Cybercriminals are continuously evolving their techniques to bypass security defenses, and one of their most effective methods is direct syscalls. By circumventing Endpoint Detection and Response (EDR) monitoring, attackers can execute malicious activities undetected. This post explores how direct syscalls work, how they evade traditional security measures, and how Cortex XDR provides a robust defense against them.
Understanding Windows Syscalls and EDR Monitoring
To appreciate the effectiveness of direct syscalls as an attack method, it’s crucial to understand how syscalls function within Windows. System calls (syscalls) act as a bridge between user-mode applications and the Windows kernel, allowing programs to request services such as file access, memory allocation, and network communication.
Normally, syscalls are accessed through Windows API functions like CreateFileW, which pass through kernel32.dll and ntdll.dll before reaching the kernel. Most EDRs monitor these calls using inline hooking, intercepting execution within these DLLs to detect suspicious activity.
How Attackers Exploit Direct Syscalls to Evade EDRs
Traditional EDR solutions rely on user-mode hooks to track API calls, but attackers have developed techniques to bypass these hooks, including:
- Manual DLL Loading: Loading a clean copy of system libraries to bypass EDR modifications.
- DLL Cloning: Creating a duplicate of a system DLL and loading it under a different name to avoid detection.
- Direct Syscalls: Instead of relying on Windows API functions, attackers implement syscall stubs themselves, calling kernel services directly and bypassing the monitoring hooks of EDR solutions.
Why Direct Syscalls Are Dangerous for Businesses
Businesses of all sizes—small, medium, and enterprise-level—are vulnerable to sophisticated cyber threats. Attackers use direct syscalls to execute stealthy attacks such as:
- Injecting malware into legitimate processes.
- Stealing sensitive data while avoiding traditional detection methods.
- Evading forensic analysis by removing traces of execution.
Cortex XDR’s Unique Approach to Detecting Malicious Direct Syscalls
Unlike traditional EDR solutions that rely solely on user-mode monitoring, Cortex XDR operates at the kernel level, intercepting syscalls before they execute. This ensures that direct syscalls do not go unnoticed, even when attackers attempt to bypass security controls.
How Cortex XDR Detects Malicious Direct Syscalls:
- Kernel-Level Monitoring: Cortex XDR captures system call execution at the KTRAP_FRAME level, tracking where the syscall originates.
- Image Tracking: Using ImageTracker, Cortex XDR verifies whether the syscall comes from a legitimate Windows DLL (
ntdll.dll) or a suspicious source (e.g., injected shellcode). - Behavioral Analytics: Instead of relying on static detection, Cortex XDR uses machine learning algorithms to analyze syscall behavior and detect anomalies.
Real-World Example: How Cortex XDR Detected Lumma Stealer
One recent example of direct syscall usage in malware is Lumma Stealer, a dangerous info-stealer that targets cryptocurrency wallets and sensitive browser data.
- The malware bypasses EDR hooks by extracting syscall indexes from
ntdll.dlland executing syscalls directly. - It then injects shellcode into legitimate Windows processes using direct syscalls, avoiding detection.
- Cortex XDR’s behavioral analytics flagged the unusual direct syscall behavior, enabling rapid identification and mitigation.
Final Thoughts: Strengthening Your Cybersecurity Strategy
Cybercriminals continuously refine their techniques to evade security measures, and direct syscalls are a prime example of these advanced evasion strategies. Businesses must adopt EDR solutions that go beyond traditional user-mode monitoring to detect stealthy threats effectively.
By leveraging kernel-level monitoring and AI-powered behavioral analytics, Cortex XDR provides businesses with an advanced defense against malicious direct syscalls.
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