Four Romanian nationals have been charged with hacking card-processing systems at more than 150 Subway restaurants and 50 other unnamed retailers, according to an indictment unsealed Thursday.

The hackers compromised the credit-card data of more than 80,000 customers and used the data to make millions of dollars of unauthorized purchases, according to the indictment (.pdf).

From 2008 until May 2011, the hackers allegedly hacked into more than 200 point-of-sale (POS) systems in order to install a keystroke logger and other sniffing software that would steal customer credit, debit and gift-card...