Security Experts:

Purple Fox Malware Squirms Like a Worm on Windows

Malware hunters at Guardicore are warning that an aggressive botnet operator has turned to SMB password brute-forcing to infect and spread like a worm across the Microsoft Windows ecosystem.

The malware campaign, dubbed Purple Fox, has been active since at least 2018 and the discovery of the new worm-like infection vector is yet another sign that consumer-grade malware continues to reap profits for cybercriminals.

According to Guardicore researcher Amit Serper, the Purple Fox operators primarily used exploit kits and phishing emails to build botnets for crypto-mining and other nefarious uses.  

Now, the new SMB brute-force method is being combined with rootkit capabilities to hide and spread widely across internet-facing Windows computers with weak passwords.

“Throughout the end of 2020 and the beginning of 2021, Guardicore Global Sensors Network (GGSN) detected Purple Fox’s novel spreading technique via indiscriminate port scanning and exploitation of exposed SMB services with weak passwords and hashes,” Serper explained.

Serper said May 2020 saw a “significant amount of malicious activity” where the number of infections climbed by roughly 600% and amounted to a total of 90,000 attacks.

Serper’s blog, which contains IOCs to help defenders hunt for signs of infection, explains the aggressiveness of the malware operator:

“While it appears that the functionality of Purple Fox hasn’t changed much post exploitation, its spreading and distribution methods – and its worm-like behavior – are much different than described in previously published articles. Throughout our research, we have observed an infrastructure that appears to be made out of a hodge-podge of vulnerable and exploited servers hosting the initial payload of the malware, infected machines which are serving as nodes of those constantly worming campaigns, and server infrastructure that appears to be related to other malware campaigns.

Serper’s team at Guardicore warned that the attackers are hosting various MSI packages on nearly 2,000 servers, most of which are compromised machines which were repurposed to host malicious payloads. 

“We have established that the vast majority of the servers, which are serving the initial payload, are running on relatively old versions of Windows Server running IIS version 7.5 and Microsoft FTP, which are known to have multiple vulnerabilities with varying severity levels,” Guardicore said in a technical blog post.

The company found the campaign spreading via two distinct mechanisms -- a worm payload after a victim machine is compromised through a vulnerable exposed service (such as SMB); or the worm payload is being sent via email through a phishing campaign.

The company is encouraging malware hunters to use public indicators of compromise to find signs of malicious activity related to this threat.

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Ryan Naraine is Editor-at-Large at SecurityWeek and host of the popular Security Conversations podcast series. Ryan is a veteran cybersecurity strategist who has built security engagement programs at major global brands, including Intel Corp., Bishop Fox and Kaspersky GReAT. He is a co-founder of Threatpost and the global SAS conference series. Ryan's past career as a security journalist included bylines at major technology publications including Ziff Davis eWEEK, CBS Interactive's ZDNet, PCMag and PC World. Ryan is a director of the Security Tinkerers non-profit, an advisor to early-stage entrepreneurs, and a regular speaker at security conferences around the world. Follow Ryan on Twitter @ryanaraine.