PDFSider Malware Used in Fortune 100 Finance Ransomware Attack

A Fortune 100 finance company was targeted by ransomware actors using a new Windows malware strain called PDFSider, built to quietly deliver malicious code during intrusions. Rather than relying on brute force, the attackers used social engineering, posing as IT support staff and convincing employees to launch Microsoft Quick Assist, enabling remote access. Resecurity researchers identified the malware during incident response, describing it as a stealth backdoor engineered to avoid detection while maintaining long-term control, with traits typically associated with advanced, high-skill intrusion activity. 

Resecurity previously told BleepingComputer that PDFSider had appeared in attacks connected to Qilin ransomware, but researchers emphasize it is not limited to a single group. Their threat hunting indicates the backdoor is now actively used by multiple ransomware operators as a delivery mechanism for follow-on payloads, suggesting it is spreading across criminal ecosystems rather than remaining a niche tool. 

The infection chain begins with spearphishing emails containing a ZIP archive. Inside is a legitimate, digitally signed executable for PDF24 Creator, developed by Miron Geek Software GmbH, paired with a malicious DLL named cryptbase.dll. Since the application expects that DLL, it loads the attacker’s version instead. This technique, known as DLL side-loading, allows the malicious code to execute under the cover of a trusted program, helping it evade security controls that focus on the

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