Home Depot says that in addition to 56 million payment cards, the attackers responsible for the breach on their POS network earlier this year also compromised 53 million email addresses.
Thursday's breach investigation update also said the attackers leveraged a third-party vendor's username and password in order to access the network.
Home Depot said the vendor's credentials were not enough to access the POS network directly, so the attackers needed to elevate their access. Using a zero-day vulnerability in Windows, they did just that, and maintained their presence on Home Depot's network for five months.
The retailer has stated previously that they believe the malware used in the attack was active on their POS network between April and September of 2014.
In a statement, Aviv Raff, the Co-Founder and CTO of Seculert, told CSO that there are way too many blind spots on a large network to prevent an attack. In Home Depot's case, the fact that the attackers were able to live on the network for five months means "it's more like a blind cavern than a blind spot."
"This is mainly because Home Depot, like other retailers that got breached, were more focused on trying to prevent an attack than trying to detect an active compromise," he added.
While a zero-day flaw in Windows ultimately allowed the foothold needed by the attackers to complete their mission, the issue of supply chain problems and third-party access once again comes to center stage, much like it did in the wake of the Target breach.
"The latest revelation about Home Depot's email leaks once again brings to light the vulnerability of passwords. Everyone in IT knows strong authentication is the answer. So why aren't we rolling it out? There is a general sentiment that implementing strong authentication is difficult, but it's not anymore," commented Incapsula's CEO, Marc Gaffan.
Home Depot has stated that they plan to improve their overall security posture, but they are focusing on the public facing aspects first.
They're recently turned to Voltage Security for help protecting card data, and the plans to implement EMV (Chip & Pin) are on track to be completed before the year's end. As far as enforcing two-factor authentication, or other protective measures in the supply chain, the company hasn't issued any statements on the matter.
Thursday's update also included a reminder that customers who used a payment card at a Home Depot store in 2014, from April on, are eligible to receive identity protection services at no cost.
For those who had their email address compromised, Home Depot said they'd be notified directly, urging them to be on guard against Phishing scams.
"The biggest threat to users who have had their e-mail stolen is the threat of Phishing," said Adam Kujawa, head of Malware Intelligence at Malwarebytes Labs, in a statement.
"Spear Phishing tactics utilizing the knowledge that the e-mail addresses belong to Home Depot customers is a likely outcome, resulting in millions of people potentially receiving fake e-mails claiming to be from Home Depot requesting either the opening of an infected / malicious file or requesting login credentials. Of course these e-mails might just be sold to a spam agency looking for more potential customers to push their advertisements and junk mail onto."