Cybersecurity researchers from Palo Alto Networks’ Unit 42 have disclosed the resurgence of the Bookworm malware, which has been linked to the Stately Taurus threat actor group. This malware, initially discovered in 2015, uses a sophisticated DLL sideloading technique that allows it to bypass security defenses and effectively infiltrate Windows systems. The new analysis highlights the continued use of this technique by Stately Taurus, confirming Bookworm’s role in the group’s ongoing cyber-espionage activities.
Bookworm operates by utilizing legitimate executables signed by automation organizations, which are then used to load malicious payloads. One such payload, identified as BrMod104.dll, communicates with the malware’s command and control (C2) server, making it difficult to detect. To further evade detection, Bookworm employs obfuscation tactics, masquerading as legitimate Windows update requests by mimicking HTTP requests aimed at Microsoft servers.
This sophisticated approach demonstrates the malware’s ability to adapt and remain effective over time.
In addition to the DLL sideloading technique, Bookworm exhibits a modular architecture that allows for flexibility and adaptability in its deployment. This modular design has remained largely consistent throughout the years, with only minimal changes observed in newer versions. The analysis also reveals overlaps between Bookworm and another backdoor variant, ToneShell, which share similar debug paths and infrastructure. This suggests that both malware families may have been developed by the same group, further linking Bookworm to the Stately Taurus group.
The resurgence of Bookworm malware underscores the persistent and evolving nature of advanced persistent threat (APT) groups like Stately Taurus. These threats continue to target government entities and organizations in Southeast Asia. Researchers emphasize the importance of employing advanced security measures, including machine learning-based detection and behavioral threat protection, to defend against these sophisticated attacks. Proactive defense strategies are essential to mitigate the risk posed by such persistent and adaptable cyber threats.
