Think of it as a mansion with a high-tech security system — but the front door wasn’t locked tight.
Using the Citigroup customer Web site as a gateway to bypass traditional safeguards and impersonate actual credit card holders, a team of sophisticated thieves cracked into the bank’s vast reservoir of personal financial data, until they were detected in a routine check in early May.
That allowed them to capture the names, account numbers, e-mail addresses and transaction histories of more than 200,000 Citi customers, security experts said, revealing for the first time details of one of the most brazen bank hacking attacks in recent years.
The case illustrates the threat posed by the rising demand for private financial information from the world of foreign hackers.
In the Citi breach, the data thieves were able to penetrate the bank’s defenses by first logging on to the site reserved for its credit card customers.
Once inside, they leapfrogged between the accounts of different Citi customers by inserting various account numbers into a string of text located in the browser’s address bar. The hackers’ code systems automatically repeated this exercise tens of thousands of times — allowing them to capture the confidential private data.
The method is seemingly simple, but the fact that the thieves knew to focus on this particular vulnerability marks the Citigroup attack as especially ingenious, security experts said.
One security expert familiar with the investigation wondered how the hackers could have known to breach security by focusing on the vulnerability in the browser. “It would have been hard to prepare for this type of vulnerability,” he said. The security expert insisted on anonymity because the inquiry was at an early stage.
The financial damage to Citigroup and its customers is not yet clear. Sean Kevelighan, a bank spokesman, declined to comment on the details of the breach, citing the ongoing criminal investigation. In a statement, he said that Citigroup discovered the breach in early May and the problem was “rectified immediately.” He added that the bank had initiated internal fraud alerts and stepped up its account monitoring.
The expertise behind the attack, according to law enforcement officials and security experts, is a sign of what is likely to be a wave of more and more sophisticated breaches by high-tech thieves hungry for credit card numbers and other confidential information.
That is because demand for the data is on the rise. In 2008, the underground market for the data was flooded with more than 360 million stolen personal records, most of them credit and debit files. That compared with 3.8 million records stolen in 2010, according to a report by Verizon and the Secret Service, which investigates credit card fraud along with other law enforcement agencies like the Federal Bureau of Investigation.
Now, as credit cards that were compromised in the vast 2008 thefts expire, thieves are stepping up efforts to find new accounts.
As a result, prices for basic credit card information could rise to several dollars from their current level of only pennies.
“If you think financially motivated breaches are huge now, just wait another year,” said Bryan Sartin, who conducts forensic investigations for Verizon’s consulting arm.
The kind of information the thieves are able to glean is shared in online forums that are a veritable marketplace for criminals. Networks that three years ago numbered several thousands users have expanded to include tens of thousands of hackers.
“These are online bazaars,” said Pablo Martinez, deputy special agent in charge of the Secret Service’s criminal investigation division. “They are growing exponentially and we have seen the entire process become more professional.”
For example, some hackers specialize in prying out customer names, account numbers and other confidential information, Mr. Martinez said. Brokers then sell that information in the Internet bazaars. Criminals use it to impersonate customers and buy merchandise. Finally, “money mules” wire home the profits through outlets like Western Union or MoneyGram.
“It’s like ‘Mission Impossible’ when they select the teams,” said Mark Rasch, a former prosecutor who is now with CSC, an information technology services firm. “And they don’t know each other, except by hacker handle and reputation.”
In the Citi attack, the hackers did not obtain expiration dates or the three-digit security code on the back of the card, which will make it harder for thieves to use the information to commit fraud.
Not every breach results in a crime. But identify theft has ranked first among complaints to the Federal Trade Commission for 11 consecutive years, with 1.34 million in 2010, twice as many as the next category, which is debt collection.
Many of these attacks have their origins in Eastern Europe, including Russia, Belarus, Ukraine and Romania. In fact, the security expert familiar with the Citi breach said it originated in the region, though he would not specify the country.
In Russia, Xakep.ru, is one of the larger forums for Eastern European hackers today, with nearly 13,300 registered members, according to Cyveillance. HackZone.ru is larger, and has more than 58,000 members. In addition, attacks by Romanian hackers have grown noticeably more advanced recently, according to security experts.
On HackZone, one seller who called himself “zoloto” promised “all cards valid 100%” and that they would be sold only one time.
Underscoring the multinational nature of these rings, American law-enforcement agencies have also been putting more investigators overseas.
“The only way to address a global issue is to address it globally with your partners,” said Gordon M. Snow, assistant director of the F.B.I.’s Cyber Division.
The Secret Service established a presence in Tallinn, Estonia, last month, and has embedded agents with Ukrainian authorities since the beginning of the year. The F.B.I. has embedded agents in the Netherlands, Estonia, Ukraine and Romania, and works closely with its counterparts in Australia, Germany and Britain.
But even officials at these agencies acknowledge that as fast as they move, the hackers’ strategies are evolving at Silicon Valley speed.
“With every takedown, they regroup,” said J. Keith Mularski, a supervisory special agent with the F.B.I.
Riva Richmond contributed reporting.