Researchers at Cisco have spotted an instance of the Nuclear exploit kit where the final payload is downloaded via the Tor network in order to make tracking more difficult.
The attack observed by Cisco Talos starts with a compromised website that acts as the exploit kit gate. When users access this site, they are redirected to a landing page via a technique known as “302 cushioning.”
302 cushioning, which has been used by exploit kits such as Angler, leverages HTTP 302 redirects to automatically take victims to malicious websites. By using 302 redirects, which are less likely to raise suspicion, instead of the traditional hidden iframes or external “script src” tags, attackers increase their chances of evading detection by security systems.
The landing page probes the victim’s system to determine if it can be exploited, and delivers the exploit page. In the Nuclear EK instance observed by Cisco, attackers leveraged an Adobe Flash Player vulnerability to push the payload. It’s worth noting that exploit code for the recently patched Flash flaw tracked as CVE-2016-1019 has been spotted in the Nuclear exploit kit.
What makes this instance of Nuclear noteworthy is that instead of directly dropping a piece of ransomware, it drops a Tor client for Windows. The client file, named “tor.exe,” is executed and a request is made via the Tor anonymity network to download a secondary payload.
“We looked at the Tor traffic and were able to find several domains listed in the network traffic. None of these domains have ever been registered and we were not able to find any DNS traffic associated with them. There also appears to be several time stamps from both 2016 and 2015 included as well,” Cisco’s Nick Biasini explained in a blog post.
By using Tor to download a second payload instead of directly dropping a malicious executable, which could easily be tracked based on command and control (C&C) communications, attackers make it difficult to track the malware back to the hosting system.
Researchers have spotted several improvements and new techniques in Nuclear EK operations over the past months. In October 2015, Morphisec reported that the exploit kit had been generating Flash Player exploits on the fly. More recently, anti-malware firm Malwarebytes spotted an attack in which the exploit kit gate used a fake CloudFlare DDoS check page to redirect victims.