A recent study has unveiled a sophisticated method that can effectively bypass Microsoft’s Windows Defender antivirus protection, utilizing a combination of direct system calls and XOR encryption techniques. This research, released this week, highlights significant vulnerabilities in one of the most commonly used security solutions that comes pre-installed with every Windows operating system.
The innovative technique takes advantage of the fundamental architecture of Windows, specifically targeting the division between user mode (Ring 3) and kernel mode (Ring 0) operations. By sidestepping the conventional execution flow of Windows, attackers can execute harmful code without triggering any defensive measures.
Understanding the Technique
According to findings published by Hackmosphere, this method circumvents the traditional execution path where applications typically call Windows API functions through libraries such as kernel32.dll. These libraries then relay requests to ntdll.dll before reaching the kernel. Instead, the attackers directly invoke the syscall instruction with the appropriate syscall number, effectively bypassing any security monitoring that occurs at the user mode level.
To enhance the effectiveness of this attack, researchers employed XOR encryption to obfuscate the malicious shellcodes. This straightforward yet powerful cryptographic technique transforms the harmful code into an unrecognizable format, allowing it to evade signature-based detection.
For instance, the researchers demonstrated how direct syscalls can be implemented in C++.
XOR Encryption
XOR encryption functions on the principle of bitwise XORing, where each bit of the plaintext code is combined with a corresponding bit from a secret key. Once the payload is prepared for execution, it is decrypted in memory, leaving no trace on the disk that antivirus solutions can detect.
In their tests, the researchers created a Meterpreter reverse shell payload using msfvenom, encrypted it with XOR, and executed it via direct syscalls. This approach successfully bypassed the latest Windows Defender protections without leaving any malicious artifacts on the disk.
Even more alarming, the researchers noted that this technique has been viable since at least 2022, with various modifications, and continues to function effectively in 2025 against the most recent updates of Windows Defender.
While Microsoft has previously addressed similar bypass techniques, asserting they have “limited practical applicability” due to often requiring user interaction, security experts argue otherwise. They contend that such methods can easily be integrated into broader attack chains.
The researchers recommend that Microsoft enhance its defenses by implementing kernel-level monitoring of syscalls, rather than relying solely on user-mode hooks. Additionally, they advise organizations to adopt supplementary security layers beyond Windows Defender, particularly those capable of monitoring behavior at the kernel level.
In light of these developments, security teams are encouraged to implement application whitelisting and restrict administrative privileges to mitigate the risks associated with these advanced bypass techniques.
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