From ee5dde1cde30ed7dd94b8199cc19a71b5bd8b7ec Mon Sep 17 00:00:00 2001 From: Joe Testa Date: Tue, 26 Sep 2017 20:46:00 -0400 Subject: [PATCH] Added RSA certificate auditing. --- ssh-audit.py | 357 +++++++++++++++++++++++++++++++++++---------------- 1 file changed, 245 insertions(+), 112 deletions(-) diff --git a/ssh-audit.py b/ssh-audit.py index a750269..ee12fb6 100755 --- a/ssh-audit.py +++ b/ssh-audit.py @@ -462,14 +462,14 @@ class SSH2(object): # pylint: disable=too-few-public-methods # type: () -> int return self.__unused - def set_rsa_hostkey_size(self, rsa_type, rsa_hostkey_size): - self.__rsa_key_sizes[rsa_type] = rsa_hostkey_size; + def set_rsa_key_size(self, rsa_type, hostkey_size, ca_size=-1): + self.__rsa_key_sizes[rsa_type] = (hostkey_size, ca_size) - def rsa_hostkey_sizes(self): + def rsa_key_sizes(self): return self.__rsa_key_sizes def set_dh_modulus_size(self, gex_alg, modulus_size): - self.__dh_modulus_sizes[gex_alg] = modulus_size + self.__dh_modulus_sizes[gex_alg] = (modulus_size, -1) def dh_modulus_sizes(self): return self.__dh_modulus_sizes @@ -522,9 +522,10 @@ class SSH2(object): # pylint: disable=too-few-public-methods # Obtains RSA host keys and checks their size. class RSAKeyTest(object): RSA_TYPES = ['ssh-rsa', 'rsa-sha2-256', 'rsa-sha2-512'] + RSA_CA_TYPES = ['ssh-rsa-cert-v01@openssh.com'] @staticmethod - def run(s, kex): + def run(s, server_kex): KEX_TO_DHGROUP = { 'diffie-hellman-group1-sha1': KexGroup1, 'diffie-hellman-group14-sha1': KexGroup14_SHA1, @@ -543,57 +544,97 @@ class SSH2(object): # pylint: disable=too-few-public-methods # Pick the first kex algorithm that the server supports, which we # happen to support as well. - selected_kex_str = None + kex_str = None kex_group = None - for server_kex_alg in kex.kex_algorithms: + for server_kex_alg in server_kex.kex_algorithms: if server_kex_alg in KEX_TO_DHGROUP: - selected_kex_str = server_kex_alg - kex_group = KEX_TO_DHGROUP[server_kex_alg]() + kex_str = server_kex_alg + kex_group = KEX_TO_DHGROUP[kex_str]() break + if kex_str is not None: + SSH2.RSAKeyTest.__test(s, server_kex, kex_str, kex_group, SSH2.RSAKeyTest.RSA_TYPES) + SSH2.RSAKeyTest.__test(s, server_kex, kex_str, kex_group, SSH2.RSAKeyTest.RSA_CA_TYPES, ca=True) + + @staticmethod + def __test(s, server_kex, kex_str, kex_group, rsa_types, ca=False): # If the server supports one of the RSA types, extract its key size. - modulus_size = 0 - if selected_kex_str is not None: - for rsa_type in SSH2.RSAKeyTest.RSA_TYPES: - if rsa_type in kex.key_algorithms: + hostkey_modulus_size = 0 + ca_modulus_size = 0 + ran_test = False - # Send the server our KEXINIT message, using only our - # selected kex and RSA type. Send the server's own - # list of ciphers and MACs back to it (this doesn't - # matter, really). - client_kex = SSH2.Kex(os.urandom(16), [selected_kex_str], [rsa_type], kex.client, kex.server, 0, 0) + # If the connection is closed, re-open it and get the kex again. + if not s.is_connected(): + s.connect() + unused1, unused2, err = s.get_banner() + if err is not None: + s.close() + return - s.write_byte(SSH.Protocol.MSG_KEXINIT) - client_kex.write(s) - s.send_packet() + # Parse the server's initial KEX. + packet_type, payload = s.read_packet() + SSH2.Kex.parse(payload) - # Do the initial DH exchange. The server responds back - # with the host key and its length. Bingo. - kex_group.send_init(s) - kex_group.recv_reply(s) - modulus_size = kex_group.get_hostkey_size() - # We only need to test one RSA type, since the others - # will all be the same. + for rsa_type in rsa_types: + if rsa_type in server_kex.key_algorithms: + ran_test = True + + # Send the server our KEXINIT message, using only our + # selected kex and RSA type. Send the server's own + # list of ciphers and MACs back to it (this doesn't + # matter, really). + client_kex = SSH2.Kex(os.urandom(16), [kex_str], [rsa_type], server_kex.client, server_kex.server, 0, 0) + + s.write_byte(SSH.Protocol.MSG_KEXINIT) + client_kex.write(s) + s.send_packet() + + # Do the initial DH exchange. The server responds back + # with the host key and its length. Bingo. + kex_group.send_init(s) + kex_group.recv_reply(s) + + hostkey_modulus_size = kex_group.get_hostkey_size() + ca_modulus_size = kex_group.get_ca_size() + + # If we're not working with the CA types, we only need to + # test one RSA key, since the others will all be the same. + if ca is False: break - if modulus_size > 0: - + if hostkey_modulus_size > 0 or ca_modulus_size > 0: # Set the hostkey size for all RSA key types since 'ssh-rsa', # 'rsa-sha2-256', etc. are all using the same host key. - for rsa_type in SSH2.RSAKeyTest.RSA_TYPES: - kex.set_rsa_hostkey_size(rsa_type, modulus_size) + # Note, however, that this may change in the future. + if ca is False: + for rsa_type in rsa_types: + server_kex.set_rsa_key_size(rsa_type, hostkey_modulus_size) + else: + server_kex.set_rsa_key_size(rsa_type, hostkey_modulus_size, ca_modulus_size) # Keys smaller than 2048 result in a failure. fail = False - if modulus_size < 2048: + if hostkey_modulus_size < 2048 or (ca_modulus_size < 2048 and ca_modulus_size > 0): fail = True # If this is a bad key size, update the database accordingly. if fail: - for rsa_type in SSH2.RSAKeyTest.RSA_TYPES: + if ca is False: + for rsa_type in SSH2.RSAKeyTest.RSA_TYPES: + alg_list = SSH2.KexDB.ALGORITHMS['key'][rsa_type] + alg_list.append(['using small %d-bit modulus' % hostkey_modulus_size]) + else: alg_list = SSH2.KexDB.ALGORITHMS['key'][rsa_type] - alg_list.append(['using small %d-bit modulus' % modulus_size]) + + min_modulus = min(hostkey_modulus_size, ca_modulus_size) + min_modulus = min_modulus if min_modulus > 0 else max(hostkey_modulus_size, ca_modulus_size) + alg_list.append(['using small %d-bit modulus' % min_modulus]) + + # If we ran any tests, close the socket, as the connection has + # been put in a state that later tests can't use. + if ran_test: + s.close() # Performs DH group exchanges to find what moduli are supported, and checks # their size. @@ -602,13 +643,15 @@ class SSH2(object): # pylint: disable=too-few-public-methods # Creates a new connection to the server. Returns an SSH.Socket, or # None on failure. @staticmethod - def reconnect(ipvo, host, port, gex_alg): - s = SSH.Socket(host, port) - s.connect(ipvo) + def reconnect(s, gex_alg): + if s.is_connected(): + return + + s.connect() unused, unused, err = s.get_banner() if err is not None: s.close() - return None + return False # Parse the server's initial KEX. packet_type, payload = s.read_packet(2) @@ -620,32 +663,31 @@ class SSH2(object): # pylint: disable=too-few-public-methods s.write_byte(SSH.Protocol.MSG_KEXINIT) client_kex.write(s) s.send_packet() - return s + return True # Runs the DH moduli test against the specified target. @staticmethod - def run(ipvo, host, port, s, kex): + def run(s, kex): 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 in GEX_ALGS: if gex_alg in kex.kex_algorithms: - # 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 not None: - s.close() - s = None - - s = SSH2.GEXTest.reconnect(ipvo, host, port, gex_alg) - if s is None: + if SSH2.GEXTest.reconnect(s, gex_alg) is False: break + kex_group = GEX_ALGS[gex_alg]() smallest_modulus = -1 @@ -657,12 +699,11 @@ class SSH2(object): # pylint: disable=too-few-public-methods # 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_modulus_size() + smallest_modulus = kex_group.get_dh_modulus_size() except Exception as e: pass finally: s.close() - s = None # Try an array of specific modulus sizes... one at a time. reconnect_failed = False @@ -673,21 +714,22 @@ class SSH2(object): # pylint: disable=too-few-public-methods if smallest_modulus > 0 and bits >= smallest_modulus: break - if s is None: - s = SSH2.GEXTest.reconnect(ipvo, host, port, gex_alg) - if s is None: - reconnect_failed = True - break + if SSH2.GEXTest.reconnect(s, gex_alg) is False: + reconnect_failed = True + break try: kex_group.send_init(s, bits, bits, bits) kex_group.recv_reply(s) - smallest_modulus = kex_group.get_modulus_size() + smallest_modulus = kex_group.get_dh_modulus_size() except Exception as e: pass 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() - s = None + if smallest_modulus > 0: kex.set_dh_modulus_size(gex_alg, smallest_modulus) @@ -699,7 +741,7 @@ class SSH2(object): # pylint: disable=too-few-public-methods # For 'diffie-hellman-group-exchange-sha256', add # a failure reason. if len(lst) == 1: - lst.append(text) + lst.append([text]) # For 'diffie-hellman-group-exchange-sha1', delete # the existing failure reason (which is vague), and # insert our own. @@ -975,6 +1017,10 @@ class ReadBuf(object): # type: () -> text_type return self._buf.readline().rstrip().decode('utf-8', 'replace') + def reset(self): + self._buf = BytesIO() + self._len = 0 + super(ReadBuf, self).reset() class WriteBuf(object): def __init__(self, data=None): @@ -1064,6 +1110,9 @@ class WriteBuf(object): self._wbuf.seek(0) return payload + def reset(self): + self._wbuf = BytesIO() + class SSH(object): # pylint: disable=too-few-public-methods class Protocol(object): # pylint: disable=too-few-public-methods @@ -1567,10 +1616,8 @@ class SSH(object): # pylint: disable=too-few-public-methods for alg_type, alg_list in alg_pair.items(): if alg_type == 'aut': continue - rec[sshv][alg_type] = {'add': {}, 'del': {}} + rec[sshv][alg_type] = {'add': {}, 'del': {}, 'chg': {}} for n, alg_desc in alg_db[alg_type].items(): - if alg_type == 'key' and '-cert-' in n: - continue versions = alg_desc[0] if len(versions) == 0 or versions[0] is None: continue @@ -1597,18 +1644,19 @@ class SSH(object): # pylint: disable=too-few-public-methods if fc > 0: faults += pow(10, 2 - i) * fc if n not in alg_list: - if faults > 0: + if faults > 0 or (alg_type == 'key' and '-cert-' in n): continue rec[sshv][alg_type]['add'][n] = 0 else: if faults == 0: continue - if n == 'diffie-hellman-group-exchange-sha256': - if software.compare_version('7.3') < 0: - continue - rec[sshv][alg_type]['del'][n] = faults + if n in ['diffie-hellman-group-exchange-sha256', 'ssh-rsa', 'rsa-sha2-256', 'rsa-sha2-512', 'ssh-rsa-cert-v01@openssh.com']: + rec[sshv][alg_type]['chg'][n] = faults + else: + rec[sshv][alg_type]['del'][n] = faults add_count = len(rec[sshv][alg_type]['add']) del_count = len(rec[sshv][alg_type]['del']) + chg_count = len(rec[sshv][alg_type]['chg']) new_alg_count = len(alg_list) + add_count - del_count if new_alg_count < 1 and del_count > 0: mf = min(rec[sshv][alg_type]['del'].values()) @@ -1626,6 +1674,8 @@ class SSH(object): # pylint: disable=too-few-public-methods del rec[sshv][alg_type]['add'] if del_count == 0: del rec[sshv][alg_type]['del'] + if chg_count == 0: + del rec[sshv][alg_type]['chg'] if len(rec[sshv][alg_type]) == 0: del rec[sshv][alg_type] if len(rec[sshv]) == 0: @@ -1770,6 +1820,7 @@ class SSH(object): # pylint: disable=too-few-public-methods raise ValueError('invalid port: {0}'.format(port)) self.__host = host self.__port = nport + self.__ipvo = () def _resolve(self, ipvo): # type: (Sequence[int]) -> Iterable[Tuple[int, Tuple[Any, ...]]] @@ -1794,10 +1845,12 @@ class SSH(object): # pylint: disable=too-few-public-methods out.fail('[exception] {0}'.format(e)) sys.exit(1) - def connect(self, ipvo=(), cto=3.0, rto=5.0): + def connect(self, ipvo=None, cto=3.0, rto=5.0): # type: (Sequence[int], float, float) -> None err = None - for af, addr in self._resolve(ipvo): + if ipvo is not None: + self.__ipvo = ipvo + for af, addr in self._resolve(self.__ipvo): s = None try: s = socket.socket(af, socket.SOCK_STREAM) @@ -1956,8 +2009,19 @@ class SSH(object): # pylint: disable=too-few-public-methods data = struct.pack('>Ib', plen, padding) + payload + pad_bytes return self.send(data) + # Returns True if this Socket is connected, otherwise False. + def is_connected(self): + return (self.__sock is not None) + def close(self): self.__cleanup() + self.reset() + self.__state = 0 + self.__header = [] + self.__banner = None + + def reset(self): + super(SSH.Socket, self).reset() def _close_socket(self, s): # type: (Optional[socket.socket]) -> None @@ -1975,6 +2039,7 @@ class SSH(object): # pylint: disable=too-few-public-methods def __cleanup(self): # type: () -> None self._close_socket(self.__sock) + self.__sock = None class KexDH(object): # pragma: nocover @@ -1988,7 +2053,8 @@ class KexDH(object): # pragma: nocover self.__hostkey_type = None self.__hostkey_e = 0 self.__hostkey_n = 0 - self.__hostkey_n_len = 0 # This is the length of the host key modulus. + self.__hostkey_n_len = 0 # Length of the host key modulus. + self.__ca_n_len = 0 # Length of the CA key modulus (if hostkey is a cert). self.__f = 0 self.__h_sig = 0 @@ -2017,47 +2083,97 @@ class KexDH(object): # pragma: nocover # TODO: change Exception to something more specific. raise Exception('Expected MSG_KEXDH_REPLY (%d) or MSG_KEXDH_GEX_REPLY (%d), but got %d instead.' % (SSH.Protocol.MSG_KEXDH_REPLY, SSH.Protocol.MSG_KEXDH_GEX_REPLY, packet_type)) - host_key_len = struct.unpack('>I', payload[0:4])[0] - ptr = 4 - - hostkey = payload[ptr:ptr + host_key_len] - ptr += host_key_len - - f_len = struct.unpack('>I', payload[ptr:ptr+4])[0] - ptr += 4 - - self.__f = payload[ptr:ptr + f_len] - ptr += f_len - - h_sig_len = struct.unpack('>I', payload[ptr:ptr+4])[0] - ptr += 4 - - self.__h_sig = payload[ptr:ptr + h_sig_len] - ptr += h_sig_len + # Get the host key blob, F, and signature. + ptr = 0 + hostkey, hostkey_len, ptr = KexDH.__get_bytes(payload, ptr) + self.__f, f_len, ptr = KexDH.__get_bytes(payload, ptr) + self.__h_sig, h_sig_len, ptr = KexDH.__get_bytes(payload, ptr) # Now pick apart the host key blob. - hostkey_type_len = struct.unpack('>I', hostkey[0:4])[0] - ptr = 4 - # Get the host key type (i.e.: 'ssh-rsa', 'ssh-ed25519', etc). - self.__hostkey_type = hostkey[ptr:ptr + hostkey_type_len] - ptr += hostkey_type_len + ptr = 0 + self.__hostkey_type, hostkey_type_len, ptr = KexDH.__get_bytes(hostkey, ptr) - hostkey_e_len = struct.unpack('>I', hostkey[ptr:ptr + 4])[0] - ptr += 4 + # If this is an RSA certificate, skip over the nonce. + if self.__hostkey_type.startswith('ssh-rsa-cert-v0'): + nonce, nonce_len, ptr = KexDH.__get_bytes(hostkey, ptr) - self.__hostkey_e = int(binascii.hexlify(hostkey[ptr:ptr + hostkey_e_len]), 16) - ptr += hostkey_e_len + # The public key exponent. + hostkey_e, hostkey_e_len, ptr = KexDH.__get_bytes(hostkey, ptr) + self.__hostkey_e = int(binascii.hexlify(hostkey_e), 16) # Here is the modulus size & actual modulus of the host key public key. - self.__hostkey_n_len = struct.unpack('>I', hostkey[ptr:ptr + 4])[0] + hostkey_n, self.__hostkey_n_len, ptr = KexDH.__get_bytes(hostkey, ptr) + self.__hostkey_n = int(binascii.hexlify(hostkey_n), 16) + + # If this is an RSA certificate, continue parsing to extract the CA + # key. + if self.__hostkey_type.startswith('ssh-rsa-cert-v0'): + # Skip over the serial number. + ptr += 8 + + # Get the certificate type. + cert_type = int(binascii.hexlify(hostkey[ptr:ptr + 4]), 16) + ptr += 4 + + # Only SSH2_CERT_TYPE_HOST (2) makes sense in this context. + if cert_type == 2: + + # Skip the key ID (this is the serial number of the + # certificate). + key_id, key_id_len, ptr = KexDH.__get_bytes(hostkey, ptr) + + # The principles, which are... I don't know what. + principles, printicples_len, ptr = KexDH.__get_bytes(hostkey, ptr) + + # The timestamp that this certificate is valid after. + valid_after = hostkey[ptr:ptr + 8] + ptr += 8 + + # The timestamp that this certificate is valid before. + valid_before = hostkey[ptr:ptr + 8] + ptr += 8 + + # TODO: validate the principles, and time range. + + # The critical options. + critical_options, critical_options_len, ptr = KexDH.__get_bytes(hostkey, ptr) + + # Certificate extensions. + extensions, extensions_len, ptr = KexDH.__get_bytes(hostkey, ptr) + + # Another nonce. + nonce, nonce_len, ptr = KexDH.__get_bytes(hostkey, ptr) + + # Finally, we get to the CA key. + ca_key, ca_key_len, ptr = KexDH.__get_bytes(hostkey, ptr) + + # Last in the host key blob is the CA signature. It isn't + # interesting to us, so we won't bother parsing any further. + # The CA key has the modulus, however... + ptr = 0 + + # 'ssh-rsa', 'rsa-sha2-256', etc. + ca_key_type, ca_key_type_len, ptr = KexDH.__get_bytes(ca_key, ptr) + + # CA's public key exponent. + ca_key_e, ca_key_e_len, ptr = KexDH.__get_bytes(ca_key, ptr) + + # CA's modulus. Bingo. + ca_key_n, self.__ca_n_len, ptr = KexDH.__get_bytes(ca_key, ptr) + + + @staticmethod + def __get_bytes(buf, ptr): + num_bytes = struct.unpack('>I', buf[ptr:ptr + 4])[0] ptr += 4 - self.__hostkey_n = int(binascii.hexlify(hostkey[ptr:ptr + self.__hostkey_n_len]), 16) - - # Returns the size of the hostkey, in bits. - def get_hostkey_size(self): - size = self.__hostkey_n_len * 8 + return buf[ptr:ptr + num_bytes], num_bytes, ptr + num_bytes + # Converts a modulus length in bytes to its size in bits, after some + # possible adjustments. + @staticmethod + def __adjust_key_size(size): + size = size * 8 # Actual keys are observed to be about 8 bits bigger than expected # (i.e.: 1024-bit keys have a 1032-bit modulus). Check if this is # the case, and subtract 8 if so. This simply improves readability @@ -2066,8 +2182,16 @@ class KexDH(object): # pragma: nocover size = size - 8 return size + # Returns the size of the hostkey, in bits. + def get_hostkey_size(self): + return KexDH.__adjust_key_size(self.__hostkey_n_len) + + # Returns the size of the CA key, in bits. + def get_ca_size(self): + return KexDH.__adjust_key_size(self.__ca_n_len) + # Returns the size of the DH modulus, in bits. - def get_modulus_size(self): + def get_dh_modulus_size(self): # -2 to account for the '0b' prefix in the string. return len(bin(self.__p)) - 2 @@ -2301,8 +2425,13 @@ def output_algorithm(alg_db, alg_type, alg_name, alg_max_len=0, alg_sizes=None): # the padding. alg_name_with_size = None if (alg_sizes is not None) and (alg_name in alg_sizes): - alg_name_with_size = '%s (%d-bit)' % (alg_name, alg_sizes[alg_name]) - padding = padding[0:-11] + hostkey_size, ca_size = alg_sizes[alg_name] + if ca_size > 0: + alg_name_with_size = '%s (%d-bit cert/%d-bit CA)' % (alg_name, hostkey_size, ca_size) + padding = padding[0:-15] + else: + alg_name_with_size = '%s (%d-bit)' % (alg_name, hostkey_size) + padding = padding[0:-11] texts = [] if len(alg_name.strip()) == 0: @@ -2439,20 +2568,24 @@ def output_recommendations(algs, software, padlen=0): for alg_type in ['kex', 'key', 'enc', 'mac']: if alg_type not in alg_rec[sshv]: continue - for action in ['del', 'add']: + for action in ['del', 'add', 'chg']: if action not in alg_rec[sshv][alg_type]: continue for name in alg_rec[sshv][alg_type][action]: p = '' if out.batch else ' ' * (padlen - len(name)) + chg_additional_info = '' if action == 'del': an, sg, fn = 'remove', '-', out.warn if alg_rec[sshv][alg_type][action][name] >= 10: fn = out.fail - else: + elif action == 'add': an, sg, fn = 'append', '+', out.good + elif action == 'chg': + an, sg, fn = 'change', '!', out.fail + chg_additional_info = ' (increase modulus size to 2048 bits or larger)' b = '(SSH{0})'.format(sshv) if sshv == 1 else '' - fm = '(rec) {0}{1}{2}-- {3} algorithm to {4} {5}' - fn(fm.format(sg, name, p, alg_type, an, b)) + fm = '(rec) {0}{1}{2}-- {3} algorithm to {4}{5} {6}' + fn(fm.format(sg, name, p, alg_type, an, chg_additional_info, b)) if len(obuf) > 0: if software is not None: title = '(for {0})'.format(software.display(False)) @@ -2511,7 +2644,7 @@ def output(banner, header, kex=None, pkm=None): title, atype = 'key exchange algorithms', 'kex' output_algorithms(title, adb, atype, kex.kex_algorithms, maxlen, kex.dh_modulus_sizes()) title, atype = 'host-key algorithms', 'key' - output_algorithms(title, adb, atype, kex.key_algorithms, maxlen, kex.rsa_hostkey_sizes()) + output_algorithms(title, adb, atype, kex.key_algorithms, maxlen, kex.rsa_key_sizes()) title, atype = 'encryption algorithms (ciphers)', 'enc' output_algorithms(title, adb, atype, kex.server.encryption, maxlen) title, atype = 'message authentication code algorithms', 'mac' @@ -2687,7 +2820,7 @@ def audit(aconf, sshv=None): elif sshv == 2: kex = SSH2.Kex.parse(payload) SSH2.RSAKeyTest.run(s, kex) - SSH2.GEXTest.run(aconf.ipvo, aconf.host, aconf.port, s, kex) + SSH2.GEXTest.run(s, kex) output(banner, header, kex=kex)