Shellcoding: Locating Kernel32 Base Address
Before the shellcode will we able to locate or use any Windows APIs, the base address of kernel32.dll must be located. Thankfully there’s a trick for that that has been around for quite some time. The method Uses the Win32 Thread Information Block (TIB) to locate the Program Environment Block (PEB) to locate InInitializationOrderModuleList, which contains the base address of kernel32.dll as it’s second list entry.
I will begin by demonstrating how to locate Kernel32.dll’s base address in a 32-bit environment. As best as I can discern, from SK Chong’s 2001 Phrack article, this method was originally documented by researchers in “VX-zine”. I was unable to locate the original source. Since SK’s article is quite old, it is possible that the author’s name(s) is/are lost to time. If someone knows the original source, it would be interesting knowledge and I would happily update this article. This method will work on all versions of Windows. It is not NULL byte free but, we will handle that in a future blog post. The following code will locate the base address of Kernel32:
1 2 3 4 5 6 7 8 9 10 11 mov eax,[fs:0x30] ; Get the memory address of the PEB ; structure and store it in EAX mov eax,[eax+0x0C] ; Get the memory address of the PEB_LDR_DATA ; structure and store it in EAX mov esi,[eax+0x1C] ; Get the memory address of the first ; LIST_ENTRY structure stored in the ; InInitializationOrderModuleList and store ; it in ESI lodsd ; Loads the memory address of the second ; LIST_ENTRY structure mov ebx,[eax+0x08] ; Save the base address of kernel32.dll in EBX
The first MOV instruction saves the address of the PEB structure in EAX. Windows uses the FS register to store the address of the TIB structure, the address of the PEB structure is located at an offset of 0x30 bytes. To illustrate what is happening here, let us take a look at some screen clips of OllyDbg with an example program loaded.
a. The FS Register contains the address 0x7FFDE000:
Figure 1: FS Register Contains 0x7FFDE000
b. If we look at that location in the memory dump, we can see that the address of the PEB is located at an offset of 0x30 (48 bytes), its value is 0x7FFDF000 in this example:
Figure 2: PEB Address at Offset 0x30
The second MOV instruction saves the address of the PEB_LDR_DATA structure in EAX. The PEB_LDR_DATA structure is located at an offset of 0x0C bytes of the PEB structure, its value is 0x00341EA0 in this example:
Figure 3: PEB_LDR_DATA Structure Address at Offset 0x0C
The third MOV instruction saves the address of the InInitializationOrderModuleList list in ESI. This list contains a list of the loaded modules. The first entry is ntdll.dll, the second is kernel32.dll, which is what we’re after. The first list entry address is at an offset of 0x1C (28 bytes), it’s value is 0x00341F58 in this example:
Figure 4: The InInitializationOrderModuleList list Address at Offset 0xC1
The LODSD instruction increments ESI, which contains the address of the first module, by one so that it now points to the kernel32.dll entry and saves the address in EAX, its value is 0x00342020 in this example:
Figure 5: The LOSD Instruction Increments ESI to Point to the Next List Entry
The final MOV instruction saves the base address of kernel32.dll in EBX. At an offset of 0x08 (8 bytes) from that location is where the base address of kernel32.dll is located, its value is 0x7C800000 in this example:
Figure 6: The Base Address of Kernel32.dll Located at Offset 0x08
Now, on to try the same thing on a 64-bit system. As I did in the first blog post, I will try the 32-bit code first to see if it works. Immediately, there is a problem:
Figure 7: fs:[0x30] is Invalid
The fs:[0x30] value points to unknown memory, which is not going to help us. After referring to the Win32 Thread Information Block (TIB, a.k.a Thread Environment Block (TEB)) information on Wikipedia, it appears that on 64-bit systems the Process Environment Block (PEB) is located at gs:[0x60] instead. Before we go any further, let’s see what else has changed. I decided to switch over to WinDBG to explore a little more.
Examining the TIB(TEB) Structure
In WinDBG lets get the base address of the TEB using the !teb command:
Figure 8: Finding the Base Address of the TEB
Next, using the base address we can examine the TEB structure with the following command:
We can confirm that the PEB structure is located at an offset of 0x60.
Figure 9: The PEB Structure is at Offset 0x60 of the TEB
Examining the PEB Structure
In WinDBG the PEB structure can be dumped using the following command:
From the results we see that the LDR structure is at an offset of 0x18 of the PEB structures base address.
Figure 10: The LDR Structure is at Offset 0x18 of the PEB
Examining the PEB_LDR_DATA List Structure
We know in that when working in 32-bit that the first list item of the PEB_LDR_DATA structure should contain information about the running program, ntdll as the second list item, and the third should contain information about kernel32. We will now attempt to verify that it is the same for a 64-bit system. First, to look at the list, issue the following command:
Iterating the LDR Data Table to Find Kernel32
We are interested in the InMemoryOrderModuleList at offset 0x20. We will begin with seeing what is there.
Figure 11: Examining the PEB_LDR_DATA Structure
To see what the first entry in the list points to, we will examine the LDR_DATA_TABLE_ENTRY structure contents by issuing the following command:
As we can see in Figure 12, the first entry is the current process. Which happens to be notepad.exe in this example.
Figure 12: Looking at the First LDR Data Table Entry
So far, so good, now we will look at the next entry. To do that, we need to evaluate what 000000007723d640+0x20 points to. Since we are looking through a doubly-linked list, this should bring us to the next entry. The following command will evaluate the value:
Figure 13: Evaluating the pointer of the Next LDR Data Table Entry
With the evaluated value, we can now look at the next LDR_DATA_TABLE_ENTRY by issuing the following command:
The next entry points to ntdll.dll. This may be useful. It should not be necessary to know this address for the final shellcode but, we can note it’s location in the list.
Figure 14: The Second LDR Data Table Entry points to ntdll.dll
To look at the next entry in the list, we need to evaluate the address of the next entry. The first element of the LDR_DATA_TABLE_ENTRY structure is a doubly-linked list. To find the FLink, we just need to find evaluate the pointer, then we can look at the next entry. The commands to do this are:
Excellent, we found the entry for kernel32.dll.
Figure 15: The Third LDR Data Table Entry points to kernel32.dll
Putting It All Together
Now that we know how to find the LDR Data Table entry that contains the details for the kernel32.dll module, it is time to write some Assembly that will duplicate what we did in the previous section so that we can locate and store the base address of kernel32 so that it can be used to locate Win32 APIs.
We begin by listing the steps we performed in WinDBG to find the base address of kernel32.
- Find the address of the PEB structure.
- Find the address of the PEB_LDR_DATA structure.
- Find the address of the InMemoryOrderModuleList list.
- Iterate to the third InMemoryOrderModuleList entry.
- Store the base address of kernel32.dll.
With this basic list of tasks we will begin to assemble some Assembly (heh…) to accomplish the required tasks.
1: Find the Address of the PEB Structure
According to the Wikipedia article on the Thread Information Block is stored in the gs register and the PEB is located at an offset of 0x60. With this information, the following Assembly instruction should store the address of the PEB in the RAX register:
2: Find the Address of the PEB_LDR_DATA Structure
The PEB_LDR_DATA structure is located at an offset of 0x18 from the base address of the PEB. The following instruction should store the address of the PEB_LDR_DATA structure in the RAX register.
3: Find the Address of the InMemoryOrderModuleList list
The InMemoryOrderModuleList is at the offset of 0x20 of the PEB_LDR_DATA structure. The following assembly will store the address in the RAX register:
4: Iterate to the Third InMemoryOrderModuleList Entry
The InMemoryOrderModuleList is a doubly-linked list. The first element of each list is the FLink, so we can use this to quickly iterate to the 3rd entry in the list by loading the first pointer twice. The following code will leave the address of the third entry in the RAX register:
5: Save the Base Address of Kernel32
The final step is to save the base address of kernel32.dll somewhere so that we can use it later to retrieve other Win32 functions. According to Microsoft’s x64 Calling Convention the R12-R15 registers are Caller/Callee Saved registers. This means that we should be able to reserve one of these registers to store the base address of kernel32.dll for the remainder of our shellcode. The base address is found at an offset of 0x20 from the beginning of the list entry. The code to store the base address in the R12 register is:
The Complete Assembly
Putting it all together, the assembly program to find the base address of kernel32.dll and store it in the R12 register is as follows:
1 2 3 4 5 6 7 8 9 10 11 12 13 [SECTION .text] BITS 64 global _start _start: mov rax, [gs:0x60] mov rax, [rax+0x18] mov rax, [rax+0x20] mov rax, [rax] mov rax, [rax] mov r12, [rax+0x20]
As you can see in Figure 16, the base address of kernel32.dll has been stored successfully in R12.
Figure 16: The Base Address of Kernel32.dll is Stored in R12
Alternately, if you needed to save the base address of ntdll.dll as well you could use a slight variation, this variation will store the base address of ntdll.dll in the R13 register:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 [SECTION .text] BITS 64 global _start _start: mov rax, [gs:0x60] mov rax, [rax+0x18] mov rax, [rax+0x20] mov rax, [rax] mov r13, [rax+0x20] mov rax, [rax] mov r12, [rax+0x20]
In this installment of my shellcoding blog series we learned how to find the base address of kernel32.dll using both x86 and x64 Assembly code. The ability to find the base address of kernel32.dll is important because it can be used to call the LoadLibrary Win32 function to locate the address of other interesting Win32 functions, and even load other DLLs to add additional functionality. Like adding ws2_32.dll to provide access to functions like: accept, bind, socket, WSASocket, and WSAStartup. The fact that our finished opcodes includes NULL bytes is acceptable since we will, in a later stage, use an encoding routine to avoid NULL bytes.