A new sophisticated method to bypass Microsoft’s Windows Defender antivirus protection by combining direct syscalls with XOR encryption techniques.
The research, published this week, reveals critical vulnerabilities in one of the most widely deployed security solutions that ships with every Windows installation.
The breakthrough technique leverages the fundamental architecture of the Windows operating system, exploiting the boundary between user mode (Ring 3) and kernel mode (Ring 0) operations.
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By circumventing the traditional Windows execution flow, attackers can execute malicious code without triggering defensive mechanisms.
Understanding the Technique
According to the research published by Hackmosphere, the technique works by avoiding the conventional execution path where applications call Windows API functions through libraries like kernel32.dll, which then forwards requests to ntdll.dll before making the actual system call to the kernel.
Instead, attackers directly execute the syscall instruction with the appropriate syscall number, bypassing any security monitoring at the user mode level.
The researchers further bolstered this attack by implementing XOR encryption to obfuscate malicious shellcodes. This simple yet effective cryptographic technique transforms the malicious code into an unrecognizable form that evades signature-based detection.
Here’s an example of how direct syscalls are implemented in C++:
XOR Encryption
XOR encryption operates on the principle of bitwise XORing, where each bit of the plaintext code is combined with a corresponding bit from a secret key.
When the payload is ready to execute, it’s decrypted in memory, leaving no trace on the disk for antivirus solutions to detect.
In testing, the researchers created a Meterpreter reverse shell payload using msfvenom, encrypted it with XOR, and executed it using direct syscalls.
The attack achieved a complete bypass of the latest Windows Defender protections without writing any malicious artifacts to disk.
Even more concerning, the researchers noted that this technique has been viable since at least 2022 with various modifications and continues to work in 2025 against the latest Windows Defender updates.
Microsoft has previously addressed similar bypass techniques, stating they have “limited practical applicability” since they often require user interaction to execute.
However, security experts disagree, pointing out that such techniques could be easily incorporated into broader attack chains.
The researchers recommend that Microsoft implement kernel-level monitoring of syscalls rather than relying solely on user-mode hooks.
They also suggest organizations deploy additional security layers beyond Windows Defender, particularly solutions that can monitor behavior at the kernel level.
For now, security teams are advised to implement application whitelisting and restrict administrative privileges to mitigate the risk of these sophisticated bypass techniques.
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