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Added RSA certificate auditing.

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This commit is contained in:
Joe Testa 2017-09-26 20:46:00 -04:00
parent 33ae2946ea
commit ee5dde1cde
1 changed files with 245 additions and 112 deletions
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ssh-audit.py
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@ -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)