The developers of the Neutrino exploit kit have added a new feature intended to thwart security researchers from studying their attacks.

The feature was discovered after Trustwave's SpiderLabs division found computers they were using for research couldn't make a connection with servers that delivered Neutrino.

"The environment seems completely fine except for when accessing Neutrino," wrote Daniel Chechik, senior security researcher.

Exploit kits are one of the most effective ways that cybercriminals can infect computers with malware. They find vulnerable websites and plant code that transparently connects with another server that tries to exploit software vulnerabilities.

If the server finds a hole, the malware is delivered, and the victim is none the wiser. Exploit kits are also sometimes delivered by malicious online ads in attacks known as malvertising.

Malware writers and cyberattackers have long used methods to try to stop security engineers from studying their methods. For example, some malware programs are designed to quit if they're running in a virtual machine.

Chechik wrote that Trustwave tried changing IP addresses and Web browsers to avoid whatever was causing the Neutrino server to not respond, but it didn't work.

But by fiddling with some data traffic that Trustwave's computers were sending to the Neutrino server, they figured out what was going on.

Neutrino has been engineered to use passive OS fingerprinting, which is a method to collect and analyze data packets without the entity that is sending the packets knowing their computers are being profiled. In this case, the computer sending the packets is a security researcher's system that's probing the hackers' server.

Passive OS fingerprinting captures "traffic coming from a connecting host going to the local network," according to a post on the SANS blog. "The fingerprinting can then be conducted without the remote host being aware that its packets are being captured."

Michal Zalewski, a security researcher who works for Google, wrote a tool specifically for passive OS fingerprinting.

It offers attackers the advantage of stealth, since active OS fingerprinting -- which involves sending direct traffic to another network -- can trigger alerts from firewalls and intrusion detection equipment.

Chechik wrote that Neutrino appears to be using passive OS fingerprinting in order to shut down connections coming from Linux machines, which are likely to be used by security researchers.

"This approach generally reduces their exposure to anything from automated scans to unwanted security researchers," he wrote.

It's a smart move by Neutrino's developers, because if a server doesn't respond, it's generally considered down.

"It is very likely that this behavior would simply be written off as a dead server and Neutrino would achieve its goal of being left alone by anyone who isn't a potential victim," Chechik wrote.