Rundll32 Execution with Log.DLL

This threat brief focuses on the abuse of rundll32.exe to execute malicious DLLs, specifically log.dll, a technique associated with the Lotus Blossom group's Chrysalis backdoor. The attacker places a rogue log.dll in a location such as %AppData%\Bluetooth and leverages rundll32.exe to invoke a specific function within the DLL (e.g., LogInit). This execution decrypts and runs shellcode. While some legitimate applications like the Bitdefender Submission Wizard also use log.dll, they are susceptible to DLL sideloading attacks, making this detection crucial for identifying malicious activity that bypasses traditional defenses. This campaign was first reported in 2026 and continues to be a relevant threat.

Attack Chain

1. The attacker gains initial access, often through social engineering or exploiting software vulnerabilities (not specified in source).
2. A malicious log.dll is placed in a writable directory, such as %AppData%\Bluetooth, mimicking a legitimate DLL location.
3. The attacker uses rundll32.exe to execute the malicious log.dll with a specific function call (e.g., rundll32.exe log.dll,LogInit).
4. Rundll32.exe loads and executes the log.dll.
5. The LogInit function in log.dll decrypts embedded shellcode.
6. The shellcode is injected into a legitimate process or executed directly, establishing persistence or escalating privileges.
7. The injected shellcode connects to a command-and-control (C2) server to download additional payloads or receive instructions.
8. The attacker performs actions on the compromised system, such as data exfiltration, lateral movement, or installing additional malware.

Impact

Successful exploitation can lead to complete system compromise, data theft, and the installation of persistent backdoors. The Lotus Blossom group has been known to target organizations across various sectors. The ability to bypass traditional security measures through DLL sideloading makes this a high-impact threat. Even legitimate software can become an attack vector.

Recommendation

• Deploy the Sigma rule Rundll32 Execution with Log.DLL to detect malicious rundll32.exe executions using log.dll (logsource: process_creation).
• Investigate any rundll32.exe process executions with log.dll as a command-line argument, especially when originating from unusual paths (Sigma rule Rundll32 Execution with Log.DLL).
• Monitor for suspicious file creations or modifications in %AppData%\Bluetooth or other common DLL sideloading locations (logsource: file_event).
• Implement application control policies to restrict the execution of rundll32.exe from untrusted locations.
• Audit systems for DLL sideloading vulnerabilities in legitimate applications.