enterprise environments

OysterLoader, a sophisticated malware loader also known as Broomstick and CleanUp, has emerged as a significant threat since mid-2024. It is a multi-stage downloader linked to ransomware attacks and data theft, particularly associated with the Rhysida ransomware group. Written in C++, it infiltrates systems through malicious websites that impersonate legitimate software download platforms, tricking victims into executing a signed Microsoft Installer (MSI) that launches the malware. OysterLoader employs a four-stage infection chain designed to evade detection. The first stage uses a packer named TextShell to load hidden code into memory, creating an illusion of legitimacy through harmless Windows API calls. The second stage decompresses a concealed payload using a modified LZMA algorithm. The third stage functions as a downloader and environment tester, establishing contact with its command-and-control (C2) server via HTTPS. In the final stage, OysterLoader installs a malicious DLL that executes every 13 minutes through the Windows Task Scheduler, communicating with multiple hardcoded servers and transmitting critical system information. The malware uses customized Base64 encoding and variable communication endpoints to evade detection. Its primary objective is to ensure persistence and facilitate the delivery of additional payloads, including ransomware and credential stealers. Security analysts predict that OysterLoader will remain a formidable threat through 2026, particularly for organizations downloading administrative tools from unverified sources. Indicators of Compromise (IOC): - Mutex: h6p#dx!&fse?%AS! - Task: COPYING3 (rundll32 DllRegisterServer) - C2 Domain: grandideapay[.]com/api/v2/facade - RC4 Key: vpjNm4FDCr82AtUfhe39EG5JLwuZszKPyTcXWVMHYnRgBkSQqxzBfb6m75HZV3UyRY8vPxDna4WC2KMAgJjQqukrFdELXeGNSws9SBFXnYJ6ExMyu97KCebD5mTwaUj42NPAvHdkGhVtczWgfrZ3sLyRZg4HuX97AnQtK8xvpLU2CWDhVq5PEfjTNz36wdFasecBrkGSDApf83d6NMyaJCsvcRBq9ZYKthjuw5S27EVzWrPHgkmUxFL4bQSgMa4F - IP: 85.239.53.66