A group of hackers used SQL injection techniques to steal huge amounts of data from Heartland Payment Systems Inc., TJX Companies Inc. and other businesses, according to court papers filed in connection with last week's indictment of the group's alleged ringleader.A federal grand jury in New Jersey indicted Albert Gonzalez and two unidentified accomplices on charges related to the theft of 130 million credit and debit card numbers from Heartland, Hannaford Bros. Co., 7-Eleven Inc. and three other retailers not identified by prosecutors.Gonzalez had been indicted earlier by grand juries in Massachusetts and New York on charges related to thefts of data from several other retailers, including TJX, Dave & Buster's Holdings, BJ's Wholesale Club, OfficeMax, Barnes & Noble and The Sports Authority.The U.S. alleges that the criminal group used SQL injection techniques to exploit poorly coded Web application software. Once they gained access to a corporate system, the hackers planted sophisticated packet-sniffing tools and other malware to detect and steal payment card data flowing over the victim companies' networks, according to court documents.SQL injection attacks take advantage of a vulnerability that appears when a Web application fails to properly filter or validate data a user enters on a Web page to order a product or communicate with a company. An attacker can send a malformed SQL query to the underlying database to break into it, plant malicious code or access other systems."We see SQL injection as the top attack technique on the Web," said Michael Petitti, chief marketing officer at Trustwave, a security firm whose clients include Heartland.Launching such attacks "doesn't require much expertise at all," said Chris Wysopal, chief technology officer at Veracode Inc., a provider of security services. "It is at the script-kiddie level to do these kinds of attacks."He added that companies using older versions of Microsoft's SQL Server database are especially vulnerable to SQL injection attacks.This version of this story originally appeared in Computerworld 's print edition.