Credit card fraud is widespread in Brazil. Duplication of credit cards has become a major problem, with hundreds of active gangs specialized on the fraud.
Can credit cards be used in Brazil? Yes, they can, taking some basic precautions. The most important thing is to understand that the duplication of the card takes places when the card is out of your sight. Think of the waiter that takes your card away, has a hidden reader, and duplicates the card on the way to and from the cash counter.
My advice is: at the time of paying, you need to be able to see where the card reader is. On a shop or supermarket, more likely than not the reader will be by the till. In cases like this, the risk of duplication is small. Just make sure before signing the value on the credit card receipt matches what you are supposed to be paying. If instead of R$100 the receipt shows R$1,000, and you sign the receipt, there will be little you can do back home to get the money back.
Quite a number of restaurants will bring a swipe machine to the table. That’s ok. If the waiter needs to take away the card, go along with him, that’s a normal practice in Brazil. If you don’t trust the place, pay with cash.
I do not advice using your credit card with manual swippers (knuckle busters).