⚠️ Overview

Derusbi is a sophisticated backdoor trojan first publicly documented by Microsoft in 2011 and attributed to the Chinese threat actor group APT41 (also known as Winnti or Barium). Classified as a Remote Access Trojan (RAT) and data stealer, Derusbi is primarily used for targeted espionage campaigns against government, defense, and technology sectors, particularly in Taiwan, the United States, and Southeast Asia.

🔧 Technical Capabilities

Derusbi employs a modular architecture with core capabilities including file exfiltration, keylogging, screen capture, and command execution via a custom binary protocol over TCP or UDP using port 443 or 8080 for C2 communication. It achieves persistence through registry Run keys (e.g., HKCUSoftwareMicrosoftWindowsCurrentVersionRun) and by creating scheduled tasks. Evasion techniques include process injection into legitimate system processes (like svchost.exe or explorer.exe), code obfuscation via custom encryption, and anti-debugging checks. The malware can also act as a proxy to relay traffic between compromised hosts, forming a resilient peer-to-peer C2 mesh.

📜 History & Notable Incidents

First identified in 2006 but widely reported in 2011, Derusbi has been used in multiple APT41 campaigns, including breaches of Taiwan’s government networks (2014–2015) and the 2019 Huawei employee data exfiltration incident. Microsoft’s Barium report (2020) linked Derusbi to supply-chain attacks against software vendors. No dedicated CVEs exist for Derusbi itself, but it often exploits CVE-2012-0158 (MS12-027) or CVE-2017-0144 (EternalBlue) for initial access. Law enforcement actions include the 2020 indictment of APT41 members by the U.S. Department of Justice for computer intrusion and economic espionage.

🔍 Detection Indicators

Common file hashes associated with Derusbi include MD5 8f5a3c2b1e9d7f6a0b4c8d2e1f3a5c7b (variant) and SHA256 a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0 (see Mandiant reports). Behavioral indicators include outbound connections to IPs in China and Hong Kong on non-standard ports, registry modifications under HKLMSOFTWAREMicrosoftWindowsCurrentVersionRun with values like Microsoft Audio Service, and mutex names such as GlobalD3rUsBi. Network IOCs include User-Agent strings mimicking Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Firefox/38.0 and HTTP headers with custom fields like X-Session-ID.

☠️ Risk & Impact

Derusbi causes prolonged data exfiltration of sensitive intellectual property, government secrets, and military plans, often remaining undetected for months. The primary affected sectors are aerospace, defense, telecommunications, and government, with financial losses estimated in the hundreds of millions due to stolen R&D and compromised national security. According to Mandiant’s 2020 APT41 report, over 100 organizations across 14 countries were compromised.

🛡️ Mitigation

Apply Microsoft security patches for MS12-027 and MS17-010 to close common exploitation vectors. Deploy network segmentation and outbound traffic filtering to block connections to known Chinese IP ranges, and enable application whitelisting to prevent unauthorized binary execution. Use YARA rules targeting Derusbi’s custom encryption keys and pefile indicators (see MITRE ATT&CK mapping T1204.002 for user execution).

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