The developers of the Neutrino exploit kit have added a new feature designed to reduce exposure to automated scans and security researchers’ analysis attempts.
Researchers at Trustwave discovered the new feature after observing a drop in Neutrino exploit kit instances in their lab environment. A closer analysis revealed that the EK instances they had been monitoring served malware as expected when accessed from a vulnerable Windows machine, but the Neutrino server was not responding to requests from the lab environment.
“The problem was that every other website (and even exploit kit) was working as intended, the environment seemed completely fine except for when accessing Neutrino,” Trustwave’s Daniel Chechik explained in a blog post. “We tried the obvious solutions of trying to change IP addresses, browsers, pretty much anything we know that exploit kits commonly use to evade security researchers, but to no avail.”
After making some modifications to the maximum transmission unit (MTU) of the TCP packets they were sending, Trustwave researchers finally got a response from the Neutrino server. While at first this appeared to be a bug in Neutrino, it turned out to be the result of a new feature added by the exploit kit’s developers.
A newly added iptables module, named OSF, provides passive operating system fingerprinting and allows the attackers to prevent certain types of machines from accessing their server.
The Neutrino servers could not be accessed from Trustwave’s lab environment because the test machine had been detected as running Linux, which is not allowed because the operating system is often used for security products, servers and other types of devices that can’t be infected by the malware delivered by the exploit kit. By filtering out Linux machines, the cybercriminals increase chances of preventing automated scanning tools and researchers from accessing their servers.
When Trustwave experts changed the MTU, the Neutrino server started detecting the test machine as “unknown OS,” allowing it to connect.
“Exploit kits often employ measures to keep security researchers and other unwanted visitors away from their servers, but most of the time these measures are handled on the HTTP level- with web servers redirecting you away or returning fake error codes; implementing this logic on a TCP level as Neutrino did is a fairly smart move on their part: generally speaking when a server doesn't respond to you at all you tend to assume that it's down,” Chechik said.
“With exploit kits changing their location as rapidly as they sometimes do, 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,” he added.
The malicious actors behind Neutrino have been working on improving their creation over the past period. Heimdal Security reported in early January that a new Neutrino campaign had been using blackhat SEO poisoning tricks in an effort to deliver ransomware and other types of malware.