Researchers from Cisco Talos reported an uptick in the use of second-tier exploit kits (EKs) for the delivery of malware.

Nuclear, Angler and Neutrino EKs — the main players in the arena — have all scaled back their operations, leaving a power void in the space. This enabled other contenders, such as the Sundown exploit kit, to make a run at the EK throne.

Introducing Sundown Exploit Kit

After a deeper look at Sundown, Talos researchers were surprised by what they found. To start, Sundown operates on a relatively small infrastructure footprint. At the same time, it runs one of the largest domain shadowing implementations Talos had ever seen. This is truly an asymmetrical approach.

While just a few IPs — researchers found only 10 — were directly linked to a campaign, Talos discovered that Sundown’s registrant accounts used more than 80,000 malicious subdomains that were associated with over 500 domains. The sheer number of domains renders traditional blacklisting solutions pretty much useless.

In addition, Sundown efficiently recycles the subdomains after use. This helps it avoid possible detection by not leaving a trail of visible past operations behind.

The Big Takeaway

Sundown is similar to other recent exploit kits in the way it operates. It features a landing page and an exploit page that contains a payload. The compromised landing page includes an iframe.

That iframe points to some location on the page that renders off screen, redirecting the browser to the exploit page. This exploit page evaluates the incoming victim for vulnerabilities before it delivers a malicious payload.

Sundown has quirks, to be sure. For one thing, it reuses exploits, and it also uses wildcard domains in its shadowing.

Lastly, the developers do not seem sophisticated enough to understand why they might want to hide the EK from sight. The colorful logo Sundown displays indicates the authors want to be seen and well-known.

“The big takeaway,” Talos researcher Nick Biasini told Threatpost, is that “Sundown is a much larger threat than people realize.”