The FBI is warning about a new online threat that can steal personal and financial information.

E-skimmers are lines of computer code that hackers can insert into online shopping carts on websites. They're akin to the physical skimmers that can be lurking inside ATMs or at gas pumps, ready to steal the financial information of unsuspecting users.

"The software is on the website, it’s on the server," consumer advocate Adam Levin explains. "So when you go to check out, the information is gathered and transmitted to the hacker."

In the past couple of years, e-skimming attacks have hit large companies including Macy's, British Airways and Ticketmaster. And unlike traditional skimmers where consumers are advised to jiggle the card reader and see if it's loose, e-skimmers are virtually undetectable.

Experts say when you shop online, always use a credit card, not a debit card; you’ll get better fraud protection. You should also check your credit card statements for unusual transactions, and it might be a good idea to sign up for transaction alerts by email.