PDFSIDER Malware Actively Used by Threat Actors to Bypass Antivirus and EDR Defenses

PDFSIDER is a sophisticated backdoor malware designed to evade modern endpoint detection and response (EDR) systems through DLL side-loading and encrypted command-and-control communications.

Threat actors are actively distributing this malware via spear-phishing campaigns that exploit legitimate PDF software vulnerabilities, establishing persistent remote access to compromised systems with minimal detectable artifacts.

Attack Delivery Mechanism

The PDFSIDER campaign initiates via targeted spear-phishing emails containing ZIP archives with a trojanized executable that claims to be the PDF24 App.

The legitimate PDF24 Creator software by Miron Geek Software GmbH provides the cover for the actual attack chain. When victims execute the EXE file, it appears inactive but immediately begins malicious operations in the background.

The malware leverages a critical technique called DLL side-loading, in which attackers place a malicious DLL alongside the legitimate PDF24.exe application.

During normal execution, PDF24.exe loads the attacker’s cryptbase.dll instead of the legitimate system library, granting complete code execution.

This approach exploits a fundamental Windows behavior, making detection difficult because the parent process appears legitimate.

The EXE file carries a valid digital signature, further disguising its malicious nature and bypassing signature-based security controls.

The rise of AI-powered coding tools has accelerated the discovery of vulnerable software, enabling attackers to identify and exploit legitimate applications more efficiently than ever before.

Once PDFSIDER gains execution, it establishes encrypted command-and-control channels using the Botan 3.0.0 cryptographic library with AES-256 in GCM mode.

The malware operates almost entirely in memory, minimizing disk artifacts that traditional antivirus solutions might detect.

All data communications are encrypted using AEAD authentication, protecting both command integrity and confidentiality during exfiltration.

The malware initializes Winsock for network communication and gathers comprehensive system information, including the username, computer name, and process identifiers.

Commands are executed through hidden cmd.exe processes using the CREATE_NO_WINDOW flag, ensuring no visible console appears to the user.

This stealthy execution, combined with encrypted communications, aligns more with state-sponsored espionage tradecraft than with financially motivated cybercrime.

PDFSIDER implements sophisticated environment detection to avoid sandbox and virtual machine execution.

The malware uses GlobalMemoryStatusEx to check available RAM, and systems with low memory trigger early termination.

Additional debugger detection through IsDebuggerPresent prevents execution within analysis environments, effectively blocking security researchers from studying the malware in controlled conditions.

Indicators of Compromise

File Name	MD5 Hash	Status
Cryptbase.dll	298cbfc6a5f6fa041581233278af9394	Malicious
About.dll	e0e674ec74d323e0588973aae901b5d2	Clean
Language.dll	80e4a29270b828c1f97d9cde9475fcbd	Clean
NotifyIcon.dll	96ff508f9be007062b1770691f489e62	Clean
Pdf24.exe	a32dc85eee2e1a579199050cd1941e1d	Clean
Settings.dll	9f9dd5a432b4dde2160c7a7170e0d069	Clean

Organizations should implement strict controls on the execution of executable files, particularly those claiming to be legitimate software updates.

User awareness training should emphasize caution with email attachments and unexpected PDF software installation requests.

Network monitoring of DNS queries on port 53, combined with encrypted traffic analysis, may detect PDFSIDER’s C2 communications.

EDR solutions should be configured to detect DLL side-loading attempts and to monitor the loading behavior of cryptbase.dll from non-system directories.

MITRE ATT&CK Mapping

Tactic	Technique	Description
Initial Access	T1574.002 – DLL Side-Loading	Malicious cryptbase.dll hijacks legitimate PDF24.exe
Execution	T1059.003 – Windows Command Shell	Hidden cmd.exe command execution
Execution	T1106 – Native API	Low-level Win32 APIs for process control
Execution	T1204 – User Execution	User runs trojanized PDF24 application
Defense Evasion	T1497 – Virtualization Evasion	CPU, RAM, and debugger checks
Defense Evasion	T1622 – Debugger Evasion	IsDebuggerPresent detection
Discovery	T1082 – System Information Discovery	Collects system identifiers and configuration
Command Control	T1095 – Non-Application Layer Protocol	Custom encrypted Winsock communications
Exfiltration	T1041 – Exfiltration Over C2	Encrypted data transmission via C2

The analysis was conducted by Resecurity and documents the malware’s delivery mechanism, technical functionality, and anti-analysis techniques.

The report also includes a comprehensive set of indicators of compromise (IOCs), mapped MITRE ATT&CK techniques, and actionable defensive recommendations, presented in a format consistent with professional cybersecurity intelligence publications.

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