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Send peer a list of supported algorithms after the banner exchange. Fixes not only the weird case of an ssh-audit client hanging against an ssh-audit server, but perhaps some real-world hangs as well.

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This commit is contained in:
Joe Testa 2020-08-11 20:11:42 -04:00
parent 6d84cfdc31
commit 1123ac718c
1 changed files with 43 additions and 8 deletions
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51
ssh-audit.py
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@ -45,7 +45,7 @@ from typing import Dict, List, Set, Sequence, Tuple, Iterable
from typing import Callable, Optional, Union, Any
VERSION = 'v2.2.1-dev'
SSH_HEADER = 'SSH-{0}-OpenSSH_8.0' # SSH software to impersonate
SSH_HEADER = 'SSH-{0}-OpenSSH_8.2' # SSH software to impersonate
GITHUB_ISSUES_URL = 'https://github.com/jtesta/ssh-audit/issues' # The URL to the Github issues tracker.
# The program return values corresponding to failure(s) encountered, warning(s) encountered, connection errors, and no problems found, respectively.
@ -1133,6 +1133,12 @@ class SSH2: # pylint: disable=too-few-public-methods
hostkey_modulus_size = 0
ca_modulus_size = 0
# If the connection still exists, close it so we can test
# using a clean slate (otherwise it may exist in a non-testable
# state).
if s.is_connected():
s.close()
# For each host key type...
for host_key_type in host_key_types:
# Skip those already handled (i.e.: those in the RSA family, as testing one tests them all).
@ -1146,7 +1152,10 @@ class SSH2: # pylint: disable=too-few-public-methods
# If the connection is closed, re-open it and get the kex again.
if not s.is_connected():
s.connect()
err = s.connect()
if err is not None:
return
unused = None # pylint: disable=unused-variable
unused2 = None # pylint: disable=unused-variable
unused, unused2, err = s.get_banner()
@ -1222,7 +1231,10 @@ class SSH2: # pylint: disable=too-few-public-methods
if s.is_connected():
return True
s.connect()
err = s.connect()
if err is not None:
return False
unused = None # pylint: disable=unused-variable
unused2 = None # pylint: disable=unused-variable
unused, unused2, err = s.get_banner()
@ -2445,7 +2457,8 @@ class SSH: # pylint: disable=too-few-public-methods
c.settimeout(self.__timeout)
self.__sock = c
def connect(self) -> None:
def connect(self) -> Optional[str]:
'''Returns None on success, or an error string.'''
err = None
for af, addr in self._resolve(self.__ipvo):
s = None
@ -2454,7 +2467,7 @@ class SSH: # pylint: disable=too-few-public-methods
s.settimeout(self.__timeout)
s.connect(addr)
self.__sock = s
return
return None
except socket.error as e:
err = e
self._close_socket(s)
@ -2463,8 +2476,7 @@ class SSH: # pylint: disable=too-few-public-methods
else:
errt = (self.__host, self.__port, err)
errm = 'cannot connect to {} port {}: {}'.format(*errt)
out.fail('[exception] {}'.format(errm))
sys.exit(PROGRAM_RETVAL_CONNECTION_ERROR)
return '[exception] {}'.format(errm)
def get_banner(self, sshv: int = 2) -> Tuple[Optional['SSH.Banner'], List[str], Optional[str]]:
if self.__sock is None:
@ -2522,6 +2534,23 @@ class SSH: # pylint: disable=too-few-public-methods
except socket.error as e:
return -1, str(e.args[-1])
def send_algorithms(self) -> None:
'''Sends the list of supported host keys, key exchanges, ciphers, and MACs. Emulates OpenSSH v8.2.'''
key_exchanges = ['curve25519-sha256', 'curve25519-sha256@libssh.org', 'ecdh-sha2-nistp256', 'ecdh-sha2-nistp384', 'ecdh-sha2-nistp521', 'diffie-hellman-group-exchange-sha256', 'diffie-hellman-group16-sha512', 'diffie-hellman-group18-sha512', 'diffie-hellman-group14-sha256']
hostkeys = ['rsa-sha2-512', 'rsa-sha2-256', 'ssh-rsa', 'ecdsa-sha2-nistp256', 'ssh-ed25519']
ciphers = ['chacha20-poly1305@openssh.com', 'aes128-ctr', 'aes192-ctr', 'aes256-ctr', 'aes128-gcm@openssh.com', 'aes256-gcm@openssh.com']
macs = ['umac-64-etm@openssh.com', 'umac-128-etm@openssh.com', 'hmac-sha2-256-etm@openssh.com', 'hmac-sha2-512-etm@openssh.com', 'hmac-sha1-etm@openssh.com', 'umac-64@openssh.com', 'umac-128@openssh.com', 'hmac-sha2-256', 'hmac-sha2-512', 'hmac-sha1']
compressions = ['none', 'zlib@openssh.com']
languages = ['']
kexparty = SSH2.KexParty(ciphers, macs, compressions, languages)
kex = SSH2.Kex(os.urandom(16), key_exchanges, hostkeys, kexparty, kexparty, False, 0)
self.write_byte(SSH.Protocol.MSG_KEXINIT)
kex.write(self)
self.send_packet()
def send_banner(self, banner: str) -> None:
self.send(banner.encode() + b'\r\n')
if self.__state < self.SM_BANNER_SENT:
@ -3659,7 +3688,11 @@ def audit(aconf: AuditConf, sshv: Optional[int] = None, print_target: bool = Fal
if aconf.client_audit:
s.listen_and_accept()
else:
s.connect()
err = s.connect()
if err is not None:
out.fail(err)
sys.exit(PROGRAM_RETVAL_CONNECTION_ERROR)
if sshv is None:
sshv = 2 if aconf.ssh2 else 1
err = None
@ -3670,6 +3703,8 @@ def audit(aconf: AuditConf, sshv: Optional[int] = None, print_target: bool = Fal
else:
err = '[exception] did not receive banner: {}'.format(err)
if err is None:
s.send_algorithms() # Send the algorithms we support (except we don't since this isn't a real SSH connection).
packet_type, payload = s.read_packet(sshv)
if packet_type < 0:
try: