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Windows Syscall Shellcode 

Aug 04, 2005 02:00 AM

by Piotr Bania


This article has been written to show that is possible to write shellcode for Windows operating systems that doesn't use standard API calls at all. Of course, as with every solution, this approach has both advantages and disadvantages. In this paper we will look at such shellcode and also introduce some example usage. IA-32 assembly knowledge is definitely required to fully understand this article.

All shellcode here has been tested on Windows XP SP1. Note that there are variations in the approach depending on the operating system and service pack level, so this will be discussed further as we progress.

Some background

Windows NT-based systems (NT/2000/XP/2003 and beyond) were designed to handle many subsystems, each having its own individual environment. For example, one of NT subsystems is Win32 (for normal Windows applications), another example would be POSIX (Unix) or OS/2. What does it mean? It means that Windows NT could actually run (of course with proper os add-ons) OS/2 and support most of it features. So what changes were made as the OS was developed? To support all of these potential subsystems, Microsoft made unified set of APIs which are called wrappers of each subsystem. In short, all subsystems have all the needed libraries for them to work. For example Win32 apps call the Win32 Subsystem APIs, which in fact call NT APIs (native APIs, or just natives). Natives don't require any subsystem to run.


From native API calls to syscalls

Is this theory true, that shellcode can be written without any standard API calls? Well, for some APIs it is for some it isn't. There are many APIs that do their job without calling native NT APIs and so on. To prove this, let's look at the GetCommandLineA API exported from KERNEL32.DLL.


.text:77E7E358 ; --------------- S U B R O U T I N E ------------------------- .text:77E7E358 .text:77E7E358 .text:77E7E358 ; LPSTR GetCommandLineA(void) .text:77E7E358 public GetCommandLineA .text:77E7E358 GetCommandLineA proc near .text:77E7E358                 mov eax, dword_77ED7614 .text:77E7E35D                 retn .text:77E7E35D GetCommandLineA endp

This API routine doesn't use any arbitary calls. The only thing it does is the return the pointer to the program command line. But let's now discuss an example that is in line with our theory. What follows is part of the TerminateProcess API's disassembly.


.text:77E616B8 ; BOOL __stdcall TerminateProcess(HANDLE hProcess,UINT uExitCode) .text:77E616B8 public TerminateProcess .text:77E616B8 TerminateProcess proc near           ; CODE XREF: ExitProcess+12 j .text:77E616B8                                      ; sub_77EC3509+DA p .text:77E616B8 .text:77E616B8 hProcess       =        dword ptr 4 .text:77E616B8 uExitCode      =        dword ptr 8 .text:77E616B8 .text:77E616B8                  cmp [esp+hProcess], 0 .text:77E616BD                  jz short loc_77E616D7 .text:77E616BF                  push [esp+uExitCode]       ; 1st param: Exit code .text:77E616C3                  push [esp+4+hProcess]      ; 2nd param: Handle of process .text:77E616C7                  call ds:NtTerminateProcess ; NTDLL!NtTerminateProcess

As you can see, the TerminateProcess API passes arguments and then executes NtTerminateProcess, exported by NTDLL.DLL. The NTDLL.DLL is the native API. In other words, the function which name starts with 'Nt' is called the native API (some of them are also ZwAPIs - just look what exports from the NTDLL library). Let's now look at NtTerminateProcess.


.text:77F5C448 public ZwTerminateProcess .text:77F5C448 ZwTerminateProcess proc near      ; CODE XREF: sub_77F68F09+D1 p .text:77F5C448                                   ; RtlAssert2+B6 p .text:77F5C448 mov eax, 101h                     ; syscall number: NtTerminateProcess .text:77F5C44D mov edx, 7FFE0300h                ; EDX = 7FFE0300h .text:77F5C452 call edx                          ; call 7FFE0300h .text:77F5C454 retn 8 .text:77F5C454 ZwTerminateProcess endp

This native API infact only puts the number of the syscall to eax and calls memory at 7FFE0300h, which is:


7FFE0300      8BD4    MOV EDX,ESP 7FFE0302      0F34    SYSENTER 7FFE0304      C3      RETN

And that shows how the story goes; EDX is now user stack pointer, EAX is the system call to execute. The SYSENTER instruction executes a fast call to a level 0 system routine, which does rest of the job.


Operating system differences

In Windows 2000 (and other NT based systems except XP and newer) no SYSENTER instruction is used. However, in Windows XP the "int 2eh" (our old way) was replaced by SYSENTER instruction. The following schema shows the syscall implementation for Windows 2000:


      MOV   EAX, SyscallNumber               ; requested syscall number       LEA   EDX, [ESP+4]                     ; EDX = params...       INT   2Eh                              ; throw the execution to the KM handler       RET   4*NUMBER_OF_PARAMS               ; return

We know already the Windows XP way, however here is the one I'm using in shellcode:


     push   fn                               ; push syscall number      pop    eax                              ; EAX = syscall number      push   eax                              ; this one makes no diff      call   b                                ; put caller address on stack b:   add    [esp],(offset r - offset b)      ; normalize stack      mov    edx, esp                         ; EDX = stack      db     0fh, 34h                         ; SYSENTER instruction r:   add    esp, (param*4)                   ; normalize stack

It seems that SYSENTER was first introduced in the Intel Pentium II processors. This author is not certain but one can guess that SYSENTER is not supported by Athlon processors. To determine if the instruction is available on a particular processor, use the CPUID instruction together with a check for the SEP flag and some specific family/model/stepping checks. Here is the example how Intel does this type of checking:


IF (CPUID SEP bit is set)     THEN IF (Family = 6) AND (Model < 3) AND (Stepping < 3)        THEN           SYSENTER/SYSEXIT_NOT_SUPPORTED        FI;     ELSE SYSENTER/SYSEXIT_SUPPORTED FI;

But of course this is not the only difference in various Windows operating systems -- system call numbers also change between the various Windows versions, as the following table shows:

Syscall symbol NtAddAtom NtAdjustPrivilegesToken NtAlertThread
Windows NT SP 3 0x3 0x5 0x7
SP 4 0x3 0x5 0x7
SP 5 0x3 0x5 0x7
SP 6 0x3 0x5 0x7
Windows 2000 SP 0 0x8 0xa 0xc
SP 1 0x8 0xa 0xc
SP 2 0x8 0xa 0xc
SP 3 0x8 0xa 0xc
SP 4 0x8 0xa 0xc
Windows XP SP 0 0x8 0xb 0xd
SP 1 0x8 0xb 0xd
SP 2 0x8 0xb 0xd
Windows 2003 Server SP 0 0x8 0xc 0xe
SP 1 0x8 0xc 0xe

The syscall number tables are available on the Internet. The reader is advised to look at the one from metasploit.com, however other sources may also be good.


Syscall shellcode advantages

There are several advantages when using this approach:


  • Shellcode doesn't require the use of APIs, due to the fact that it doesn't have to locate API addresses (there is no kernel address finding/no export section parsing/import section parsing, and so on). Due to this "feature" it is able to bypass most of ring3 "buffer overflow prevention systems." Such protection mechanisms usually don't stop the buffer overflow attacks in itself, but instead they mainly hook the most used APIs and check the caller address. Here, such checking would be of no use.
  • Since you are sending the requests directly to the kernel handler and you "jump over" all of those instructions from the Win32 Subsystem, the speed of execution highly increases (although in the era of modern processors, who truly cares about speed of shellcode?).

Syscall shellcode disadvantages

There are also several disadvantages to this approach:


  • Size -- this is the main disadvantage. Becase we are "jumping over" all of those subsytem wrappers, we need to code our own ones, and this increases the size of shellcode.
  • Compability -- as has been written above, there exist various implementations from "int 2eh" to "sysenter," depending on the operating system version. Also, the system call number changes together with each Windows version (for more see the References section).

The ideas

The shellcode at the end of this article dumps a file and then writes an registry key. This action causes execution of the dropped file after the computer reboots. Many of you may ask me why we would not to execute the file directly without storing the registry key. Well, executing win32 application by syscalls is not a simple task -- don't think that NtCreateProcess will do the job; let's look at what CreateProcess API must do to execute an application:


  1. Open the image file (.exe) to be executed inside the process.
  2. Create the Windows executive process object.
  3. Create the initial thread (stack, context, and Windows executive thread object).
  4. Notify the Win32 subsystem of the new process so that it can set up for the new process and thread.
  5. Start execution of the initial thread (unless the CREATE_SUSPENDED flag was specified).
  6. In the context of the new process and thread, complete the initialization of the address space (such as load required DLLs) and begin execution of the program.

Therefore, it is clearly much easier and quicker to use the registry method. The following shellcode that concludes this article drops a sample MessageBox application (mainly, a PE struct which is big itself so the size increases) however there are plenty more solutions. Attacker can drop some script file (batch/vbs/others) and download a trojan/backdoor file from an ftp server, or just execute various commands such as: "net user /add piotr test123" & "net localgroup /add administrators piotr". This idea should help the reader with optimizations, now enjoy the proof of concept shellcode.

If you experience formatting issues with the code as listed below, an archive of this proof of concept is available for download from SecurityFocus.


The shellcode - Proof Of Concept
 comment $             -----------------------------------------------             WinNT (XP) Syscall Shellcode - Proof Of Concept             -----------------------------------------------             Written by: Piotr Bania 


Final words

The author hopes you have enjoyed the article. If you have any comments don't hesitate to contact him; also remember that code was developed purely for educational purposes only.

Further reading


  1. "Inside the Native API" by Mark Russinovich
  2. "MSDN" from Microsoft
  3. Interactive Win32 syscall page from Metasploit

About the author

Piotr Bania is an independent IT Security/Anti-Virus Researcher from Poland with over five years of experience. He has discovered several highly critical security vulnerabilities in popular applications like RealPlayer. More information can be found on his website.

This article originally appeared on SecurityFocus.com -- reproduction in whole or in part is not allowed without expressed written consent.

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