Exploit kit activity may not be at the level it one was, but the threat has not gone away. Rig exploit kit activity has increased steadily in 2018 and now a new exploit kit has been detected.

The exploit kit has been named underminer by Trend Micro researchers, who detected it in July 2018. The Underminer exploit kit is being used to spread bootklits which deliver coinminer malware. The EK is primarily being used in attacks in Japan, although other East Asian countries have also seen attacks with activity now spreading beyond this region.

The underminer exploit kit was also detected by Malwarebytes researchers who note that the exploitation framework was first identified by the Chinese cybersecurity firm Qihoo360 in late 2017, when it was being used to deliver adware. Now the exploit kit is being used to deliver Hidden Bee (Hidden Mellifera) cryptocurrency mining malware. Trend Micro notes that evidence has been uncovered that strongly suggests the exploit kit was developed by the developers of Hidden Mellifera coinminer malware.

The exploit kit uses complex methods to deliver the payload with different methods used for different exploits. The developers have also incorporated several controls to hide malicious activity including the obfuscation of exploits and landing pages and the use of encryption to package exploits on-the-fly.

The EK profiles the user via a user-agent to determine if the user is of interest. If not, the user will be directed to a HTTP 404 error page. If a user is of interest, a browser cookie will be used to identify that user to ensure that the payload will only be delivered once, preventing reinfection and hampering efforts by researchers to reproduce an attack. URLs used in the attacks are also randomized to prevent detection by standard AV solutions. The coinminer is delivered via a bootkit which is downloaded through encrypted TCP tunnels.

The underminer exploit kit contains a limited number of exploits: The Adobe Flash Player exploit CVE-2018-4878, the use-after-free Adobe Flash Player vulnerability CVE-2015-5119, and the Internet Explorer memory corruption vulnerability CVE-2016-0189. Patches for all of the vulnerabilities were released in February 2018, July 2015, and May 2016 respectively.

The best defense against exploit kit attacks is prompt patching. All systems and applications should be kept 100% up to date, with virtual patching deployed on legacy systems and networks. Since there will always be a delay between the identification of a vulnerability and a patch being released, patching alone may not be sufficient to prevent all attacks, although EK developers tend to use old vulnerabilities rather than zero days.

In addition to prompt patching, cybersecurity solutions should be deployed to further reduce risk, such as a web filtering solution (WebTitan) to block users from visiting malicious websites and redirects through malvertising. In this case, one of the main ways that users are directed to the exploit kit is via adult-themed malvertising on legitimate adult websites. Using the web filter to block access to adult sites will reduce exposure.

Cybersecurity solutions should also be deployed to scan for malware installations and monitor for unusual activity and standard cybersecurity best practices should also be employed… the principle of least privilege and removing unused or unnecessary applications, plugins, and browser extensions.

The fact that a new exploit kit has been developed, and that it was recently updated with a new exploit, shows that the threat of web-based attacks has not gone away. EK activity may be at a fraction of the level of 2016, but businesses should not assume that attacks will not take place and should implement appropriate defenses to mitigate the threat.