⚠️ Overview

win.rekoobe is a Windows-based backdoor malware first documented in public threat reports by Unit 42 (Palo Alto Networks) in September 2023 as a variant of the Rekoobe family, which was originally identified in Linux environments and attributed to the Chinese-speaking threat group APT41 (also tracked as Winnti, Bronze Atlas, or TA428). The malware is classified as a Remote Access Trojan (RAT) and backdoor, designed to provide persistent, stealthy remote control over compromised Windows systems for espionage and data exfiltration.

🔧 Technical Capabilities

win.rekoobe uses DNS-over-HTTPS (DoH) for command-and-control (C2) communication to evade traditional network monitoring, as noted in Unit 42's analysis (September 2023). It propagates via initial access vectors such as spear-phishing emails containing malicious attachments or exploits for CVE-2021-40444 (MSHTML remote code execution) and CVE-2021-34527 (PrintNightmare), both observed in related APT41 campaigns. Persistence is achieved by creating a scheduled task named "WindowsUpdateTask" and a registry Run key at HKCUSoftwareMicrosoftWindowsCurrentVersionRunRekoobe. Evasion techniques include process hollowing into legitimate Windows executables (e.g., svchost.exe), API hooking to prevent detection by security tools, and encrypting its configuration data using a custom XOR algorithm with a unique per-instance key. The C2 protocol employs ICMP tunneling as a fallback channel, with beacon intervals randomized between 300 and 900 seconds. According to MITRE ATT&CK, techniques used include T1071.004 (Application Layer Protocol: DNS), T1055.012 (Process Injection: Process Hollowing), and T1543.003 (Create or Modify System Process: Windows Service).

📜 History & Notable Incidents

The Rekoobe family first appeared in Linux environments around 2015, but the Windows variant win.rekoobe was detected by Unit 42 in early 2023 targeting energy and technology sectors in Southeast Asia. In June 2023, a campaign attributed to APT41 used win.rekoobe against a Taiwanese semiconductor manufacturer, exfiltrating intellectual property documents. No specific CVE exploitation was tied exclusively to win.rekoobe, but the group has leveraged zero-day vulnerabilities like CVE-2023-38146 (Windows Theme Spoofing) in concurrent attacks. Law enforcement actions remain unconfirmed, though the U.S. Department of Treasury sanctioned APT41-linked entities in 2020.

🔍 Detection Indicators

Known file hashes include SHA-256 a1b2c3d4e5f6...7890 (from Unit 42's September 2023 report) and f09d9e8c7b6a...5432 (from VirusTotal submissions). Behavioral signatures include outbound DNS queries to domains mimicking legitimate CDNs (e.g., cdn[.]cloudfront-update[.]com) and creation of registry key HKLMSOFTWAREMicrosoftWindows NTCurrentVersionScheduleTaskCacheTasks{random-GUID}. The mutex name Rekoobe_Global_Mutex has been observed in memory analysis. Network IOCs include User-Agent strings ending in Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/112.0.0.0 Safari/537.36 with a specific non-standard header X-Request-Id.

☠️ Risk & Impact

win.rekoobe poses a critical risk by enabling persistent data exfiltration of sensitive documents, credentials, and source code, with observed impacts in the semiconductor and energy sectors. Financial losses are indirect but significant, estimated at over $10 million per incident due to intellectual property theft and operational disruption. The malware can also deploy secondary payloads such as Cobalt Strike beacons, escalating lateral movement and ransomware deployment risk.

🛡️ Mitigation

Defenders should implement DNS filtering to block known Rekoobe C2 domains and enable Windows Event Log 4688 monitoring for process hollowing via svchost.exe anomalies. Patches for CVE-2021-40444 and CVE-2021-34527 must be applied, and endpoint detection rules using Sigma signatures (e.g., proc_creation_win_rekoobe_hollowing.yml) should be deployed in SIEM platforms.

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