One of Brazil's most popular payment methods may have been compromised for over two years according to a US security firm.
The heist, which affects Boleto Bancario payments, could have resulted in the theft of up to $3.75 billion (£2.18 billion), making it the largest electronic theft in history. The exact amount stolen has not yet been verified.
Boletos allow an individual to pay an exact amount to a merchant and can be used and generated for both online and offline transactions. Boletos are the second most popular payment method in Brazil and were responsible for approximately 18 per cent of all purchases in 2012.
US security firm RSA described the attack as "a major fraud operation and a serious cybercrime threat to banks, merchants and banking customers in Brazil".
While researchers do not yet know if the fraudsters were successful in collecting money from all their infiltrations, 192,227 PCs have been infected by the scam and an additional 83,506 email credentials have been stolen.
The attack is known colloquially as the man-in-the-browser threat and works by injecting malware into a user's web browser after they have clicked on a seemingly genuine email link. Once the link has been clicked, scammers can intercept and alter Boleto details, without the user's knowledge.
Computer security analyst Graham Cluley said that there were a number of reasons why the attack had been successful on such a large scale.
"Sadly Brazilian computers aren't always necessarily running the very latest anti-virus software, and because Boletos aren't used outside of Brazil it might have made security companies less vigilant about the threat."
In order to avoid becoming a victim of this kind of scam, Mr Cluley advises users to "be cautious about opening unsolicited email attachments or clicking on unknown links, and keep your computer updated with security patches and the latest anti-virus."