The malicious software that unknown thieves used to steal credit and debit card numbers in the data breach at Home Depot this year was installed mainly on payment systems in the self-checkout lanes at retail stores, according to sources close to the investigation. The finding could mean thieves stole far fewer cards during the almost five-month breach than they might have otherwise.

Since news of the Home Depot breach first broke on Sept. 2, this publication has been in constant contact with multiple financial institutions that are closely monitoring daily alerts from Visa and MasterCard for reports about new batches of accounts that the card associations believe were compromised in the break-in. Many banks have been bracing for a financial hit that is much bigger than the exposure caused by the breach at Target, which lasted only three weeks and exposed 40 million cards.

But so far, banking sources say Visa and MasterCard have been reporting far fewer compromised cards than expected given the length of the Home Depot exposure.

Sources now tell KrebsOnSecurity that in a conference call with financial institutions today, officials at MasterCard shared several updates from the ongoing forensic investigation into the breach at the nationwide home improvement store chain. The card brand reportedly told banks that at this time it is believed that only self-checkout terminals were impacted in the breach, but stressed that the investigation is far from complete.

MasterCard also reportedly relayed that the investigation to date found evidence of compromise at approximately 1,700 of the nearly 2,200 U.S. stores, with another 112 stores in Canada potentially affected.

Officials at MasterCard declined to comment. Home Depot spokeswoman Paula Drake also declined to comment, except to say that, “Our investigation is continuing, and unfortunately we’re not going to comment on other reports right now.”

Tags: Home Depot breach, mastercard, Paula Drake, self-checkout, target data breach, Visa