Last week, federal prosecutors indicted Albert Gonzalez, a man already charged with stealing nearly 100 million credit- and debit-card accounts from retailer TJX, for allegedly working with three other people to hack into another five companies. Gonzalez and his cohorts allegedly stole at least an additional 130 million credit- and debit-card accounts.

In each case, the initial compromise was through the victim’s Web site using a technique, known as SQL injection, that is rarely talked about outside of computer security circles.

The attack takes advantage of website components that allow user input, such as search boxes and login pages. If the Web application does not adequately check the validity of the string of characters, an attacker can enter a specially formatted string that, when processed, will be converted into a database command. Since most Web databases use the structured query language, or SQL, the attack is known as a SQL injection.

“It is a medium-level threat that the rest of the industry has ignored for so long, that the attackers have realized it’s a wide-open field,” says Dan Holden, product manager for IBM’s X-Force vulnerability research team.

Because Web developers are not typically programmers–and most programmers are not adequately taught security practices–online applications are rife with SQL injection flaws.

Big Blue has seen the number of SQL injection attacks double from the first to the second quarter of 2009. In the past few years, vulnerabilities that allow SQL injection to happen have occupied one of the top-three places in the annual list of flaws. Last year, about 20 percent of the 5,600 vulnerabilities entered into the National Vulnerability Database were related to SQL injection.

“Developers are working in high-level programming languages and they just aren’t taught to deal with vulnerabilities,” Holden says. “Bugs and vulnerabilities occur because people make mistakes, and it’s people that program applications.”

Underscoring the danger, security firm ScanSafe announced this week that it had found nearly 100,000 Web pages that had been compromised using a SQL-injection attack to include malicious code.

“It is like it has hit puberty,” Holden says. “SQL injection has started to come into its own.”