電腦攻擊與防禦

The Attack and Defense of Computers

Dr. 許 富 皓

Rootkit

for Windows by [Bryce Cogswell et al. ]

Categories of Rootkits – Windows

User-mode RootkitsKernel-mode Rootkits

User-mode Rootkits

Windows API [wikipedia]

The Windows API, informally WinAPI, is the name given by Microsoft to the core set of application programming interfaces available in the Microsoft Windows operating systems. It is designed for use by C/C++ programs and is the most direct way to interact with a Windows system for software applications.

Windows API [developerfusion]

As you know, windows can do lots of things. Manage hardware, run programs, display icons. Much of these functions are carried out by DLL files. DLLs (Dynamic Linked Libraries) store functions, so other programs can access them.

The advantage of using DLLs is that the same file can be accessed at the same time by different programs.

The functions stored in the windows DLLs are called Windows API.

Native API[wikipedia]

Lower level access to a Windows system, mostly required for device drivers, is provided by the Native API in current versions of Windows.

Windows Library Files -- user32.dll [answers.com]

user32.dll is a DLL that implements the Windows User API Client Library. It is a core file for several versions of the Microsoft Windows operating system. If this file is damaged or deleted, the operating system will not work.

Windows Library Files -- ntdll.dll [answers.com]

The Native API (with capitalized N) is the publicly mostly undocumented application programming interface used internally by the Windows NT family of operating systems produced by Microsoft.Most of them are in ntdll.dll and ntoskrnl.exe (and it's variants).

User-mode Rootkits – Utilizing Windows APIs

A user-mode rootkit might intercept all calls to the Windows FindFirstFile/FindNextFile APIs, which are used by file system exploration utilities, including Explorer and the command prompt, to enumerate the contents of file system directories. When an application performs a directory listing that would otherwise return results that contain entries identifying the files associated with the rootkit, the rootkit intercepts and modifies the output to remove the entries.

API Hooking [craigheffner]

In Windows, all applications must communicate with the kernel through API functions; as such, these functions are critical to even the simplest Windows application. Thus, the ability to intercept, monitor, and modify a program's API calls, commonly called API hooking, effectively gives one full control over that process. This can be useful for a multitude of reasons, including debugging, reverse engineering, and hacking.

Intercept API CallsWhile there are several methods which can be used to intercept, monitor, and modify a program's API calls, one of them is DLL redirection.

DLL Redirection [craigheffner]

Since an executable imports API functions from DLL files, DLL redirection allows us to tell a program that the DLLs it needs are located in a different directory than the originals; in this way we can create a DLL with the same name as the original, which exports the same function names as the original, but each function may contain whatever code we like.

User-mode Rootkits – Utilizing Windows Native APIs

More sophisticated user-mode rootkits intercept file system, Registry, and process enumeration functions of the Native API.This prevents their detection by scanners that compare the results of a Windows API enumeration with that returned by a Native API enumeration.

Registry [Microsoft] A central hierarchical database used in Microsoft Windows 9x, Windows CE, Windows NT, and Windows 2000 used to store information necessary to configure

the system for one or more users. applications. hardware devices.

Registry data is stored in binary files.

Information Contained in the Registry [Microsoft]

The Registry contains information that Windows continually references during operation, such as

profiles for each user. the applications installed on the computer.the types of documents that each can create.property sheet settings for folders and application icons.what hardware exists on the system and the ports that are being used.

Description of the Registry [Microsoft]

The Registry replaces most of the text-based .ini files used in Windows 3.x and MS-DOS configuration files, such as the Autoexec.bat and Config.sys.Although the Registry is common to several Windows operating systems, there are some differences among them.

What Does the Registry Look Like -- [Tim Smith] ?

The Registry is stored on your hard disk in several files but the only way to look at it and make changes is to use the regedit program.

To access this, click on the Start Button and then on the Run option. Type regedit into the box that appears and press Enter. This will launch regedit and you will now have your first sight of the Registry.

Organization of Registry [Tim Smith]

The Registry is organized much like the files on a disk and will look familiar if you have ever used the Folders view in Windows Explorer. In the Registry, however, these folders are called keys.

To open a key, simply click on the small plus (+) symbol next to it. You will then see that each key contains either more keys - called subkeys - or values.

Key Overview [Tim Smith]

The keys are organized logically but there are thousands of them, which can be daunting the first time you sneak a peek at the Registry. To simplify things, be aware that there are five root keys and these are the basic building blocks of the Registry.

Predefined Keys [Microsoft]

What follows is the predefined keys that are used by the system. HKEY_CURRENT_USER (abbr. HKCU) HKEY_USERS (abbr. HKU) HKEY_LOCAL_MACHINE (abbr. HKLM) HKEY_CLASSES_ROOT (abbr. HKCR) HKEY_CURRENT_CONFIG (abbr. HKCC)

• The maximum size of a key name is 255 characters.

Key Value [wikipedia]

Each of the predefined keys is divided into subkeys, which may contain further subkeys, and so on. Any key may contain values. These values can be:

String Value Binary Value (0 and 1's)DWORD Value, a 32 bit unsigned integer (numbers between 0 and 4,294,967,295 [232 – 1]) Multi-String value Expandable String Value

Key Hierarchy [wikipedia]

Each key has a default value, which is in effect a value with the same name as the key. Registry keys and values are specified with a syntax similar to Windows' filenames, using backslashes to indicate levels of hierarchy.

E.g. HKEY_LOCAL_MACHINE\Software\Microsoft\Windows refers to the subkey "Windows" of the subkey "Microsoft" of the subkey "Software" of the HKEY_LOCAL_MACHINE key.

Example (1)

HKCU has subkeys and values. By pressing the + before the HKCU you can see its subkeys.

Example (2)

Key Specifying Applications to Run When a User Logs in [wikipedia]

HKLM\Software\Microsoft\Windows\CurrentVersion\Run (and the HKCU equivalent) specifies applications to run whenever a user logs in. These can include desirable programs, such as printer monitoring programs or frequently-used tools, but a lot of malware uses this registry key to ensure it is automatically run. This key is a good place to start looking for evidence of malware if you think your computer has been infected.

Example

Spyware and Registry [Tim Smith] Spyware often installs values in the Registry to make sure that it's launched to monitor your computer when Windows starts up.

When looking for advice on how to remove these programs you may be told to edit the Registry. Always make sure that the advice is coming from a trustworthy source such as Registry Guide for Windows or Systweak.com.

Sometimes the spyware also installs a small program to monitor the Registry and replace keys that you delete, so you should use software such as Spybot Search and Destroy to clean your computer entirely.

Kernel-mode Rootkits

Kernel-mode RootkitsKernel-mode rootkits can be even more powerful since, not only can they intercept the Native API in kernel-mode, but they can also directly manipulate kernel-mode data structures. A common technique for hiding the presence of a malware process is to remove the process from the kernel's list of active processes. Since process management APIs rely on the contents of the list, the malware process will not display in process management tools like Task Manager or Process Explorer.

Rootkit Techniques by [Ivo Ivanov]

Techniques InvolvedStep 1: Injecting techniques Step 2: Interception Mechanisms

Injecting Techniques [Ivo Ivanov]

Injecting TechniquesMethod 1: Registry Method 2: Global Windows HooksOther Methods(omitted in this lecture)

Injecting DLL by using CreateRemoteThread() API functionImplanting through BHO add-insMS Office add-ins

Inject by Registry

In order to inject a DLL into processes that link with USER32.DLL, you simply can add the DLL name to the value of the following registry key:

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_DLLs Its value contains a single DLL name or group of DLLs separated either by comma or spaces.According to MSDN documentation, all DLLs specified by the value of that key are loaded by each Windows-based application running within the current logon session.

Inject a DLL into Processes

Invoke Registry Editor

Select the Appropriate Key

Edit the Selected Key

Load USER32-Related DLLs It is interesting that the actual loading of these DLLs occurs as a part of USER32's initialization.USER32 reads the value of mentioned registry key and calls LoadLibrary() for these DLLs in its DllMain code. Restrictions:

However this trick applies only to applications that use USER32.DLL. Another restriction is that this built-in mechanism is supported only by NT and 2K operating systems.

ShortcomingsIn order to activate/deactivate the injection process you have to reboot Windows. The DLL you want to inject will be mapped only into these processes that use USER32.DLL, thus you cannot expect to get your hook injected into console applications, since they usually don't import functions from USER32.DLL. On the other hand you don't have any control over the injection process. It means that it is implanted into every single GUI application, regardless you want it or not. It is a redundant overhead especially if you intend to hook few applications only.

Inject by Hooks [Chris Cummings ]

What Are Hooks?Put shortly, a hook is a function you can create as part of a dll or your application to monitor the 'goings on' inside the windows operating system. The idea is to write a function that is called every time a certain event in windows occurs –

for example when a user presses a key on the keyboard or moves the mouse.

Hooks were provided by Microsoft primarily to help program writers with the debugging of their applications, but they can be put to use in many different ways –

for example, write hidden key logging program to find out other users’ passwords to the internet!

Types of HooksThere are 2 types of hooks - global or local.

A local hook is one that monitors things happening only for a specific program (or thread).A global hook monitors the entire system (all threads).

Both types of hooks are set up in the same way, the main difference being that for a local hook, the function to be called can be within the program it is monitoring, but with a global hook the function must be stored and loaded from a separate dll.

Hook-related Functions

The SetWindowsHookEx Function

SetWindowsHookEx is the function provided by Microsoft to install a hook. It accepts the following arguments:

SetWindowsHookEx returns a handle (i.e. an identifier) for the current hook, so you can use UnhookWindowsHookEx to remove the hook later on.

SetWindowsHookEx Example [Michel Leunen]

// Hood Function (Callback Procedure) Declaration LRESULT CALLBACK MouseProc(int code, WPARAM wParam, LPARAM lParam);

// Global variables HHOOK HookHandle; HINSTANCE DllInstance; bool IsInRect=false; 

bool InstallMouseHook() { HookHandle=SetWindowsHookEx(WH_MOUSE,     reinterpret_cast<HOOKPROC>(MouseProc),DllInstance,0); if (HookHandle==NULL) return false;   else  return true;}//--------------------------------------------------------------------bool RemoveMouseHook(){  if(UnhookWindowsHookEx(HookHandle)==0)        return false;   else  return true;} 

Types of Hooks used in idHook Parameter of a Hook Function

Types of Hooks appearing in a hook function.

The Hook FunctionThe hook function is the procedure to be called by windows when the event we specify happens. A hook for any event always takes the same form, but the values passed to it by windows can mean different things.

For example if the hook is type WH_KEYBOARD, windows will pass information to it relating to which key was pressed. Your hook procedure should accept the following arguments:

A hook function returns a value of type longword. What you should set it to depends on the type of hook, or you can just set it to the value that CallNextHookEx returns.

Hook Function Example

LRESULT CALLBACK MouseProc(int code, WPARAM wParam, LPARAM lParam) {   if (code<0)   {     return CallNextHookEx(HookHandle,code,wParam,lParam);   }   //Define a rectangle   POINT MousePos;   RECT UpperRightCorner=Rect(Screen->Width-2,0,Screen->Width,2);   //Get the mouse position   GetCursorPos(&MousePos);   //Check if the mouse is in the rectangle   if((PtInRect(&UpperRightCorner,MousePos))&&(IsInRect==false))   {     //if the mouse is in the rectangle, launch the screensaver     IsInRect=true;     SendMessage(GetDesktopWindow(),WM_SYSCOMMAND,SC_SCREENSAVE,0);  }   else IsInRect=false;   //Call the next hook in the chain   return CallNextHookEx(HookHandle,code,wParam,lParam); }

The CallNextHookEx FunctionThis function is to do with hook chains. When a hook is installed for a certain event, there may be others like it already installed –

for example 2 programs at once might be trying to log keyboard input. When you install a hook with SetWindowsHookEx it adds your hook procedure to the front of a list of hook procedures. CallNextHookEx simply calls the next procedure in the list. When your hook procedure is finished, it can run CallNextHookEx, and then return the value it gets from it or a different one depending on the type of hook. CallNextHookEx takes exactly the same form as a hook procedure plus one extra –

the handle returned by SetWindowsHookEx identifying the hook. The other values you pass to it should be the values your hook procedure was called with. How you should use it depends on the type of hook

The UnhookWindowsHookEx Function

This function simply removes your hook. The only argument you pass to it is the hook handle returned by SetWindowsHookEx.

Global Windows Hooks

Global HooksThe global hook is slightly more complicated than a local hook. To create a global hook you need 2 steps,

to make the executable file that executes SetWindowsHookEx() to make a Dll to contain the hook procedure.

System-wide HooksA system-wide hook is registered just once when SetWindowsHookEx() is executed. If no error occurs a handle to the hook is returned.

The returned value is required at the end of the custom hook function when a call to CallNextHookEx() has to be made.

After a successful call to SetWindowsHookEx() , the operating system injects the DLL automatically (but not necessary immediately) into all processes that meet the requirements for this particular hook.

Processes that use hook functions with the same type as this particular hook.

Global Variables vs. Shared DataOnce an application installs a system-wide hook, the operating system maps the DLL into the address space in each of its client processes. Therefore global variables within the DLL will be per-process and cannot be shared among the processes that have loaded the hook DLL. All variables that contain shared data must be placed in a shared data section.

A Global Hook Is Loaded by Multiple Processes That Don't Share the Same Address Space

For instance hook handle sg_hGetMsgHook, that is obtained by SetWindowsHookEx() and is used as parameter in CallNextHookEx() must be used virtually in all address spaces. It means that its value must be shared among hooked processes as well as the Hook Server application. In order to make this variable "visible" to all processes we should store it in the shared data section.

Define Variables Shared by All Processes

ExampleInstalling a mouse hook.

Interception Mechanisms [Ivo Ivanov]

Interception MechanismsInjecting a DLL into the address space of an external process is a key element of a spying system. It provides an excellent opportunity to have a control over process's thread activities. However it is not sufficient to have the DLL injected if you want to intercept API function calls within the process. In terms of the level where the hook is applied, there are two mechanisms for API spying –

Kernel level spying, User level spying.

The Module Relationships and Their Dependencies on Windows 2K and Interception Points

NT Kernel Level Hooking

NT Kernel Level HookingThere are several methods for achieving hooking of NT system services in kernel mode.

The most popular interception mechanism was originally demonstrated by Mark Russinovich and Bryce Cogswell in their article "Windows NT System-Call Hooking".

• Their basic idea is to inject an interception mechanism for monitoring NT system calls just bellow the user mode. This technique is very powerful and provides an extremely flexible method for hooking the point that all user-mode threads pass through before they are serviced by the OS kernel.

However, all these hooking strategies, remain out of the scope of this course.

Win32 User Level Hooking

Win32 User Level HookingProxy DLL (Trojan DLL)Spying by altering of the Import Address TableOther Approaches (not covered in this lecture)

Code overwritingSpying by a debuggerWindows sub-classing

Function Forwarder [Jeffrey Richter]

Define a Function Forwarder

// Function forwarders to functions in DllWork#pragma comment(linker, "/ export:SomeFunc=DllWork.SomeFunc")

This pragma tells the linker that the stub DLL should export a function called SomeFunc, but that the actual implementation for the function is in a function SomeFunc contained in the DllWork.dll. You'll have to have one pragma line for each function exported by your DllWork.dll for a program to call your functions correctly.

Defined in a stub DLL (A stub DLL refers to an entire DLL of unimplemented functions.)

Stub DLL and the Real DLL

__declspec(dllexport) void ShutdownLibrary();

__declspec(dllexport) void SomeFunc();

void SomeFunc()

{ MessageBox(NULL, "Doing something", "SomeFunc in DllWork", MB_OK); }

void ShutdownLibrary()

{ // Notify the worker threads to shutdown

SetEvent(g_hEventTerminate); }

BOOL WINAPI DllMain (HINSTANCE hinstDll, DWORD fdwReason, LPVOID fImpLoad)

{ int nThread;

:

}

The real DLL

#pragma comment(linker, "/export:SomeFunc=DllWork.SomeFunc")

BOOL WINAPI DllMain (HINSTANCE hinstDll, DWORD fdwReason, LPVOID p)

{ :

ShutdownLibrary();

:

return(TRUE);

}

stub DLLThe DLL used by an application

LoadLibrary loads the stub DLL, then the OS loader automatically loads your real DLL.

_declspec [wikipedia]

the _declspec keyword is a strange new keyword that is not part of the ANSI C standard, but that most compilers will understand anyway. _declspec allows a variety of non-standard options to be specified, that will affect the way a program runs. specifically, there are two _declspec identifiers that we want to discuss:

_declspec(dllexport) _declspec(dllimport)

Dllexport [wikipedia]

When writing a DLL, we need to use the dllexport keyword to denote functions that are going to be available to other programs. Functions without this keyword will only be available for use from inside the library itself.

DllMain [wikipedia]

When Windows links a DLL to a program, Windows calls the libraries' DllMain function. This means that every DLL needs to have a DllMain function.

Function Forwarder Example

Function ForwarderA function forwarder is an entry in a DLL's export table that redirects a function call to another function in another DLL.

For example, if you run the Visual C++® DumpBin utility on the Windows NT Kernel32.dll, you'll see a part of the output that looks like this:

C:\winnt\system32>DumpBin -Exports Kernel32.dll (some output omitted) 360 167 HeapAlloc (forwarded to NTDLL.RtlAllocateHeap) 361 168 HeapCompact (000128D9) 362 169 HeapCreate (000126EF) 363 16A HeapCreateTagsW (0001279E) 364 16B HeapDestroy (00012750) 365 16C HeapExtend (00012773) 366 16D HeapFree (forwarded to NTDLL.RtlFreeHeap) 367 16E HeapLock (000128ED) 368 16F HeapQueryTagW (000127B8) 369 170 HeapReAlloc (forwarded to NTDLL.RtlReAllocateHeap) 370 171 HeapSize (forwarded to NTDLL.RtlSizeHeap) (remainder of output omitted)

How a Function Forwarder Forwards the Execution of a Function to Another Function

This output of the previous slide shows four forwarded functions. Whenever your application calls HeapAlloc, HeapFree, HeapReAlloc, or HeapSize, your executable is dynamically linked with Kernel32.dll. When you invoke your executable, the loader loads Kernel32.dll and sees that there are forwarded functions that are actually contained inside NTDLL.dll, so the loader also loads the NTDLL.dll module. When your executable calls HeapAlloc, it is actually calling the RtlAllocateHeap function inside NTDLL.dll. The HeapAlloc function does not actually exist anywhere in the system!

If you call

GetProcAddress(GetModuleHandle("Kernel32"), "HeapAlloc");

GetProcAddress looks in Kernel32's export table, sees that HeapAlloc is a forwarded function, and calls GetProcAddress recursively looking for RtlAllocateHeap inside NTDLL.dll's export table.

Proxy DLL (Trojan DLL) RootkitsAn easy way for hacking API is just to replace a DLL with one that has the same name and exports all the symbols of the original one. This technique can be effortlessly implemented using function forwarders. A function forwarder basically is an entry in the DLL's export section that delegates a function call to another DLL's function. You can accomplish this task by simply using #pragma comment:

#pragma comment(linker, "/export:DoSomething=DllImpl.ActuallyDoSomething")

However, if you decide to employ this method, you should take the responsibility of providing compatibilities with newer versions of the original library.

Portable Executable File Format [Ashkbiz Danehkar ]

Portable Executable File FormatThe Portable Executable file format was defined to provide the best way for the Windows Operating System to execute code and also to store the essential data which is needed to run a program, for example constant data, variable data, import library links, and resource data. It consists of

MS-DOS file information, Windows NT file information, Section Headers, and Section images.

Portable Executable File Format Structure (1)

Portable Executable File Format Structure TypesMS-DOS Information IMAGE_DOS_HEADER

MS-DOS Stub Program Windows NT Information Signature

(IMAGE_NT_HEADERS) IMAGE_FILE_HEADER

IMAGE_OPTIONAL_HEADER32

IMAGE_DATA_DIRECTORY[16] IMAGE_SECTION_HEADER[0]

Sections Information IMAGE_SECTION_HEADER[n] SECTION[0]

SECTION[n]

:

:

Using Ordinal Numbers to Identify DLL Procedures

In addition to a name, all DLL procedures can be identified by an ordinal number that specifies the procedure in the DLL. Some DLLs do not include the names of their procedures and require you to use ordinal numbers when declaring the procedures they contain.

MS-DOS Information

e_lfanew is the offset which refers to the position of the Windows NT data.

offset value

Structure IMAGE_NT_HEADERStypedef struct _IMAGE_NT_HEADERS { DWORD Signature; IMAGE_FILE_HEADER FileHeader; IMAGE_OPTIONAL_HEADER OptionalHeader;} IMAGE_NT_HEADERS, *PIMAGE_NT_HEADERS;[1]

[2]

If you assume that the pMem pointer relates the start point of the memory space for a selected portable executable file, you can retrieve the MS-DOS header and also the Windows NT header by the following lines,

IMAGE_DOS_HEADER image_dos_header; IMAGE_NT_HEADERS image_nt_headers; PCHAR pMem; … memcpy(&image_dos_header, pMem, sizeof(IMAGE_DOS_HEADER)); memcpy(&image_nt_headers, pMem+image_dos_header.e_lfanew, sizeof(IMAGE_NT_HEADERS));

Access the MS-DOS header and the Windows NT Header

Signature and IMAGE_FILE_HEADER of Windows NT Information

typedef struct _IMAGE_OPTIONAL_HEADER{ WORD Magic; BYTE MajorLinkerVersion; BYTE MinorLinkerVersion; DWORD SizeOfCode; DWORD SizeOfInitializedData; DWORD AddressOfEntryPoint;

: DWORD ImageBase; DWORD SizeOfStackReserve; DWORD SizeOfStackCommit; DWORD SizeOfHeapReserve; DWORD SizeOfHeapCommit; DWORD LoaderFlags; DWORD NumberOfRvaAndSizes; IMAGE_DATA_DIRECTORY DataDirectory[IMAGE_NUMBEROF_DIRECTORY_ENTRIES]; } IMAGE_OPTIONAL_HEADER,*PIMAGE_OPTIONAL_HEADER;

Structure IMAGE_OPTIONAL_HEADER

A pointer to the first IMAGE_DATA_DIRECTORY structure in the data directory.

AddressOfEntryPoint and ImageBaseAddressOfEntryPoint

A pointer to the entry point function, relative to the image base address.

ImageBaseThe preferred address of the first byte of the image when it is loaded in memory. This value is a multiple of 64K bytes.

• The default value for DLLs is 0x10000000.• The default value for applications is 0x00400000.

RemarksThe actual structure in Winnt.h is named IMAGE_OPTIONAL_HEADER32 and IMAGE_OPTIONAL_HEADER is defined as IMAGE_OPTIONAL_HEADER32. However, if _WIN64 is defined, then IMAGE_OPTIONAL_HEADER is defined as IMAGE_OPTIONAL_HEADER64.

IMAGE_OPTIONAL_HEADER32 of Windows NT Information

IMAGE_DATA_DIRECTORYRepresents the data directory.

typedef struct _IMAGE_DATA_DIRECTORY { DWORD VirtualAddress; DWORD Size;

} IMAGE_DATA_DIRECTORY, *PIMAGE_DATA_DIRECTORY; Members

VirtualAddress • The relative virtual address of the table.

Size • The size of the table, in bytes.

points to a IMAGE_IMPORT_DESCRIPTOR structure

List of the Data Directories

IMAGE_DATA_DIRECTORY of Windows NT Information

type

points to a IMAGE_IMPORT_DESCRIPTOR structure.

Each DLL file has a corresponding IMAGE_IMPORT_DESCRIPTOR structure.

Example [stinson]

If we have a PE that imports two modules, we'll have: one IMAGE_DATA_DIRECTORY structure for the Import Symbols (i.e. the Import Table) let's say that struct's VirtualAddress is 0xc7d8 -- recall this is an RVA.then 0xc7d8 is where the first IMAGE_IMPORT_DESCRIPTOR lives since we are importing two modules, there will be 3 IMAGE_IMPORT_DESCRIPTOR structs in our array (which starts at 0xc7d8, recall)

• --> the 3rd IMAGE_IMPORT_DESCRIPTOR is all zeroes then the Size field above (for this IMAGE_DATA_DIRECTORY) will be 3 * sizeof( IMAGE_IMPORT_DESCRIPTOR )

IMAGE_SECTION_HEADER

Represents the image section header format.

typedef struct _IMAGE_SECTION_HEADER {

BYTE Name[IMAGE_SIZEOF_SHORT_NAME]; union

{ DWORD PhysicalAddress; DWORD VirtualSize; } Misc; DWORD VirtualAddress; DWORD SizeOfRawData; DWORD PointerToRawData; DWORD PointerToRelocations; DWORD PointerToLinenumbers; WORD NumberOfRelocations; WORD NumberOfLinenumbers; DWORD Characteristics;

} IMAGE_SECTION_HEADER, *PIMAGE_SECTION_HEADER;

IMAGE_SECTION_HEADER

The address of the first byte of the section when loaded into memory, relative to the image base.

Section Names

IMAGE_SECTION_HEADER Array of Section Information

SECTION Array of Section Information

Import Address TableWithin a PE file, there's an array of data structures, one per imported DLL. Each of these structures gives the name of the imported DLL and points to an array of function pointers. The array of function pointers is known as the import address table (IAT). Each imported API has its own reserved spot in the IAT where the address of the imported function is written by the Windows loader. This last point is particularly important: once a module is loaded, the IAT contains the address that is invoked when calling imported APIs.

The beauty of the IAT is that there's just one place in a PE file where an imported API's address is stored. No matter how many source files you scatter calls to a given API through, all the calls go through the same function pointer in the IAT.Let's examine what the call to an imported API looks like.CALL 0x0040100C •••CALL 0x0040100C ••• 0x0040100C: JMP DWORD PTR [0x00405030]

• Here, 0x405030 is an entry within the IAT.

Imported API Calls and the IAT (1)

Imported API Calls and the IAT (2)

The CALL in previous slide transfers control to a small stub. The stub is a JMP to the address whose value is at 0x405030

The Imports Section [Matt Pietrek]

Section .idataSection .idata contains information about Import Address Table. This part of the PE structure is particularly very crucial for building a spy program based on altering IAT.

Location of Section .idata in a PE File

IMAGE_IMPORT_DESCRIPTOR Structure

The anchor of the imports data is the IMAGE_IMPORT_DESCRIPTOR structure.There's one IMAGE_IMPORT_DESCRIPTOR for each imported executable (e.g. DLL file).The end of the IMAGE_IMPORT_DESCRIPTOR array is indicated by an entry with fields all set to 0.

Structure IMAGE_IMPORT_DESCRIPTORtypedef struct _IMAGE_IMPORT_DESCRIPTOR

{ DWORD OriginalFirstThunk; DWORD TimeDateStamp; DWORD ForwarderChain; DWORD Name; DWORD FirstThunk;} IMAGE_IMPORT_DESCRIPTOR, *PIMAGE_IMPORT_DESCRIPTOR;

Members of Structure IMAGE_IMPORT_DESCRIPTOR

An Executable Importing Some APIs from USER32.DLL

Both arrays have elements of type IMAGE_THUNK_DATA, which is a pointer-sized union. Each IMAGE_THUNK_DATA element corresponds to one imported function from the executable. The ends of both arrays are indicated by an IMAGE_THUNK_DATA element with a value of zero.

IMAGE_THUNK_DATA

Structure IMAGE_THUNK_DATAtypedef struct _IMAGE_IMPORT_BY_NAME { WORD Hint; BYTE Name[1];} IMAGE_IMPORT_BY_NAME, *PIMAGE_IMPORT_BY_NAME;

typedef struct _IMAGE_THUNK_DATA { union { PDWORD Function; PIMAGE_IMPORT_BY_NAME AddressOfData; } u1;} IMAGE_THUNK_DATA, *PIMAGE_THUNK_DATA;

The IMAGE_THUNK_DATA union is a DWORD with these interpretations:

DWORD Function; // Memory address of the imported functionDWORD Ordinal; // Ordinal value of imported API DWORD AddressOfData; // RVA to an IMAGE_IMPORT_BY_NAME with // the imported API nameDWORD ForwarderString; // RVA to a forwarder string

Interpretation of Fields of Structure IMAGE_THUNK_DATA

The IMAGE_THUNK_DATA structures within the IAT lead a dual-purpose life.

In the executable file, they contain either the ordinal of the imported API or an RVA to an IMAGE_IMPORT_BY_NAME structure.

• The IMAGE_IMPORT_BY_NAME structure is just a WORD, followed by a string naming the imported API. The WORD value is a hint to the loader as to what the ordinal of the imported API might be.

When the loader brings in the executable, it overwrites each IAT entry with the actual address of the imported function.

IMAGE_THUNK_DATA Structures of the IAT

BindingWhen an executable is bound (via the bind program, for instance), the IMAGE_THUNK_DATA structures in the IAT are overwritten with the actual address of the imported function. The executable file on disk has the actual in-memory addresses of APIs in other DLLs in its IAT. When loading a bound executable, the Windows loader can bypass the step of looking up each imported API and writing it to the IAT. The correct address is already there.

Spying by Altering of the Import Address Table

Here are the logical steps of a replacing cycle: Locate the import section from the IAT of each loaded by the process DLL module as well as the process itself Find the IMAGE_IMPORT_DESCRIPTOR chunk of the DLL that exports that function. Practically speaking, usually we search this entry by the name of the DLL Locate the IMAGE_THUNK_DATA which holds the original address of the imported function Replace the function address with the user supplied one

By changing the address of the imported function inside the IAT, we ensure that all calls to the hooked function will be re-routed to the function interceptor.

Case Study 1

A Simple RootkitA simple script put in Perl’s string context, compiled and named netstat.exe may be an example of a trivial rootkit.

A real system netstat could be named oldnetstat.exe. The principle of operation of the new netstat is that

• once the command line will call the real netstat (now oldnetstat.exe), it will be directed to a temporary text file.

• Then the rootkit searches that file for any information about the listening port to remove it (according to the procedure predefined in the rootkit code).

• After modification, the result is displayed on the screen and the old file is removed. This principle is both simple and efficient and provides an interesting possibility – it may be used to spoof output data acting from any other tool available through the command line – for example, tlist, or dir.

There are many programs of this type available on the Web. Some of them did not display, for example, information on listening ports such as 666, 27374, 12345, 31337 – i.e. well-known Trojan horse ports.

Case Study 2

A Windows Rootkit Example -- Rootki

The idea of a first enhanced rootkit, Rootki , for the Windows environment was born in due time. The originator was Greg Hoglund.

Rootki 0.40 – Existing Form and Activating MethodsThis rootkit has been designed as a kernel mode driver that runs with system privileges right at the core of the system kernel. Given this fact, it has access to all resources of the operating system, thus having a broad field of action. In order to install it one requires the administrator’s permissions whilst simple net start/net stop commands are sufficient to activate/disactivate it respectively.

Rootki 0.40 – Hiding ApproachOnce the rootkit has been loaded, the hacker can hide directories and files on the victim’s disk.

This method is efficient provided that the object to be hidden has a name prefixed with _root_ – for example, _root_directory_name. How does this work?

• The rootkit, by patching the kernel, intercepts all system calls for the listing of the disk content and all objects beginning with the sequence _root_ – are hidden from display.

• The same applies to the searching process – all files and directories with the above sequence of characters are hidden from the search.

Rootki 0.40 – Hiding ProcessesThis rootkit feature can also be used to hide processes running as well as to do the same with the system registry entries, by prefixing all keys and entries with _root_ .

This enables the hacker to install, for example, services which will become a backdoor, thus being as invisible for the system administrator as services or registry entries or processes running in the system memory.

Rootki 0.40 – Key LoggerThe rootkit can also intercept all key strokes typed at the system console. This may be carried out by hooking into the keyboard driver and issuing the ‘sniffkeys’ command.

Case Study 3

A Famous Rootkit CaseThe word "rootkit" came to public awareness in the 2005 Sony CD copy protection scandal, in which Sony BMG music CDs surreptitiously placed a rootkit on Microsoft Windows PCs when the CD was played on the computer. Sony provided no mention of this in the CD or its packaging, referring only to security rights management measures.

Protect Systems against Rootkits

Guarding against the Rootkit – Checking from Other Hosts

“vulnerability” of a rootkit: objects are only hidden from the environment of the compromised machine and they can easily be seen from another computer. Mapping a Network Drive remotely from another machine (or using net use command) is a means to see everything, which has been hidden for a local user. This is because the remote machine is using a clean kernel to view the files and directories on the compromised machine, avoiding the rootkits filtration process.

Guarding against the Rootkit – Renaming Check Utilities

A rootkit, however, cannot affect processes that have _root_ in their names. In other words, when a system administrator, is analyzing the system log using regedit.exe, he cannot see hidden entries, but just by changing its name to  _root_regedit.exe, it will be enough for him to see all of them as well as hidden keys and registry entries. This is true for all programs – for example, Task Manager.

Appendix (could be omitted)

List the Names of ALL Import Functions of a PE File (1) [Iczelion]

Verify that the file is a valid PE From the DOS header, go to the PE header Obtain the address of the data directory in OptionalHeader Go to the 2nd member of the data directory. Extract the value of VirtualAddress Use that value to go to the first IMAGE_IMPORT_DESCRIPTOR structure Check the value of OriginalFirstThunk. If it's not zero, follow the RVA in OriginalFirstThunk to the RVA array. If OriginalFirstThunk is zero, use the value in FirstThunk instead. Some linkers generate PE files with 0 in OriginalFirstThunk. This is considered a bug. Just to be on the safe side, we check the value in OriginalFirstThunk first.

List the Names of ALL Import Functions of a PE File (2) [Iczelion]

For each member in the array, we check the value of the member against IMAGE_ORDINAL_FLAG32. If the most significant bit of the member is 1, then the function is exported by ordinal and we can extract the ordinal number from the low word of the member. If the most significant bit of the member is 0, use the value in the member as the RVA into the IMAGE_IMPORT_BY_NAME, skip Hint, and you're at the name of the function. Skip to the next array member, and retrieve the names until the end of the array is reached (it's null -terminated). Now we are done extracting the names of the functions imported from a DLL. We go to the next DLL. Skip to the next IMAGE_IMPORT_DESCRIPTOR and process it. Do that until the end of the array is reached (IMAGE_IMPORT_DESCRIPTOR array is terminated by a member with all zeroes in its fields).