Application Security Testing: An Integral Part of DevOps
The Portable Executable Format is the data structure that describes how the various parts of a Win32 executable file are held together. It allows the operating system to load the executable and locate the dynamically linked libraries required to run that executable and to navigate the code, data, and resource sections compiled into that executable.
Getting Over DOS
The PE Format was created for Windows but Microsoft had to make sure that running such an executable in DOS would yield a meaningful error message and exit. To this end, the very first bit of a Windows executable file is actually a DOS executable (sometimes known as the stub) that writes "This program requires Windows" or similar, then exits.
Private Type IMAGE_DOS_HEADER e_magic As Integer ''\\ Magic number e_cblp As Integer ''\\ Bytes on last page of file e_cp As Integer ''\\ Pages in file e_crlc As Integer ''\\ Relocations e_cparhdr As Integer ''\\ Size of header in paragraphs e_minalloc As Integer ''\\ Minimum extra paragraphs needed e_maxalloc As Integer ''\\ Maximum extra paragraphs needed e_ss As Integer ''\\ Initial (relative) SS value e_sp As Integer ''\\ Initial SP value e_csum As Integer ''\\ Checksum e_ip As Integer ''\\ Initial IP value e_cs As Integer ''\\ Initial (relative) CS value e_lfarlc As Integer ''\\ File address of relocation table e_ovno As Integer ''\\ Overlay number e_res(0 To 3) As Integer ''\\ Reserved words e_oemid As Integer ''\\ OEM identifier (for e_oeminfo) e_oeminfo As Integer ''\\ OEM information; e_oemid specific e_res2(0 To 9) As Integer ''\\ Reserved words e_lfanew As Long ''\\ File address of new exe header End Type
The only field of this structure that is of interest to Windows is e_lfanew; it is the file pointer to the new Windows executable header. To skip over the DOS part of the program, set the file pointer to the value held in this field:
Private Sub SkipDOSStub(ByVal hfile As Long) Dim BytesRead As Long '\\ Go to start of file... Call SetFilePointer(hfile, 0, 0, FILE_BEGIN) If Err.LastDllError Then Debug.Print LastSystemError End If Dim stub As IMAGE_DOS_HEADER Call ReadFileLong(hfile, VarPtr(stub), Len(stub), BytesRead, ByVal 0&) Call SetFilePointer(hfile, stub.e_lfanew, 0, FILE_BEGIN) End Sub
The NT Header
The NT header holds the information needed by the Windows program loader to load the program. It consists of the PE File signature followed by an IMAGE_FILE_HEADER and IMAGE_OPTIONAL_HEADER records.
For applications designed to run under Windows (in other words, not OS/2 or VxD files) the four bytes of the PE File signature should equal &h4550. The other defined signatures are:
Public Enum ImageSignatureTypes IMAGE_DOS_SIGNATURE = &H5A4D ''\\ MZ IMAGE_OS2_SIGNATURE = &H454E ''\\ NE IMAGE_OS2_SIGNATURE_LE = &H454C ''\\ LE IMAGE_VXD_SIGNATURE = &H454C ''\\ LE IMAGE_NT_SIGNATURE = &H4550 ''\\ PE00 End Enum
Following the PE file signature is the IMAGE_NT_HEADERS structure that stores information about the target environment of the executable. The structure is:
Private Type IMAGE_FILE_HEADER Machine As Integer NumberOfSections As Integer TimeDateStamp As Long PointerToSymbolTable As Long NumberOfSymbols As Long SizeOfOptionalHeader As Integer Characteristics As Integer End Type
The Machine member describes what target CPU the executable was compiled for. It can be one of the following:
Public Enum ImageMachineTypes IMAGE_FILE_MACHINE_I386 = &H14C ''\\ Intel 386. ''\\ MIPS little-endian, = &H160 big-endian IMAGE_FILE_MACHINE_R3000 = &H162 IMAGE_FILE_MACHINE_R4000 = &H166 ''\\ MIPS little-endian IMAGE_FILE_MACHINE_R10000 = &H168 ''\\ MIPS little-endian IMAGE_FILE_MACHINE_WCEMIPSV2 = &H169 ''\\ MIPS little-endian WCE v2 IMAGE_FILE_MACHINE_ALPHA = &H184 ''\\ Alpha_AXP IMAGE_FILE_MACHINE_POWERPC = &H1F0 ''\\ IBM PowerPC Little-Endian IMAGE_FILE_MACHINE_SH3 = &H1A2 ''\\ SH3 little-endian IMAGE_FILE_MACHINE_SH3E = &H1A4 ''\\ SH3E little-endian IMAGE_FILE_MACHINE_SH4 = &H1A6 ''\\ SH4 little-endian IMAGE_FILE_MACHINE_ARM = &H1C0 ''\\ ARM Little-Endian IMAGE_FILE_MACHINE_IA64 = &H200 ''\\ Intel 64 End Enum
The SizeOfOptionalHeader member indicates the size (in bytes) of the IMAGE_OPTIONAL_HEADER structure that immediately follows it. In practice, this structure is not optional, so that is a bit of a misnomer. This structure is defined as:
Private Type IMAGE_OPTIONAL_HEADER Magic As Integer MajorLinkerVersion As Byte MinorLinkerVersion As Byte SizeOfCode As Long SizeOfInitializedData As Long SizeOfUninitializedData As Long AddressOfEntryPoint As Long BaseOfCode As Long BaseOfData As Long End Type
and this in turn is immediately followed by the IMAGE_OPTIONAL_HEADER_NT structure:
Private Type IMAGE_OPTIONAL_HEADER_NT ImageBase As Long SectionAlignment As Long FileAlignment As Long MajorOperatingSystemVersion As Integer MinorOperatingSystemVersion As Integer MajorImageVersion As Integer MinorImageVersion As Integer MajorSubsystemVersion As Integer MinorSubsystemVersion As Integer Win32VersionValue As Long SizeOfImage As Long SizeOfHeaders As Long CheckSum As Long Subsystem As Integer DllCharacteristics As Integer SizeOfStackReserve As Long SizeOfStackCommit As Long SizeOfHeapReserve As Long SizeOfHeapCommit As Long LoaderFlags As Long NumberOfRvaAndSizes As Long DataDirectory(0 To 15) As IMAGE_DATA_DIRECTORY End Type
The most useful field of this structure (to my purposes, anyhow) are the 16 IMAGE_DATA_DIRECTORY entries. These describe where (if at all) the particular sections of the executable are located. The structure is defined thus:
Private Type IMAGE_DATA_DIRECTORY VirtualAddress As Long Size As Long End Type
And the directories are held in order, thus:
Public Enum ImageDataDirectoryIndexes IMAGE_DIRECTORY_ENTRY_EXPORT = 0 ''\\ Export Directory IMAGE_DIRECTORY_ENTRY_IMPORT = 1 ''\\ Import Directory IMAGE_DIRECTORY_ENTRY_RESOURCE = 2 ''\\ Resource Directory IMAGE_DIRECTORY_ENTRY_EXCEPTION = 3 ''\\ Exception Directory IMAGE_DIRECTORY_ENTRY_SECURITY = 4 ''\\ Security Directory IMAGE_DIRECTORY_ENTRY_BASERELOC = 5 ''\\ Base Relocation Table IMAGE_DIRECTORY_ENTRY_DEBUG = 6 ''\\ Debug Directory IMAGE_DIRECTORY_ENTRY_ARCHITECTURE = 7 ''\\ Architecture Specific Data IMAGE_DIRECTORY_ENTRY_GLOBALPTR = 8 ''\\ RVA of GP IMAGE_DIRECTORY_ENTRY_TLS = 9 ''\\ TLS Directory ''\\ Load Configuration Directory IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG = 10 ''\\ Bound Import Directory in headers IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT = 11 IMAGE_DIRECTORY_ENTRY_IAT = 12 ''\\ Import Address Table ''\\ Delay Load Import Descriptors IMAGE_DIRECTORY_ENTRY_DELAY_IMPORT = 13 End Enum
Note: If an executable does not contain one of the sections (as is often the case), there will be an IMAGE_DATA_DIRECTORY for it, but the address and size will both be zero.
The Image Data Directories
The exports directory
The exports directory holds details of the functions exported by this executable. For example, if you were to look in the exports directory of the MSVBVM50.dll it would list all the functions it exports, that make up the Visual Basic 5 runtime environment.
This directory consists of some info to tell you how many exported functions there are, followed by three parallel arrays that give you the address, name, and ordinal of the functions respectively. The structure is defined thus:
Private Type IMAGE_EXPORT_DIRECTORY Characteristics As Long TimeDateStamp As Long MajorVersion As Integer MinorVersion As Integer lpName As Long Base As Long NumberOfFunctions As Long NumberOfNames As Long lpAddressOfFunctions As Long '\\ Three parallel arrays...(LONG) lpAddressOfNames As Long '\\ (LONG) lpAddressOfNameOrdinals As Long '\\ (INTEGER) End Type
And you can read this info from the executable thus:
Private Sub ProcessExportTable(ExportDirectory As IMAGE_DATA_DIRECTORY) Dim deThis As IMAGE_EXPORT_DIRECTORY Dim lBytesWritten As Long Dim lpAddress As Long Dim nFunction As Long If ExportDirectory.VirtualAddress > 0 And ExportDirectory.Size > 0 Then '\\ Get the true address from the RVA lpAddress = AbsoluteAddress(ExportDirectory.VirtualAddress) '\\ Copy the image_export_directory structure... Call ReadProcessMemoryLong(DebugProcess.Handle, lpAddress, _ VarPtr(deThis), Len(deThis), lBytesWritten) With deThis If .lpName <> 0 Then image.Name = StringFromOutOfProcessPointer(DebugProcess.Handle,_ image.AbsoluteAddress(.lpName), 32, False) End If If .NumberOfFunctions > 0 Then For nFunction = 1 To .NumberOfFunctions lpAddress = LongFromOutOfprocessPointer_ (DebugProcess.Handle, _ image.AbsoluteAddress(.lpAddressOfNames)_ + ((nFunction - 1) * 4)) fExport.Name = StringFromOutOfProcessPointer_ (DebugProcess.Handle, _ image.AbsoluteAddress(lpAddress), 64, False) fExport.Ordinal = .Base + _ IntegerFromOutOfprocessPointer(DebugProcess.Handle, _ image.AbsoluteAddress(.lpAddressOfNameOrdinals) + _ ((nFunction - 1) * 2)) fExport.ProcAddress = LongFromOutOfprocessPointer_ (DebugProcess.Handle,_ image.AbsoluteAddress(.lpAddressOfFunctions) + _ ((nFunction - 1) * 4)) Next nFunction End If End With End If End Sub
The Imports Directory
The imports directory lists the dynamic link libraries that this executable depends on and which functions it imports from that dynamic link library. It consists of an array of IMAGE_IMPORT_DESCRIPTOR structures terminated by an instance of this structure where the lpName parameter is zero. The structure is defined as:
Private Type IMAGE_IMPORT_DESCRIPTOR lpImportByName As Long ''\\ 0 for terminating null import descriptor TimeDateStamp As Long ''\\ 0 if not bound, ''\\ -1 if bound, and real date\time stamp ''\\ in IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT ''\\ (new BIND) ''\\ O.W. date/time stamp of DLL bound to ''\\ (Old BIND) ForwarderChain As Long ''\\ -1 if no forwarders lpName As Long ''\\ RVA to IAT (if bound this IAT has actual addresses) lpFirstThunk As Long End Type
And you can walk the import directory thus:
Private Sub ProcessImportTable(ImportDirectory As IMAGE_DATA_DIRECTORY) Dim lpAddress As Long Dim diThis As IMAGE_IMPORT_DESCRIPTOR Dim byteswritten As Long Dim sName As String Dim lpNextName As Long Dim lpNextThunk As Long Dim lImportEntryIndex As Long Dim nOrdinal As Integer Dim lpFuncAddress As Long '\\ If the image has an imports section... If ImportDirectory.VirtualAddress > 0 And ImportDirectory.Size > 0 Then '\\ Get the true address from the RVA lpAddress = AbsoluteAddress(ImportDirectory.VirtualAddress) Call ReadProcessMemoryLong(DebugProcess.Handle, lpAddress, _ VarPtr(diThis), Len(diThis), byteswritten) While diThis.lpName <> 0 '\\ Process this import directory entry sName = StringFromOutOfProcessPointer(DebugProcess.Handle, _ image.AbsoluteAddress(diThis.lpName), 32, False) '\\ Process the import file's functions list If diThis.lpImportByName <> 0 Then lpNextName = LongFromOutOfprocessPointer(DebugProcess.Handle,_ image.AbsoluteAddress(diThis.lpImportByName)) lpNextThunk = LongFromOutOfprocessPointer(DebugProcess.Handle,_ image.AbsoluteAddress(diThis.lpFirstThunk)) While (lpNextName <> 0) And (lpNextThunk <> 0) '\\ get the function address lpFuncAddress = LongFromOutOfprocessPointer_ (DebugProcess.Handle, lpNextThunk) nOrdinal = IntegerFromOutOfprocessPointer_ (DebugProcess.Handle, lpNextName) '\\ Skip the two-byte ordinal hint lpNextName = lpNextName + 2 '\\ Get this function's name sName = StringFromOutOfProcessPointer(DebugProcess.Handle, _ image.AbsoluteAddress(lpNextName), 64, False) If Trim$(sName) <> "" Then '\\ Get the next imported function... lImportEntryIndex = lImportEntryIndex + 1 lpNextName = LongFromOutOfprocessPointer_ (DebugProcess.Handle, _ image.AbsoluteAddress(diThis.lpImportByName _ + (lImportEntryIndex * 4))) lpNextThunk = LongFromOutOfprocessPointer_ (DebugProcess.Handle,_ image.AbsoluteAddress(diThis.lpFirstThunk_ + (lImportEntryIndex * 4))) Else lpNextName = 0 End If Wend End If '\\ And get the next one lpAddress = lpAddress + Len(diThis) Call ReadProcessMemoryLong(DebugProcess.Handle, lpAddress, _ VarPtr(diThis), Len(diThis), byteswritten) Wend End If End Sub
The Resource Directory
The structure of the resource directory is somewhat more involved. It consists of a root directory (defined by the structure IMAGE_RESOURCE_DIRECTORY) immediately followed by a number of resource directory entries (defined by the structure IMAGE_RESOURCE_DIRECTORY_ENTRY). These are defined thus:
Private Type IMAGE_RESOURCE_DIRECTORY Characteristics As Long '\\Seems to be always zero? TimeDateStamp As Long MajorVersion As Integer MinorVersion As Integer NumberOfNamedEntries As Integer NumberOfIdEntries As Integer End Type Private Type IMAGE_RESOURCE_DIRECTORY_ENTRY dwName As Long dwDataOffset As Long CodePage As Long Reserved As Long End Type
Each resource directory entry can either point to the actual resource data or to another layer of resource directory entries. If the highest bit of dwDataOffset is set, this points to a directory. Otherwise, it points to the resource data.
How Is This Information Useful?
Once you know how an executable is put together, you can use this information to peer into its workings. You can view the resources compiled into it, the DLLs it depends on, and the actual functions it imports from them. More importantly, you can attach a debugger to the executable and track down any of those really troublesome general protection faults.