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 signed executable rather than the substituted library.
In some cases, attackers increase the likelihood of execution using decoy documents crafted to appear relevant to targets. One example involved a file claiming authorship from a Chinese government entity. Once launched, the malicious DLL inherits the same privileges as the legitimate executable that loaded it, increasing the attacker’s ability to operate within the system.
Resecurity notes that while the EXE remains validly signed, attackers exploited weaknesses in the PDF24 software to load the malware and bypass EDR tools more effectively. The firm also warns that AI-assisted coding is making it easier for cybercriminals to identify and exploit vulnerable software at scale.
After execution, PDFSider runs primarily in memory to reduce disk traces, using anonymous pipes to issue commands through CMD.
Each infected device is assigned a unique identifier, system details are collected, and the data is exfiltrated to an attacker-controlled VPS through DNS traffic on port 53. For command-and-control security, PDFSider uses Botan 3.0.0 and encrypts communications with AES-256-GCM, decrypting inbound data only in memory to limit its footprint. It also applies AEAD authentication in GCM mode, a cryptographic approach commonly seen in stealthy remote shell backdoors designed for targeted operations.
The malware includes anti-analysis checks such as RAM size validation and debugger detection, terminating early when it suspects sandboxing. Based on its behavior and design, Resecurity assesses PDFSider as closer to espionage-grade tradecraft than typical financially motivated ransomware tooling, built to quietly preserve covert access, execute remote commands flexibly, and keep communications protected.
