Time-of-check Time-of-use Race Condition).

The core issue stems from the temporal gap between when Sylius validates whether a promotion or coupon can be used and when it actually records that usage. During the eligibility check phase, the system reads the current usage count from an in-memory Doctrine entity. However, the actual increment of this counter happens later in the OrderPromotionsUsageModifier during order completion.

The vulnerability is exacerbated by two implementation decisions in the codebase. First, Doctrine flushes an absolute value (SET used = 1) rather than performing an atomic increment (SET used = used + 1). Second, the affected entities lack optimistic locking mechanisms that would detect concurrent modifications. This combination allows multiple concurrent requests to all read the same stale usage count, pass eligibility validation simultaneously, and complete their orders successfully.

Root Cause

The root cause lies in the absence of database-level synchronization primitives between the validation and modification phases of promotion usage tracking. When multiple requests arrive simultaneously, each request independently validates against the same in-memory state of the Doctrine entities. Since no pessimistic or optimistic locking strategy is employed, and the database writes use absolute values rather than atomic increments, the system cannot detect or prevent concurrent over-redemption.

The lack of transactional isolation at the appropriate level means that the check (reading the used counter) and the use (incrementing and persisting the counter) operate as separate, non-atomic operations vulnerable to interleaving by concurrent processes.

Attack Vector

An attacker can exploit this vulnerability by preparing multiple shopping carts, each configured to use the same limited-use promotion or coupon code. By firing simultaneous PATCH /api/v2/shop/orders/{token}/complete requests across these carts, all requests pass through the usage limit validation before any of them record their usage. This allows a single-use promotion or coupon to be redeemed multiple times—potentially unlimited times depending on how many concurrent requests the attacker can orchestrate.

The per-customer usage limit can be bypassed using the same technique: a single customer prepares multiple orders with the same coupon and completes them concurrently, exceeding their allotted redemption count.

For detailed technical information, refer to the Sylius Security Advisory.

Detection Methods for CVE-2026-31824

Indicators of Compromise

• Multiple orders completed within milliseconds of each other using the same promotion or coupon code
• Promotion or coupon used counters exceeding their configured limits
• Order audit logs showing simultaneous completion timestamps for orders with identical coupon codes
• Customer accounts with redemption counts exceeding per-customer coupon limits

Detection Strategies

• Implement database-level monitoring for promotion and coupon tables to detect usage counter anomalies
• Analyze web application logs for burst patterns of /api/v2/shop/orders/{token}/complete requests with the same coupon codes
• Set up alerts for promotions or coupons where recorded redemptions exceed the configured usageLimit value
• Monitor for automated or scripted traffic patterns targeting the order completion endpoint

Monitoring Recommendations

• Enable detailed request logging for the Sylius order completion API endpoints
• Implement rate limiting on the order completion endpoint to reduce the window for race condition exploitation
• Deploy application-level metrics tracking promotion and coupon usage patterns in real-time
• Review historical order data for signs of past exploitation, particularly orders with identical timestamps and shared coupons

How to Mitigate CVE-2026-31824

Immediate Actions Required

• Upgrade Sylius to a patched version immediately: 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 or later
• Audit recent order history for signs of promotion abuse, particularly orders with identical completion timestamps using limited-use coupons
• Review financial impact by identifying orders that may have exploited this vulnerability
• Consider temporarily disabling limited-use promotions and coupons until patching is complete

Patch Information

Sylius has released security patches addressing this TOCTOU race condition across all supported version branches. The fix implements proper database-level locking and atomic increment operations to prevent concurrent requests from bypassing usage limits.

Patched versions include: 1.9.12, 1.10.16, 1.11.17, 1.12.23, 1.13.15, 1.14.18, 2.0.16, 2.1.12, and 2.2.3. Organizations should upgrade to the appropriate patched version for their deployment. For detailed patch information, see the Sylius Security Advisory.

Workarounds

• Implement external rate limiting on the order completion API endpoint to reduce concurrent request throughput
• Add application-layer mutex locks around the order completion process for orders using limited-use coupons
• Temporarily increase coupon usage limits to absorb potential abuse while preparing for upgrade
• Implement server-side request queuing for order completion operations to serialize processing

# Example: Nginx rate limiting configuration for Sylius order completion endpoint
# Add to your nginx server block to limit concurrent requests

limit_req_zone $binary_remote_addr zone=order_complete:10m rate=2r/s;

location ~ ^/api/v2/shop/orders/.*/complete$ {
    limit_req zone=order_complete burst=5 nodelay;
    # ... existing proxy/fastcgi configuration
}