ssh-audit/src/ssh_audit/gextest.py

"""
   The MIT License (MIT)

   Copyright (C) 2017-2023 Joe Testa (jtesta@positronsecurity.com)

   Permission is hereby granted, free of charge, to any person obtaining a copy
   of this software and associated documentation files (the "Software"), to deal
   in the Software without restriction, including without limitation the rights
   to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   copies of the Software, and to permit persons to whom the Software is
   furnished to do so, subject to the following conditions:

   The above copyright notice and this permission notice shall be included in
   all copies or substantial portions of the Software.

   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
   THE SOFTWARE.
"""
import traceback

# pylint: disable=unused-import
from typing import Dict, List, Set, Sequence, Tuple, Iterable  # noqa: F401
from typing import Callable, Optional, Union, Any  # noqa: F401

from ssh_audit.kexdh import KexDHException, KexGroupExchange_SHA1, KexGroupExchange_SHA256
from ssh_audit.ssh2_kexdb import SSH2_KexDB
from ssh_audit.ssh2_kex import SSH2_Kex
from ssh_audit.ssh_socket import SSH_Socket
from ssh_audit.outputbuffer import OutputBuffer
from ssh_audit import exitcodes


# Performs DH group exchanges to find what moduli are supported, and checks
# their size.
class GEXTest:

    # Creates a new connection to the server.  Returns True on success, or False.
    @staticmethod
    def reconnect(out: 'OutputBuffer', s: 'SSH_Socket', kex: 'SSH2_Kex', gex_alg: str) -> bool:
        if s.is_connected():
            return True

        err = s.connect()
        if err is not None:
            out.v(err, write_now=True)
            return False

        _, _, err = s.get_banner()
        if err is not None:
            out.v(err, write_now=True)
            s.close()
            return False

        # Send our KEX using the specified group-exchange and most of the
        # server's own values.
        s.send_kexinit(key_exchanges=[gex_alg], hostkeys=kex.key_algorithms, ciphers=kex.server.encryption, macs=kex.server.mac, compressions=kex.server.compression, languages=kex.server.languages)

        try:
            # Parse the server's KEX.
            _, payload = s.read_packet(2)
            SSH2_Kex.parse(out, payload)
        except KexDHException:
            out.v("Failed to parse server's kex.  Stack trace:\n%s" % str(traceback.format_exc()), write_now=True)
            return False

        return True

    @staticmethod
    def granular_modulus_size_test(out: 'OutputBuffer', s: 'SSH_Socket', kex: 'SSH2_Kex', bits_min: int, bits_pref: int, bits_max: int, modulus_dict: Dict[str, List[int]]) -> int:
        '''
        Tests for granular modulus sizes.
        Builds a dictionary, where a key represents a DH algorithm name and the
        values are the modulus sizes (in bits) that have been returned by the
        target server.
        Returns an exitcodes.* flag.
        '''

        retval = exitcodes.GOOD

        out.d("Starting modulus_size_test...")
        out.d("Bits Min:  " + str(bits_min))
        out.d("Bits Pref: " + str(bits_pref))
        out.d("Bits Max:  " + str(bits_max))

        GEX_ALGS = {
            'diffie-hellman-group-exchange-sha1': KexGroupExchange_SHA1,
            'diffie-hellman-group-exchange-sha256': KexGroupExchange_SHA256,
        }

        # Check if the server supports any of the group-exchange
        # algorithms.  If so, test each one.
        for gex_alg, kex_group_class in GEX_ALGS.items():
            if gex_alg not in kex.kex_algorithms:
                out.d('Server does not support the algorithm "' + gex_alg + '".', write_now=True)
            else:
                kex_group = kex_group_class(out)
                out.d('Preparing to perform DH group exchange using ' + gex_alg + ' with min, pref and max modulus sizes of ' + str(bits_min) + ' bits, ' + str(bits_pref) + ' bits and ' + str(bits_max) + ' bits...', write_now=True)

                # It has been observed that reconnecting to some SSH servers
                # multiple times in quick succession can eventually result
                # in a "connection reset by peer" error. It may be possible
                # to recover from such an error by sleeping for some time
                # before continuing to issue reconnects.
                if GEXTest.reconnect(out, s, kex, gex_alg) is False:
                    out.fail('Reconnect failed.')
                    return exitcodes.FAILURE
                try:
                    modulus_size_returned = None
                    kex_group.send_init_gex(s, bits_min, bits_pref, bits_max)
                    kex_group.recv_reply(s, False)
                    modulus_size_returned = kex_group.get_dh_modulus_size()
                    out.d('Modulus size returned by server: ' + str(modulus_size_returned) + ' bits', write_now=True)
                except KexDHException:
                    out.d('[exception] ' + str(traceback.format_exc()), write_now=True)
                finally:
                    # The server is in a state that is not re-testable,
                    # so there's nothing else to do with this open
                    # connection.
                    s.close()

                if modulus_size_returned is not None:
                    if gex_alg in modulus_dict:
                        if modulus_size_returned not in modulus_dict[gex_alg]:
                            modulus_dict[gex_alg].append(modulus_size_returned)
                    else:
                        modulus_dict[gex_alg] = [modulus_size_returned]

        return retval

    # Runs the DH moduli test against the specified target.
    @staticmethod
    def run(out: 'OutputBuffer', s: 'SSH_Socket', kex: 'SSH2_Kex') -> None:
        GEX_ALGS = {
            'diffie-hellman-group-exchange-sha1': KexGroupExchange_SHA1,
            'diffie-hellman-group-exchange-sha256': KexGroupExchange_SHA256,
        }

        # The previous RSA tests put the server in a state we can't
        # test.  So we need a new connection to start with a clean
        # slate.
        if s.is_connected():
            s.close()

        # Check if the server supports any of the group-exchange
        # algorithms.  If so, test each one.
        for gex_alg, kex_group_class in GEX_ALGS.items():
            if gex_alg in kex.kex_algorithms:
                out.d('Preparing to perform DH group exchange using ' + gex_alg + ' with min, pref and max modulus sizes of 512 bits, 1024 bits and 1536 bits...', write_now=True)

                if GEXTest.reconnect(out, s, kex, gex_alg) is False:
                    break

                kex_group = kex_group_class(out)
                smallest_modulus = -1

                # First try a range of weak sizes.
                try:
                    kex_group.send_init_gex(s, 512, 1024, 1536)
                    kex_group.recv_reply(s, False)

                    # Its been observed that servers will return a group
                    # larger than the requested max.  So just because we
                    # got here, doesn't mean the server is vulnerable...
                    smallest_modulus = kex_group.get_dh_modulus_size()
                    out.d('Modulus size returned by server: ' + str(smallest_modulus) + ' bits', write_now=True)

                except KexDHException:
                    out.d('[exception] ' + str(traceback.format_exc()), write_now=True)
                finally:
                    s.close()

                # Try an array of specific modulus sizes... one at a time.
                reconnect_failed = False
                for bits in [512, 768, 1024, 1536, 2048, 3072, 4096]:

                    # If we found one modulus size already, but we're about
                    # to test a larger one, don't bother.
                    if bits >= smallest_modulus > 0:
                        break

                    out.d('Preparing to perform DH group exchange using ' + gex_alg + ' with min, pref and max modulus sizes of ' + str(bits) + ' bits...', write_now=True)

                    if GEXTest.reconnect(out, s, kex, gex_alg) is False:
                        reconnect_failed = True
                        break

                    try:
                        kex_group.send_init_gex(s, bits, bits, bits)
                        kex_group.recv_reply(s, False)
                        smallest_modulus = kex_group.get_dh_modulus_size()
                        out.d('Modulus size returned by server: ' + str(smallest_modulus) + ' bits', write_now=True)
                    except KexDHException as e:
                        out.d('Exception when testing DH group exchange ' + gex_alg + ' with modulus size ' + str(bits) + '.  (Hint: this is probably normal since the server does not support this modulus size.): ' + str(e), write_now=True)
                    finally:
                        # The server is in a state that is not re-testable,
                        # so there's nothing else to do with this open
                        # connection.
                        s.close()

                if smallest_modulus > 0:
                    kex.set_dh_modulus_size(gex_alg, smallest_modulus)

                    # We flag moduli smaller than 2048 as a failure.
                    if smallest_modulus < 2048:
                        text = 'using small %d-bit modulus' % smallest_modulus
                        lst = SSH2_KexDB.get_db()['kex'][gex_alg]
                        # For 'diffie-hellman-group-exchange-sha256', add
                        # a failure reason.
                        if len(lst) == 1:
                            lst.append([text])
                        # For 'diffie-hellman-group-exchange-sha1', delete
                        # the existing failure reason (which is vague), and
                        # insert our own.
                        else:
                            del lst[1]
                            lst.insert(1, [text])

                    # Moduli smaller than 3072 get flagged as a warning.
                    elif smallest_modulus < 3072:
                        lst = SSH2_KexDB.get_db()['kex'][gex_alg]

                        # Ensure that a warning list exists for us to append to, below.
                        while len(lst) < 3:
                            lst.append([])

                        # Ensure this is only added once.
                        text = '2048-bit modulus only provides 112-bits of symmetric strength'
                        if text not in lst[2]:
                            lst[2].append(text)

                if reconnect_failed:
                    break