In September 2020, we published a blog describing how the PurpleFox Exploit Kit used Cloudflare services to maintain an infrastructure resilient to blocking and detection attempts. Since then, PurpleFox has been maintaining this strategy while at same time improving its attack chain by incorporating the latest public vulnerabilities into its arsenal.
Recently, we found that PurpleFox added a very old tactic to increase its delivering performance. This time PurpleFox EK is making use of WPAD domains to infect users. While a WPAD abuse attack is a technique that has been around for approximately 14 years, it still works. Initiatives to prevent this attack help, but they are not sufficient.
Our systems started detecting victims accessing the “wpad.id” domain, which makes use of the Indonesian top level domain (*.id). We did not find any other country top level domain affected. Using this technique, a zero-click attack can be implemented, as the WPAD URL is accessed whenever the system starts, without any user input.
PurpleFox WPAD landing page
The custom shellcode downloads the next stage from the URL http://9kf[.]me/in[.]php?id=1. The domain “9kf.me” was no longer accessible by the time we analyzed the samples, but we were be able to find two more active domains, “2kf.me” and “6kf.me,” that contained the same payload.
Following the request logic, we retrieved the full chain used in this deployment. The PurpleFox chain is designed with multiple complicated stages abusing PowerShell and MSI files as previously described by Trend Micro and other researchers. This post will not go into the details; we will limit ourselves to showing how the two domains are chained to deliver the full attack chain.
The domain resolution and access to the attack chain artifacts are all being proxied through Cloudflare servers, as shown in the Figure 4.
Analysis of the full chain revealed that the following CVEs were being exploited: CVE-2020-1054, CVE-2018-8120, as well as an exploit for MS15-051. The binary exploiting the MS15-051 leak the symbols path C:\Users\K8team\Desktop\ms15-051\ms15-051\ms15-051\Win32\ms15-05, suggesting that PurpleFox is reusing tools from K8team, which is responsible for maintaining public repositories of CVE exploits POCs and hack tools.
Defending against PurpleFox
The PurpleFox Exploit Kit continues to be very active and appear to be looking for new infection tactics. Our feedback shows that this specific attempt is not only affecting Indonesian victims, as users in other countries who are using the Indonesian TLD are being affected as well. At same time, PurpleFox is trying to reach servers where the user interaction is minimal but are potentially affected by the WPAD technique, such as unattended machines.
Continuous vigilance against threat groups is an important aspect of keeping up with — if not staying one step ahead of — threats. To protect systems from this type of threat, users can use multilayered security solutions like Trend Micro Protection Suites that help detect and block attacks. Trend Micro Vision One™️ also provides visibility, correlated detection, and behavior monitoring across multiple layers, such as emails, endpoints, servers, and cloud workloads. This ensures that no significant incidents go unnoticed and allows faster response to threats before they can do any real damage to the system.
Indicators of Compromise
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