Introduction
Some time ago, I accidentally came across some interesting behaviour in PE files while debugging an unrelated project. I noticed that setting the SectionAlignment value in the NT header to a value lower than the page size (4096) resulted in significant differences in the way that the image is mapped into memory. Rather than following the usual procedure of parsing the section table to construct the image in memory, the loader appeared to map the entire file, including the headers, into memory with read-write-execute (RWX) permissions - the individual section headers were completely ignored.
As a result of this behaviour, it is possible to create a PE executable without any sections, yet still capable of executing its own code. The code can even be self-modifying if necessary due to the write permissions that are present by default.
One way in which this mode could potentially be abused would be to create a fake section table - on first inspection, this would appear to be a normal PE module containing read-write/read-only data sections, but when launched, the seemingly NX data becomes executable.
While I am sure that this technique will have already been discovered (and potentially abused) in the past, I have been unable to find any documentation online describing it. MSDN does briefly mention that the SectionAlignment value can be less than the page size, but it doesn’t elaborate any further on the implications of this.
Inside the Windows kernel
A quick look in the kernel reveals what is happening. Within MiCreateImageFileMap, we can see the parsing of PE headers - notably, if the SectionAlignment value is less than 0x1000, an undocumented flag (0x200000) is set prior to mapping the image into memory:
if(v29->SectionAlignment < 0x1000)
{
if((SectionFlags & 0x80000) != 0)
{
v17 = 0xC000007B;
MiLogCreateImageFileMapFailure(v36, v39, *(unsigned int *)(v29 + 64), DWORD1(v99));
ImageFailureReason = 55;
goto LABEL_81;
}
if(!MiLegacyImageArchitecture((unsigned __int16)v99))
{
v17 = 0xC000007B;
ImageFailureReason = 56;
goto LABEL_81;
}
SectionFlags |= 0x200000;
}
v40 = MiBuildImageControlArea(a3, v38, v29, (unsigned int)&v99, SectionFlags, (__int64)&FileSize, (__int64)&v93);
If the aforementioned flag is set, MiBuildImageControlArea treats the entire file as one single section:
if((SectionFlags & 0x200000) != 0)
{
SectionCount = 1;
}
else
{
SectionCount = a4->NumberOfSections + 1;
}
v12 = MiAllocatePool(64, 8 * (7 * SectionCount + (((unsigned __int64)(unsigned int)MiFlags >> 13) & 1)) + 184, (SectionFlags & 0x200000) != 0 ? 0x61436D4D : 0x69436D4D);
As a result, the raw image is mapped into memory with all PTEs assigned MM_EXECUTE_READWRITE protection. As mentioned previously, the IMAGE_SECTION_HEADER list is ignored, meaning a PE module using this mode can have a NumberOfSections value of 0. There are no obvious size restrictions on PE modules using this mode either - the loader will allocate memory based on the SizeOfImage field and copy the file contents accordingly. Any excess memory beyond the size of the file will remain blank.
Demonstration #1 - Executable PE with no sections
The simplest demonstration of this technique would be to create a generic “loader” for position-independent code. I have created the following sample headers by hand for testing:
// (64-bit EXE headers)
BYTE bHeaders64[328] =
{
0x4D, 0x5A, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x40, 0x00, 0x00, 0x00, 0x50, 0x45, 0x00, 0x00, 0x64, 0x86, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0xF0, 0x00, 0x22, 0x00, 0x0B, 0x02, 0x0E, 0x1D, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x48, 0x01, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x40, 0x01, 0x00, 0x00, 0x00,
0x00, 0x02, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x06, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x06, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x10, 0x00, 0x48, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x02, 0x00, 0x60, 0x81, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00,
// (code goes here)
};
BYTE bHeaders32[304] =
{
0x4D, 0x5A, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x40, 0x00, 0x00, 0x00, 0x50, 0x45, 0x00, 0x00, 0x4C, 0x01, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0xE0, 0x00, 0x02, 0x01, 0x0B, 0x01, 0x0E, 0x1D, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x30, 0x01, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x40, 0x00,
0x00, 0x02, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x06, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x06, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x10, 0x00, 0x30, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x02, 0x00, 0x40, 0x81, 0x00, 0x00, 0x10, 0x00, 0x00, 0x10, 0x00, 0x00,
0x00, 0x00, 0x10, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00,
// (code goes here)
};These headers contain a SectionAlignment value of 0x200 (rather than the usual 0x1000), a SizeOfImage value of 0x100000 (1MB), a blank section table, and an entry-point positioned immediately after the headers. Aside from these values, there is nothing special about the remaining fields:
(DOS Header)
e_magic : 0x5A4D
...
e_lfanew : 0x40
(NT Header)
Signature : 0x4550
Machine : 0x8664
NumberOfSections : 0x0
TimeDateStamp : 0x0
PointerToSymbolTable : 0x0
NumberOfSymbols : 0x0
SizeOfOptionalHeader : 0xF0
Characteristics : 0x22
Magic : 0x20B
MajorLinkerVersion : 0xE
MinorLinkerVersion : 0x1D
SizeOfCode : 0x0
SizeOfInitializedData : 0x0
SizeOfUninitializedData : 0x0
AddressOfEntryPoint : 0x148
BaseOfCode : 0x0
ImageBase : 0x140000000
SectionAlignment : 0x200
FileAlignment : 0x200
MajorOperatingSystemVersion : 0x6
MinorOperatingSystemVersion : 0x0
MajorImageVersion : 0x0
MinorImageVersion : 0x0
MajorSubsystemVersion : 0x6
MinorSubsystemVersion : 0x0
Win32VersionValue : 0x0
SizeOfImage : 0x100000
SizeOfHeaders : 0x148
CheckSum : 0x0
Subsystem : 0x2
DllCharacteristics : 0x8160
SizeOfStackReserve : 0x100000
SizeOfStackCommit : 0x1000
SizeOfHeapReserve : 0x100000
SizeOfHeapCommit : 0x1000
LoaderFlags : 0x0
NumberOfRvaAndSizes : 0x10
DataDirectory[0] : 0x0, 0x0
...
DataDirectory[15] : 0x0, 0x0
(Start of code)
For demonstration purposes, we will be using some position-independent code that calls MessageBoxA. As the base headers lack an import table, this code must locate and load all dependencies manually - user32.dll in this case. This same payload can be used in both 32-bit and 64-bit environments:
BYTE bMessageBox[939] =
{
0x8B, 0xC4, 0x6A, 0x00, 0x2B, 0xC4, 0x59, 0x83, 0xF8, 0x08, 0x0F, 0x84,
0xA0, 0x01, 0x00, 0x00, 0x55, 0x8B, 0xEC, 0x83, 0xEC, 0x3C, 0x64, 0xA1,
0x30, 0x00, 0x00, 0x00, 0x33, 0xD2, 0x53, 0x56, 0x57, 0x8B, 0x40, 0x0C,
0x33, 0xDB, 0x21, 0x5D, 0xF0, 0x21, 0x5D, 0xEC, 0x8B, 0x40, 0x1C, 0x8B,
0x00, 0x8B, 0x78, 0x08, 0x8B, 0x47, 0x3C, 0x8B, 0x44, 0x38, 0x78, 0x03,
0xC7, 0x8B, 0x48, 0x24, 0x03, 0xCF, 0x89, 0x4D, 0xE8, 0x8B, 0x48, 0x20,
0x03, 0xCF, 0x89, 0x4D, 0xE4, 0x8B, 0x48, 0x1C, 0x03, 0xCF, 0x89, 0x4D,
0xF4, 0x8B, 0x48, 0x14, 0x89, 0x4D, 0xFC, 0x85, 0xC9, 0x74, 0x5F, 0x8B,
0x70, 0x18, 0x8B, 0xC1, 0x89, 0x75, 0xF8, 0x33, 0xC9, 0x85, 0xF6, 0x74,
0x4C, 0x8B, 0x45, 0xE8, 0x0F, 0xB7, 0x04, 0x48, 0x3B, 0xC2, 0x74, 0x07,
0x41, 0x3B, 0xCE, 0x72, 0xF0, 0xEB, 0x37, 0x8B, 0x45, 0xE4, 0x8B, 0x0C,
0x88, 0x03, 0xCF, 0x74, 0x2D, 0x8A, 0x01, 0xBE, 0x05, 0x15, 0x00, 0x00,
0x84, 0xC0, 0x74, 0x1F, 0x6B, 0xF6, 0x21, 0x0F, 0xBE, 0xC0, 0x03, 0xF0,
0x41, 0x8A, 0x01, 0x84, 0xC0, 0x75, 0xF1, 0x81, 0xFE, 0xFB, 0xF0, 0xBF,
0x5F, 0x75, 0x74, 0x8B, 0x45, 0xF4, 0x8B, 0x1C, 0x90, 0x03, 0xDF, 0x8B,
0x75, 0xF8, 0x8B, 0x45, 0xFC, 0x42, 0x3B, 0xD0, 0x72, 0xA9, 0x8D, 0x45,
0xC4, 0xC7, 0x45, 0xC4, 0x75, 0x73, 0x65, 0x72, 0x50, 0x66, 0xC7, 0x45,
0xC8, 0x33, 0x32, 0xC6, 0x45, 0xCA, 0x00, 0xFF, 0xD3, 0x8B, 0xF8, 0x33,
0xD2, 0x8B, 0x4F, 0x3C, 0x8B, 0x4C, 0x39, 0x78, 0x03, 0xCF, 0x8B, 0x41,
0x20, 0x8B, 0x71, 0x24, 0x03, 0xC7, 0x8B, 0x59, 0x14, 0x03, 0xF7, 0x89,
0x45, 0xE4, 0x8B, 0x41, 0x1C, 0x03, 0xC7, 0x89, 0x75, 0xF8, 0x89, 0x45,
0xE8, 0x89, 0x5D, 0xFC, 0x85, 0xDB, 0x74, 0x7D, 0x8B, 0x59, 0x18, 0x8B,
0x45, 0xFC, 0x33, 0xC9, 0x85, 0xDB, 0x74, 0x6C, 0x0F, 0xB7, 0x04, 0x4E,
0x3B, 0xC2, 0x74, 0x22, 0x41, 0x3B, 0xCB, 0x72, 0xF3, 0xEB, 0x5A, 0x81,
0xFE, 0x6D, 0x07, 0xAF, 0x60, 0x8B, 0x75, 0xF8, 0x75, 0x8C, 0x8B, 0x45,
0xF4, 0x8B, 0x04, 0x90, 0x03, 0xC7, 0x89, 0x45, 0xEC, 0xE9, 0x7C, 0xFF,
0xFF, 0xFF, 0x8B, 0x45, 0xE4, 0x8B, 0x0C, 0x88, 0x03, 0xCF, 0x74, 0x35,
0x8A, 0x01, 0xBE, 0x05, 0x15, 0x00, 0x00, 0x84, 0xC0, 0x74, 0x27, 0x6B,
0xF6, 0x21, 0x0F, 0xBE, 0xC0, 0x03, 0xF0, 0x41, 0x8A, 0x01, 0x84, 0xC0,
0x75, 0xF1, 0x81, 0xFE, 0xB4, 0x14, 0x4F, 0x38, 0x8B, 0x75, 0xF8, 0x75,
0x10, 0x8B, 0x45, 0xE8, 0x8B, 0x04, 0x90, 0x03, 0xC7, 0x89, 0x45, 0xF0,
0xEB, 0x03, 0x8B, 0x75, 0xF8, 0x8B, 0x45, 0xFC, 0x42, 0x3B, 0xD0, 0x72,
0x89, 0x33, 0xC9, 0xC7, 0x45, 0xC4, 0x54, 0x65, 0x73, 0x74, 0x51, 0x8D,
0x45, 0xC4, 0x88, 0x4D, 0xC8, 0x50, 0x50, 0x51, 0xFF, 0x55, 0xF0, 0x6A,
0x7B, 0x6A, 0xFF, 0xFF, 0x55, 0xEC, 0x5F, 0x5E, 0x5B, 0xC9, 0xC3, 0x90,
0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90,
0x48, 0x89, 0x5C, 0x24, 0x08, 0x48, 0x89, 0x6C, 0x24, 0x10, 0x48, 0x89,
0x74, 0x24, 0x18, 0x48, 0x89, 0x7C, 0x24, 0x20, 0x41, 0x54, 0x41, 0x56,
0x41, 0x57, 0x48, 0x83, 0xEC, 0x40, 0x65, 0x48, 0x8B, 0x04, 0x25, 0x60,
0x00, 0x00, 0x00, 0x33, 0xFF, 0x45, 0x33, 0xFF, 0x45, 0x33, 0xE4, 0x45,
0x33, 0xC9, 0x48, 0x8B, 0x48, 0x18, 0x48, 0x8B, 0x41, 0x30, 0x48, 0x8B,
0x08, 0x48, 0x8B, 0x59, 0x10, 0x48, 0x63, 0x43, 0x3C, 0x8B, 0x8C, 0x18,
0x88, 0x00, 0x00, 0x00, 0x48, 0x03, 0xCB, 0x8B, 0x69, 0x24, 0x44, 0x8B,
0x71, 0x20, 0x48, 0x03, 0xEB, 0x44, 0x8B, 0x59, 0x1C, 0x4C, 0x03, 0xF3,
0x8B, 0x71, 0x14, 0x4C, 0x03, 0xDB, 0x85, 0xF6, 0x0F, 0x84, 0x80, 0x00,
0x00, 0x00, 0x44, 0x8B, 0x51, 0x18, 0x33, 0xC9, 0x45, 0x85, 0xD2, 0x74,
0x69, 0x48, 0x8B, 0xD5, 0x0F, 0x1F, 0x40, 0x00, 0x0F, 0xB7, 0x02, 0x41,
0x3B, 0xC1, 0x74, 0x0D, 0xFF, 0xC1, 0x48, 0x83, 0xC2, 0x02, 0x41, 0x3B,
0xCA, 0x72, 0xED, 0xEB, 0x4D, 0x45, 0x8B, 0x04, 0x8E, 0x4C, 0x03, 0xC3,
0x74, 0x44, 0x41, 0x0F, 0xB6, 0x00, 0x33, 0xD2, 0xB9, 0x05, 0x15, 0x00,
0x00, 0x84, 0xC0, 0x74, 0x35, 0x0F, 0x1F, 0x00, 0x6B, 0xC9, 0x21, 0x8D,
0x52, 0x01, 0x0F, 0xBE, 0xC0, 0x03, 0xC8, 0x42, 0x0F, 0xB6, 0x04, 0x02,
0x84, 0xC0, 0x75, 0xEC, 0x81, 0xF9, 0xFB, 0xF0, 0xBF, 0x5F, 0x75, 0x08,
0x41, 0x8B, 0x3B, 0x48, 0x03, 0xFB, 0xEB, 0x0E, 0x81, 0xF9, 0x6D, 0x07,
0xAF, 0x60, 0x75, 0x06, 0x45, 0x8B, 0x23, 0x4C, 0x03, 0xE3, 0x41, 0xFF,
0xC1, 0x49, 0x83, 0xC3, 0x04, 0x44, 0x3B, 0xCE, 0x72, 0x84, 0x48, 0x8D,
0x4C, 0x24, 0x20, 0xC7, 0x44, 0x24, 0x20, 0x75, 0x73, 0x65, 0x72, 0x66,
0xC7, 0x44, 0x24, 0x24, 0x33, 0x32, 0x44, 0x88, 0x7C, 0x24, 0x26, 0xFF,
0xD7, 0x45, 0x33, 0xC9, 0x48, 0x8B, 0xD8, 0x48, 0x63, 0x48, 0x3C, 0x8B,
0x94, 0x01, 0x88, 0x00, 0x00, 0x00, 0x48, 0x03, 0xD0, 0x8B, 0x7A, 0x24,
0x8B, 0x6A, 0x20, 0x48, 0x03, 0xF8, 0x44, 0x8B, 0x5A, 0x1C, 0x48, 0x03,
0xE8, 0x8B, 0x72, 0x14, 0x4C, 0x03, 0xD8, 0x85, 0xF6, 0x74, 0x77, 0x44,
0x8B, 0x52, 0x18, 0x0F, 0x1F, 0x44, 0x00, 0x00, 0x33, 0xC0, 0x45, 0x85,
0xD2, 0x74, 0x5B, 0x48, 0x8B, 0xD7, 0x66, 0x0F, 0x1F, 0x44, 0x00, 0x00,
0x0F, 0xB7, 0x0A, 0x41, 0x3B, 0xC9, 0x74, 0x0D, 0xFF, 0xC0, 0x48, 0x83,
0xC2, 0x02, 0x41, 0x3B, 0xC2, 0x72, 0xED, 0xEB, 0x3D, 0x44, 0x8B, 0x44,
0x85, 0x00, 0x4C, 0x03, 0xC3, 0x74, 0x33, 0x41, 0x0F, 0xB6, 0x00, 0x33,
0xD2, 0xB9, 0x05, 0x15, 0x00, 0x00, 0x84, 0xC0, 0x74, 0x24, 0x66, 0x90,
0x6B, 0xC9, 0x21, 0x8D, 0x52, 0x01, 0x0F, 0xBE, 0xC0, 0x03, 0xC8, 0x42,
0x0F, 0xB6, 0x04, 0x02, 0x84, 0xC0, 0x75, 0xEC, 0x81, 0xF9, 0xB4, 0x14,
0x4F, 0x38, 0x75, 0x06, 0x45, 0x8B, 0x3B, 0x4C, 0x03, 0xFB, 0x41, 0xFF,
0xC1, 0x49, 0x83, 0xC3, 0x04, 0x44, 0x3B, 0xCE, 0x72, 0x92, 0x45, 0x33,
0xC9, 0xC7, 0x44, 0x24, 0x20, 0x54, 0x65, 0x73, 0x74, 0x4C, 0x8D, 0x44,
0x24, 0x20, 0xC6, 0x44, 0x24, 0x24, 0x00, 0x48, 0x8D, 0x54, 0x24, 0x20,
0x33, 0xC9, 0x41, 0xFF, 0xD7, 0xBA, 0x7B, 0x00, 0x00, 0x00, 0x48, 0xC7,
0xC1, 0xFF, 0xFF, 0xFF, 0xFF, 0x41, 0xFF, 0xD4, 0x48, 0x8B, 0x5C, 0x24,
0x60, 0x48, 0x8B, 0x6C, 0x24, 0x68, 0x48, 0x8B, 0x74, 0x24, 0x70, 0x48,
0x8B, 0x7C, 0x24, 0x78, 0x48, 0x83, 0xC4, 0x40, 0x41, 0x5F, 0x41, 0x5E,
0x41, 0x5C, 0xC3
};As a side note, several readers have asked how I created this sample code (previously used in another project) which works correctly in both 32-bit and 64-bit modes. The answer is very simple: it begins by storing the original stack pointer value, pushes a value onto the stack, and compares the new stack pointer to the original value. If the difference is 8, the 64-bit code is executed - otherwise, the 32-bit code is executed. While there are certainly more efficient approaches to achieve this outcome, this method is sufficient for demonstration purposes:
mov eax, esp ; store stack ptr
push 0 ; push a value onto the stack
sub eax, esp ; calculate difference
pop ecx ; restore stack
cmp eax, 8 ; check if the difference is 8
je 64bit_code
32bit_code:
xxxx
64bit_code:
xxxx
By appending this payload to the original headers above, we can generate a valid and functional EXE file. The provided PE headers contain a hardcoded SizeOfImage value of 0x100000 which allows for a maximum payload size of almost 1MB, but this can be increased if necessary. Running this program will display our message box, despite the fact that the PE headers lack any executable sections, or any sections at all in this case:
Demonstration #2 - Executable PE with spoofed sections
Perhaps more interestingly, it is also possible to create a fake section table using this mode as mentioned earlier. I have created another EXE which follows a similar format to the previous samples, but also includes a single read-only section:
The main payload has been stored within this read-only section and the entry-point has been updated to 0x1000. Under normal circumstances, you would expect the program to crash immediately with an access-violation exception due to attempting to execute read-only memory. However, this doesn’t occur here - the target memory region contains RWX permissions and the payload is executed successfully:
Notes
The sample EXE files can be downloaded here.
The proof-of-concepts described above involve appending the payload to the end of the NT headers, but it is also possible to embed executable code within the headers themselves using this technique. The module will fail to load if the AddressOfEntryPoint value is less than the SizeOfHeaders value, but this can easily be bypassed since the SizeOfHeaders value is not strictly enforced. It can even be set to 0, allowing the entry-point to be positioned anywhere within the file.
It is possible that this feature was initially designed to allow for very small images, enabling the headers, code, and data to fit within a single memory page. As memory protection is applied per-page, it makes sense to apply RWX to all PTEs when the virtual section size is lower than the page size - it would otherwise be impossible to manage protections correctly if multiple sections resided within a single page.
I have tested these EXE files on various different versions of Windows from Vista to 10 with success in all cases. Unfortunately it has very little practical use in the real world as it won’t deceive any modern disassemblers - nonetheless, it remains an interesting concept.