OysterLoader is a highly sophisticated malware loader that has emerged as a serious threat in the cybersecurity landscape. First identified by Rapid7 in June 2024, this C++ based loader is primarily distributed through fake websites impersonating legitimate software such as PuTTy, WinSCP, Google Authenticator, and popular AI tools. The malware is delivered as malicious MSI installers, often digitally signed to appear trustworthy. Once executed, OysterLoader follows a four-stage infection chain involving a TextShell packer, custom shellcode execution, and deployment of the final payload. Researchers have linked OysterLoader mainly to Rhysida ransomware campaigns, though it has also been used to deliver commodity malware like Vidar infostealer, significantly increasing its impact. The effectiveness of OysterLoader lies in its advanced evasion and obfuscation techniques. It employs API hammering, dynamic API resolution using custom hashing, and timing-based sandbox detection to evade analysis. Sekoia analysts observed that the malware uses a two-tier command-and-control (C2) architecture, separating delivery servers from final C2 servers. A notable technique used is steganography, where encrypted payloads are hidden inside icon image files using RC4 encryption. Once decrypted, the payload is dropped as a DLL in the user’s AppData directory and executed via scheduled tasks every 13 minutes, ensuring persistence and continued attacker access. Organizations should block access to fake software download websites and enforce strict application allowlisting. Users must only download software from verified vendor sources. Security teams should deploy EDR solutions capable of detecting staged loaders, shellcode execution, and steganographic payload delivery. Monitoring scheduled task creation, unusual MSI executions, and encrypted HTTPS traffic is critical. Keeping operating systems, installers, and security tools up to date, combined with user awareness training, will significantly reduce exposure to OysterLoader-based attacks.
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