Card testing attacks, also known as carding attacks, are a significant concern for online payment handlers as they involve fraudsters using bots to validate stolen or generated credit card numbers through small, fake authorizations. These attacks can lead to increased processing costs, higher chargeback rates, and penalties from payment processors while compromising customer trust. Attackers use sophisticated methods, including leveraging botnets, headless browsers, and automation frameworks to execute thousands of attempts per minute, making detection challenging. Effective defense requires implementing layered security measures such as rate limiting, device intelligence, and behavioral analytics to identify suspicious activities without affecting legitimate users. Real-time risk scoring and ongoing collaboration with payment processors are recommended to keep defenses up-to-date and effective against evolving threats. Integrating solutions like Fingerprint can help businesses detect and prevent these attacks by using device intelligence and smart payment controls without disrupting genuine customer transactions.