Cybersecurity researchers have discovered a new and previously undocumented Windows backdoor that uses the Background Intelligent Transfer Service (BITS) for command-and-control (C2) communication. Dubbed BITSLOTH by Elastic Security Labs, this malware was identified on June 25, 2024, during a cyber attack targeting a South American government’s Foreign Ministry, tracked under the name REF8747. BITSLOTH is a highly advanced backdoor with 35 functions, including keylogging, screen capture, and capabilities for system discovery, enumeration, and command execution. Development of this malware began in December 2021, and it is primarily used for data gathering. Analysis of the code hints that the creators may be Chinese speakers based on certain coding characteristics. To evade detection, BITSLOTH utilizes RingQ, an open-source encryption tool that encrypts its payload and runs it directly in memory. The malware also exploits compromised web servers to drop web shells, which then deploy additional malicious payloads, including cryptocurrency miners, using RingQ. These attacks have been linked to Chinese-speaking threat actors. BITSLOTH stands out for its use of STOWAWAY to proxy encrypted C2 traffic over HTTP and a port forwarding tool called iox, previously used by the Chinese cyber espionage group Bronze Starlight in Cheerscrypt ransomware attacks. The malware is delivered as a DLL file named "flengine.dll" through DLL side-loading via the legitimate FL Studio executable ("fl.exe"). Its latest version includes a scheduling feature for precise operation timings, similar to other modern malware like EAGERBEE. BITSLOTH’s capabilities to execute commands, manage files, and collect sensitive data, combined with its use of BITS, make it particularly difficult for organizations to detect and monitor.
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