The RIG exploit kit recently stopped distributing Tofsee and cybercriminals have decided to use the botnet’s own spamming capabilities to deliver the malware, Cisco’s Talos team reported on Thursday.
Tofsee, a multi-purpose malware that has been around since 2013, allows cybercriminals to conduct various activities, including click fraud, cryptocurrency mining, DDoS attacks and sending spam.
Up until June 2016, cybercriminals distributed the malware using the RIG exploit kit and malvertising campaigns. Then, after the notorious Angler exploit kit disappeared from the scene, cybercriminals started leveraging RIG to deliver other payloads, which experts believe might have been more profitable.
After RIG stopped delivering Tofsee, cybercriminals turned to email spam campaigns to infect computers. Typically, the Tofsee botnet has been used to send spam emails advertising adult dating and pharmaceutical websites. However, in August, researchers noticed that the spam messages had changed and started delivering Tofsee malware downloaders.
The volume of these spam emails has gradually increased since mid-August, reaching more than 2,000 messages on some days in September, Cisco Talos reported.
The spammy emails are adult-themed and they purport to come from women in Russia and Ukraine. Recipients are instructed to download and open the ZIP archive attached to the messages as it allegedly contains pictures of the sender.
Instead of pictures, the archive contains an obfuscated JavaScript file that includes a WScript downloader designed to fetch and run an executable from a remote server controlled by the attacker. Once the file is executed, the system becomes infected with Tofsee.
The malware connects to various SMTP relays, which it uses to send spam emails. The threat also initiates HTTP connections as it simulates clicking on ads as part of its click fraud mechanism.
Since the demise of Angler, the RIG exploit kit has been used to deliver the SmokeLoader (aka Dofoil) backdoor and other malware. Its developers have been working on improving the kit with new exploits and command and control (C&C) patterns that could help it evade detection.
Earlier this week, researchers reported that RIG had taken the place of Neutrino in a massive malvertising campaign that delivered CryptMIC ransomware. The campaign had previously used Neutrino, which took the leading position after Angler disappeared.
Related: Cisco Targets RIG Exploit Kit
Related: Kaspersky Confirms Lurk Gang Developed Angler Exploit Kit
Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.
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