By Ian Bush

By KYW tech editor Ian Bush

PHILADELPHIA (CBS) — Target is investigating a breach that may have exposed millions of customers’ credit and debit card details.

CBS 3 has learned the Secret Service is investigating.

The hack may have happened at the height of the holiday shopping season.

The report comes from respected security blogger Brian Krebs at (See Related Story).

He says sources at two top card issuers reveal the breach started on or around Black Friday and could have extended to this past Sunday. Stolen, the sources say, were data on the cards’ magnetic stripes — typically your name, account number, expiration date, and security code.

Krebs reports the criminals may have grabbed card details from Target customers at locations across the country, though appears to be unaffected.

Krebs quotes one source as saying this “will put its mark up there with some of the largest retail breaches to date.”

“I almost never use cash anymore. I almost use a credit card all the time, so that scares me,” shopper Alicia Travis said.

‘All the time. I only use my debit card. It makes me very nervous,” shopper Robyn Nichols said.

“The fact that you know it happened when you were there makes you a little more skeptical, but it’s not going to change anything that I do day to day,” shopper Doug Allen said.

Target officials said customer who suspect unauthorized activity should contact Target at: 866-852-8680.