A new Trojan is successfully attacking online recruiting sites and already has accessed data on hundreds of thousands of users, researchers said last Friday.

Researchers from Symantec and SecureWorks separately reported finding surprisingly effective penetrations by the new Trojan, called Infostealer.Monstres, which was attempting to access the online recruitment Website, Monster.com. Other versions of the Trojan, which is a variant of the Prg Trojan, were also found to be attacking other online job sites.

Interestingly, Monster.com and a security business partner, Cyveillance, warned the industry about increasing attacks on recruiting sites less than a month ago. (See Help Wanted: ID Theft Victims.)

The new Trojan, which is usually delivered via phishing messages that Monster.com and Cyveillance warned users about, has allowed attackers to collect as many as 1.6 million pieces of data affecting "several hundred thousand" users on Monster.com alone, according to Symantec. Working independently, SecureWorks last Friday reported finding at least a dozen caches of personal information, totaling about 100,000 identities.

"The Trojan appears to be using the (probably stolen) credentials of a number of recruiters to login to the [Monster.com] Website and perform searches for resumes of candidates located in certain countries or working in certain fields," Symantec says in its blog. "The Trojan sends HTTP commands to the Monster.com Website to navigate to the Managed Folders section. It then parses the output from a pop-up window containing the profiles of the candidates that match this recruiter’s saved searches."

The personal data is then extracted from the resumes and uploaded to a remote server, Symantec says. The Symantec researchers found all of the 1.6 million pieces of compromised data on a single server, but SecureWorks found at least a dozen smaller caches, so the number of users affected likely is higher than either of the research teams has reported so far.

"Such a large database of highly personal information is a spammer’s dream," Symantec says. "In fact, we found the Trojan can be instructed to send spam email using a mail template downloadable from the command and control server."

The latest exploit is not the first instance of a Trojan attacking Monster.com, Symantec reports. "The main file used by Infostealer.Monstres, ntos.exe, is also commonly used by Trojan.Gpcoder.E, and both also have a similar icon for the executable file that reproduces the Monster.com company logo. [This is] hardly a coincidence."

"Furthermore, Trojan.Gpcoder.E has reportedly been spammed in Monster.com phishing emails," Symantec says. "These emails were very realistic, containing personal information of the victims. They requested that the recipient download a Monster Job Seeker Tool, which in fact was a copy of Trojan.Gpcoder.E.

"This Trojan will encrypt files in the affected computer and leaves a text file requesting money to be paid to the attackers in order to decrypt the files" Symantec explains. "The code for Gpcoder is rather similar to that of Monstres, which may indicate the same hacker group is behind both Trojans."

The researchers say they have informed Monster.com of the exploits so that the presumably-stolen recruiter accounts can be shut down. In the meantime, they advise users not to put personal information -- such as Social Security numbers -- into their online job postings. Users should not give out this sort of data until they have established that a potential employer is legitimate, they say.