Windows Defender Anti-virus Bypassed Using Direct Syscalls & XOR Encryption
"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."
Additional Information:
https://www.hackmosphere.fr/bypass-windows-defender-antivirus-2025-part-1/
https://www.hackmosphere.fr/bypass-windows-defender-antivirus-2025-part-2/
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Anything like this for the typical home user?
Sure, bring back Crowdstrike, that went well...
Btw I wasn't aware XOR was encrytion...
XOR cleartext once with a key you get ciphertext. XOR the ciphertext with the same key you get the original cleartext. At its core this is the way the old DES cipher works.
A bit of useful trivia: If you XOR any number with itself, you get all zeros. You can see this in practice when an assembly programmer XOR's a register with itself to clear it out.
That's how it was done in the old days to save a few cycles in Z80 assembly. XOR A instead of LD A, 0.
I use that daily in my accelerator work.
Once you learn the trick, you just use it naturally.
Apple had this undocumented function for screenshotting back on iOS 3.1, and kind of let you use it while waiting for better frameworks in iOS 4.0
At some point they started rejecting your app automatically if they found the symbol for that function in your app. I didn't want to leave my 3.1 users in the dust for no reason, so I did the same trick to obfuscate the symbol name before dynamically linking it in.
It worked right up until they stopped supporting iOS 3.1 completely.
Ftfy. not all CPUs have an xor register with itself instruction.
There are a lot more architectures than just x86 that are capable of XORing a register with itself (ie. ARM and RISC-V), and if you took OP to mean the accumulation register specifically, pretty much all CPUs going back as far as I can think have had that functionality.
Yes, but it's not universal that xoring a register with itself is more performant than simply loading it with 0.
I never made that claim, nor did the person you corrected.
Yes, but that's why x86 assembly programmers do it...
https://en.wikipedia.org/wiki/One_time_pad
XOR may be the only encryption system that cannot be cracked. The length of the key is a PITA though.
It technically counts. It's a cipher that uses the same key for encryption and decryption.
A one time pad, I think it called.
It's even better than ROT13, because you always need to apply ROT13 twice for getting the good results...
I cannot find a link to the original research, anybody has the link to the original research?
https://www.hackmosphere.fr/bypass-windows-defender-antivirus-2025-part-1/
https://www.hackmosphere.fr/bypass-windows-defender-antivirus-2025-part-2/
This is what I could find.
Wasn’t there something a few months ago about Microsoft handing out secret API calls to developers of other antivirus products so they can quietly disable Defender during the installation of their product? Some guy had this reverse engineered from an installer…
It’s not a secret. It’s a regkey. You need privs to do it though.
Saw this attack in the wild this week. Huntress MDR detected and shut it down.
That Vitus must be pretty bad.
Thanks! Fixed!