From Krebs on Security, the Target thieves apparently gained access via credentials from the subcontractor who did heating and a/c work for the retailer. I always wondered how they could've ever gained access to the actual point of sale units in the stores. This is probably not the whole story, but it goes a long way toward explaining it.
Last week, Target told reporters at The Wall Street Journal and Reuters that the initial intrusion into its systems was traced back to network credentials that were stolen from a third party vendor. Sources now tell KrebsOnSecurity that the vendor in question was a refrigeration, heating and air conditioning subcontractor that has worked at a number of locations at Target and other top retailers.
Sources close to the investigation said the attackers first broke into the retailer's network on Nov. 15, 2013 using network credentials stolen from Fazio Mechanical Services, a Sharpsburg, Penn.-based provider of refrigeration and HVAC systems.
Fazio president Ross Fazio confirmed that the U.S. Secret Service visited his company's offices in connection with the Target investigation, but said he was not present when the visit occurred. Fazio Vice President Daniel Mitsch declined to answer questions about the visit.... [more at the link]