Recent hacking attacks on Sony Corp. and Lockheed Martin Corp. grabbed headlines. What happened at City Newsstand Inc. last year did not.

Unbeknownst to owner Joe Angelastri, cyber thieves planted a software program on the cash registers at his two Chicago-area magazine shops that sent customer credit-card numbers to Russia. MasterCard Inc. demanded an investigation, at Mr. Angelastri's expense, and the whole ordeal left him out about $22,000.

His experience highlights a growing threat to small businesses. Hackers are expanding their sights beyond multinationals to include any business that stores data in electronic form. Small companies, which are making the leap to computerized systems and digital records, have now become hackers' main target.

"Who would want to break into us?" asked Mr. Angelastri, who says the breach cut his annual profit in half. "We're not running a bank."

With limited budgets and few or no technical experts on staff, small businesses generally have weak security. Cyber criminals have taken notice. In 2010, the U.S. Secret Service and Verizon Communications Inc.'s forensic analysis unit, which investigates attacks, responded to a combined 761 data breaches, up from 141 in 2009. Of those, 482, or 63%, were at companies with 100 employees or fewer. Visa Inc. estimates about 95% of the credit-card data breaches it discovers are on its smallest business customers.