Multiple hacker groups are using a "construction kit" supplied by the author of a Trojan horse program discovered in October 2006 to develop and unleash more dangerous variants of the original malware.

Already such variants have stolen sensitive information belonging to at least 10,000 individuals and sent the data to rogue servers in China, Russia and the US, according to Don Jackson, a security researcher at SecureWorks based in the US.

The stolen data includes Social Security numbers, online account information, bank account and credit card numbers, usernames and passwords, and other data that users would usually input during an SSL session.

The hackers are literally infecting thousands of users with one particular variant and once that version of the Trojan is blocked by antivirus, the hackers simply launch a new one in its place

Don Jackson - researcher, SecureWorks

The Prg Trojan, as it has been dubbed by SecureWorks, is a variant of another Trojan called wnspoem that was unearthed in October 2006. Similar to wnspoem, the Prg Trojan and its variants are designed to sniff sensitive data from Windows internal memory buffers before the data is encrypted and sent to SSL-protected Web sites.

The Trojans are programmed to send the stolen data to multiple servers around the world where it is stored in encrypted fashion and sold to others looking for such information. An analysis of log files on the servers storing the stolen data shows that a lot of the information is coming from corporate PCs, Jackson said.

The variants include a new function that allows them to listen on TCP port 6081 and wait for a remote attacker to connect and issue commands for forwarding data or for rummaging through files on the compromised system, Jackson said.

The newer variants are also more configurable and can be programmed to send stolen data to its final destination via a chain of proxy servers.

The new Prg variants encrypt stolen data differently from the original version, making older analysis tools obsolete, he said.

What makes the threat from the Prg Trojan especially potent is the availability of a construction toolkit that allows hackers to develop and release new versions of the code faster than antivirus vendors can devise applications, Jackson said.

The toolkit allows hackers to recompile and pack the malicious code in countless subtly different ways so as to evade detection by antivirus engines typically looking for specific signatures to identify and block threats, Jackson said.

The toolkit appears to have been developed by the Russian authors of the original wnspoem Trojan and comes complete with a three-page instruction manual in Russian instructing buyers how to use it. Originally, the kit appears to have been sold to other hacker groups for around $US1000. But more recently it appears to have been posted on an underground site, where others have been downloading and using it, Jackson said.

"The hackers are literally infecting thousands of users with one particular variant and once that version of the Trojan is blocked by antivirus, the hackers simply launch a new one in its place," Jackson said.

One of the groups using the construction kit has been naming its attacks after makes of cars, including Ford, Bugatti and Mercedes, according to a SecureWorks description of the Trojan. The group has been spreading versions of the Trojan by taking advantage of vulnerabilities in the ADODB database wrapper library and other components of Windows and Internet Explorer, according to SecureWorks.

That group alone may have snared data from more than 8000 victims. Data stolen by this group's Trojans are sent to servers based in the US and China, according to SecureWorks.

Another group using the toolkit has been naming its attacks using the letter "H" and has sent its variants via spam e-mails to various individuals, SecureWorks said.

One recent attack involved an e-mail with a subject line reading "HAPPY FATHER'S DAY". Data stolen by this group's Trojans is being sent back to servers in Russia. According to Jackson, many of those servers have separate staging areas on them with multiple versions of Prg Trojan programs that can be released as older versions get detected by antivirus software.