“This backdoor enables any entity with knowledge of it to compromise all user-defined keys on these cards without prior knowledge, simply by accessing the card for a few minutes.”
The FM11RF08S was the MIFARE Classic clone built to beat every known card-only attack. It shipped with a hardware backdoor, and Teuwen cracked the key. Older cards from several manufacturers shared a second backdoor key. The leading Chinese maker of unlicensed chips sold security as the feature, and the card is gaining market share worldwide.