There’s a scam out there where somebody calls pretending to be from Visa or Mastercard Security, investigating a possible misuse of your credit card. They sound very legit, even saying you are free to call Visa or Mastercard’s security department to check on it. In a series of questions they win your confidence and get you to reveal your credit card’s security number.
If someone calls you claiming to be from your credit card’s security department, don’t give them any info right away. Instead call them back, starting with the phone number on the back of your credit card.