Stealth Syscall Technique Allows Hackers to Evade Event Tracing and EDR Detection

Advanced threat actors have developed sophisticated stealth syscall execution techniques that successfully bypass modern security infrastructure, including Event Tracing for Windows (ETW), Sysmon monitoring, and Endpoint Detection and Response (EDR) systems.

These techniques combine multiple evasion methods such as call stack spoofing, ETW API hooking, and encrypted syscall execution to render traditional detection mechanisms ineffective, presenting significant challenges for cybersecurity defenders.

The core of these stealth techniques centers around executing system calls indirectly through dynamically allocated heap memory rather than standard Windows API functions.

Security researchers have documented how threat actors dynamically resolve syscall numbers at runtime from ntdll.dll, encrypt syscall stubs using XOR cipher operations, and decrypt them immediately before execution.

This approach effectively circumvents user-mode hooks that EDR solutions typically place on standard Windows APIs to monitor suspicious behavior.

The encryption methodology involves creating syscall stubs with specific assembly instructions, including “mov r10, rcx” for standard syscall setup, followed by “mov eax, syscallNumber” and the actual syscall instruction.

These stubs are encrypted with keys such as 0x5A and stored in heap-allocated memory, making static analysis tools like IDA Pro and Ghidra less effective at pattern recognition.

The dynamic nature of this execution prevents security tools from detecting known syscall patterns in memory, as the encrypted stubs only exist in their decrypted form for brief moments during execution.

Call Stack Manipulation

Sophisticated attackers have been observed implementing true stack spoofing techniques using Vectored Exception Handlers (VEH) to obscure call stack traces that security tools rely upon for threat detection.

The cyber espionage group APT41 has demonstrated expertise in constructing fake call stacks to mimic legitimate operations, successfully evading EDR systems that depend on call stack analysis for malicious activity identification.

This technique involves manipulating thread context records to redirect execution flow while maintaining the appearance of normal program operation.

Hardware breakpoint spoofing represents another critical component of these evasion techniques.

Attackers systematically clear debug registers Dr0 through Dr7 to prevent debuggers like x64dbg and WinDbg from setting effective breakpoints.

By modifying thread context flags and zeroing out these hardware registers, malicious code can execute without triggering debugger-based detection mechanisms that security researchers and automated analysis systems frequently employ.

ETW Neutralization

Perhaps most concerning is the systematic disabling of Event Tracing for Windows through direct function patching.

Attackers have developed methods to patch the NtTraceEvent function by replacing its initial instruction with a simple return (RET) instruction, effectively neutralizing ETW’s logging capabilities.

This technique falls under the MITRE ATT&CK framework as technique T1562.001: Impair Defenses: Disable or Modify Tools, where adversaries disable security monitoring to avoid detection.

According to the Report, The ETW disabling process involves using encrypted syscall stubs to modify memory protection of the NtTraceEvent function, making it executable and writable, then patching the function with a 0xC3 byte (RET instruction).

This approach prevents system-wide logging of suspicious activities that tools like Sysmon typically capture, creating significant blind spots in security monitoring infrastructure.

These advanced stealth techniques collectively create a formidable challenge for traditional security detection methods.

The combination of encrypted syscall execution, stack spoofing, hardware breakpoint clearing, and ETW disablement represents an evolution in adversary capabilities that requires defenders to develop more sophisticated detection mechanisms.

Security professionals must understand these techniques to develop effective countermeasures, including behavioral analysis that doesn’t rely solely on call stack inspection or ETW logging, and implement multi-layered detection strategies that can identify these evasion attempts through alternative indicators of compromise.