While the financial sector in Brazil continues to wrestle with “bolware” attacks – malware targeting a popular payment method in the country called “Boleto,” new findings on a fraud ring furthering the schemes have surfaced.

On Wednesday, RSA released a report uncovering the extent of one gang’s exploits. According to the security firm, over a two-year period, a bolware fraud ring may have compromised 495,753 Boletos transactions, which are estimated to be worth up to $3.75 billion.

RSA added that it could not determine how many Boletos were actually paid out by victims, or whether hijacked transactions were successfully carried out in all cases – but that “fraudsters behind this operation may have had the potential to cash out these modified Boletos,” that could account for losses in the billions.

Boletos issued online essentially allow consumers to make electronic payments to merchants, whether for bills, taxes, or a variety of other purposes. Bolware targeting such transactions have been seen in the wild since late 2012, but RSA’s findings revealed the extent of the attacks, as well as other discoveries, like the number of compromised computers and email credentials stolen by the malware.

Boleto attackers target Windows PCs running three browsers: Chrome, Firefox and Internet Explorer. Using a man-in-the-browser (MitB) attack, the malware is able to modify Boleto information so that funds are instead redirected to mule accounts set up by attackers, the report said.

RSA detected that 192,227 computers were compromised by bolware, and that over 83,000 email credentials were hijacked by the malware. In addition, 34 banks were impacted the fraud ring, which has devised 19 bolware variants over time.

On Wednesday, Avivah Litan, vice president and distinguished analyst at research firm Gartner, told SCMagazine.com that bolware uses an IFRAME which hides the fraudulent transaction from bank customers.

“It overrides the transfer and works behind the scenes,” Litan said. “It puts up an iFrame and it hides what it’s doing from the consumer, changing the destination account.”

More noteworthy, however, are the findings that researchers can now share with law enforcement, who’ve been hard pressed to get a handle of the threat, Litan explained.

“We are always talking about Zeus and SpyEye, but this gang really dwarfs those. And Brazil is really ahead of the U.S. and Europe in cyber crime. The Brazilian consumers are victimized far more than any other consumers around the world, and it’s generally the work of Brazilian [cyber] thieves,” Litan said.

In addition to data shedding light on the extent of the bolware botnet and compromised credentials associated with the operation, RSA also turned over a “significant number of fraudulent Boleto ID numbers and indicators of compromise” to the FBI and Brazil’s Federal Police department, the report said.

The security firm is also working with the country’s banking industry to advise financial institutions on mitigation steps, which can aid in blacklisting fraudulent Boleto IDs and thwarting further attacks.