OpenMage magento-lts before 20.19.0 and 21.0.0-beta1 before 21.0.0-beta3 contains a race condition in shopping cart price rule usage limits, because Mage_SalesRule_Model_Validator checks coupon limits without locking and the usage counters are later incremented non-atomically. Unauthenticated attackers, including guest shoppers, can submit concurrent checkouts to redeem a limited coupon more times than its cap allows.