The 46,000 people reportedly infected by ads on job sites may be only a fraction of the victims of an ambitious, multistage attack that has stolen data belonging to several hundred thousand people who posted resumes on Monster.com, a researcher said this weekend.

According to Symantec security analyst Amado Hidalgo, a new Trojan horse called Infostealer.Monstres by Symantec has stolen more than 1.6 million records belonging to several hundred thousand people from Monster Worldwide's job search service. That data is then used to target the Monster.com users with credible phishing mail that plants more malware on their machines.

"We are investigating the reports related to this Trojan and will take any necessary steps indicated by that investigation," Monster.com spokesman Steve Sylven said Sunday in an e-mail.

The personal information filched from Monster.com includes names, e-mail addresses, home address, phone numbers and resume identification numbers, said Hidalgo, who traced the data to a remote server used by the attackers to store the stolen information. Infostealer.Monstres ripped off Monster.com by using legitimate log-ins, likely stolen from recruiters and human resource personnel who have access to the "Monster for employers" areas of the site.

Once inside, the Trojan horse ran automated searches for resumes of candidates located in certain countries or working in certain fields. The results were then uploaded to the attackers' remote server.

"Such a large database of highly personal information is a spammer's dream," said Hidalgo. In fact, that's exactly what the attackers are using their newly-acquired data for.

"The attackers first gather e-mail address and other personal information from resumes posted to Monster.com with Infostealer.Monstres," Hidalgo said. "Next, they will try to infect the computers of those candidates by sending targeted Monster.com phishing mails which install [Banker.c or Gpcoder.e]."

The first piece of malware, dubbed Banker.c by Symantec, is a run-of-the-mill information-stealing Trojan horse that monitors the infected PC for log-ons to online banking accounts. When it sniffs a log-on in process, Banker.c records the username and password, then transmits the data back to hacker HQ. Gpcoder.e, on the other hand, is "ransomware," the name given to Trojan horses that encrypt files on the hacked computer, then hold those files hostage until the user pays a fee to unlock the data.

Although both Banker.c and Gpcoder.e may be distributed in other ways -- SecureWorks last week said it had spotted something like the former coming from infected ads placed on job search sites -- Infostealer.Monstres' built-in mailing code and template lets it send messages posing as missives from Monster.com straight to the job-site users it finds in its automated searches.

Infostealer.Monstres' second-stage attack, which uses Gpcoder, is especially insidious. Realistic-looking e-mails that contain convincing personal information -- the very information stolen from Monster.com -- instruct the recipient to download a program called "Monster Job Seeker Tool." There is no tool, of course; victims download the ransomware Gpcoder.e instead.

Hidalgo's research led him to conclude that the three pieces of code -- Infostealer.Monstres, Banker.c, and Gpcoder.e -- are related, and probably the work of a single group.

"While their final purpose is different, their modus operandi is very similar, using identical file names, creating the same system folder, injecting code into the same processes and hooking the same system functions using root-kit techniques to gain control of network functionalities and to steal sensitive information," said Hidalgo. "They share code and a number of traits that could indicate they were developed by the same group or perhaps created using a kit."

Monster.com's Sylven defended the service's automated searches and said that although the company monitors database activity, stolen credentials have been used in the past to access the system. Moreover, he said, it's difficult to tell a valid automated search generated by a real person from one cranked out by software. "Many of our larger customers rely heavily on our database, and their use may be similar to programmatic or scripted access," said Sylven.

He could not confirm that the stolen accounts had been disabled, although Hidalgo noted in a blog entry posted Friday afternoon that Symantec had notified Monster of the compromised log-ins. "When unusual access is detected, we do terminate that access and investigate if possible," Sylven said.