mirror of
https://github.com/jtesta/ssh-audit.git
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DH GEX Modulus Size Testing
This commit is contained in:
Adam Russell
Joe Testa
parent
0a6ac5de54
commit
5ac0ffa8f1
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@ -59,6 +59,7 @@ class AuditConf:
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self.lookup = ''
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self.manual = False
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self.debug = False
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self.gex_test = ''
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def __setattr__(self, name: str, value: Union[str, int, float, bool, Sequence[int]]) -> None:
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valid = False
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@ -86,7 +87,7 @@ class AuditConf:
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if value == -1.0:
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raise ValueError('invalid timeout: {}'.format(value))
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valid = True
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elif name in ['ip_version_preference', 'lookup', 'policy_file', 'policy', 'target_file', 'target_list']:
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elif name in ['ip_version_preference', 'lookup', 'policy_file', 'policy', 'target_file', 'target_list', 'gex_test']:
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valid = True
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elif name == "threads":
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valid, num_threads = True, Utils.parse_int(value)
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@ -33,6 +33,7 @@ from ssh_audit.ssh2_kexdb import SSH2_KexDB
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from ssh_audit.ssh2_kex import SSH2_Kex
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from ssh_audit.ssh_socket import SSH_Socket
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from ssh_audit.outputbuffer import OutputBuffer
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from ssh_audit import exitcodes
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# Performs DH group exchanges to find what moduli are supported, and checks
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@ -70,6 +71,76 @@ class GEXTest:
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return True
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# Tests for modulus size in bits against the specified target.
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@staticmethod
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def 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:
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'''
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Tests for modulus size in bits against the target target.
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Builds a dictionary, where a key represents a DH algorithm name and the
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values are the modulus sizes (in bits) that have been returned by the
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target server.
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Returns an exitcodes.* flag.
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'''
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retval = exitcodes.GOOD
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out.d("Starting modulus_size_test...")
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out.d("Bits Min: " + str(bits_min))
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out.d("Bits Pref: " + str(bits_pref))
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out.d("Bits Max: " + str(bits_max))
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if all(x < 0 for x in (bits_min, bits_pref, bits_max)):
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out.fail("min, pref and max values cannot be negative.")
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return exitcodes.FAILURE
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GEX_ALGS = {
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'diffie-hellman-group-exchange-sha1': KexGroupExchange_SHA1,
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'diffie-hellman-group-exchange-sha256': KexGroupExchange_SHA256,
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}
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# Check if the server supports any of the group-exchange
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# algorithms. If so, test each one.
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for gex_alg in GEX_ALGS:
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if gex_alg not in kex.kex_algorithms:
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out.d('Server does not support the algorithm "' + gex_alg + '".', write_now=True)
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else:
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kex_group = GEX_ALGS[gex_alg]()
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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)
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# It has been observed that reconnecting to some SSH servers
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# multiple times in quick succession can eventually result
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# in a "connection reset by peer" error. It may be possible
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# to recover from such an error by sleeping for some time
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# before continuing to issue reconnects.
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if GEXTest.reconnect(out, s, kex, gex_alg) is False:
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out.fail('Reconnect failed.')
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return exitcodes.FAILURE
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try:
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modulus_size_returned = None
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kex_group.send_init_gex(s, bits_min, bits_pref, bits_max)
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kex_group.recv_reply(s, False)
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modulus_size_returned = kex_group.get_dh_modulus_size()
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out.d('Modulus size returned by server: ' + str(modulus_size_returned) + ' bits', write_now=True)
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except Exception as e:
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out.d('[exception] ' + str(e), write_now=True)
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# import traceback
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# print(traceback.format_exc())
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pass
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finally:
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# The server is in a state that is not re-testable,
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# so there's nothing else to do with this open
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# connection.
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s.close()
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if modulus_size_returned is not None:
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if gex_alg in modulus_dict:
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if modulus_size_returned not in modulus_dict[gex_alg]:
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modulus_dict[gex_alg].append(modulus_size_returned)
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else:
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modulus_dict[gex_alg] = [modulus_size_returned]
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return retval
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# Runs the DH moduli test against the specified target.
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@staticmethod
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def run(out: 'OutputBuffer', s: 'SSH_Socket', kex: 'SSH2_Kex') -> None:
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@ -88,7 +159,8 @@ class GEXTest:
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# algorithms. If so, test each one.
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for gex_alg, kex_group_class in GEX_ALGS.items():
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if gex_alg in kex.kex_algorithms:
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out.d('Preparing to perform DH group exchange using ' + gex_alg + '...', write_now=True)
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weak_sizes = 512, 1024, 1536
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out.d('Preparing to perform DH group exchange using ' + gex_alg + ' with min, pref and max modulus sizes of ' + str(weak_sizes[0]) + ' bits, ' + str(weak_sizes[1]) + ' bits and ' + str(weak_sizes[2]) + ' bits...', write_now=True)
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if GEXTest.reconnect(out, s, kex, gex_alg) is False:
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break
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@ -98,15 +170,17 @@ class GEXTest:
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# First try a range of weak sizes.
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try:
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kex_group.send_init_gex(s, 512, 1024, 1536)
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kex_group.send_init_gex(s, weak_sizes[0], weak_sizes[1], weak_sizes[2])
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kex_group.recv_reply(s, False)
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# Its been observed that servers will return a group
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# larger than the requested max. So just because we
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# got here, doesn't mean the server is vulnerable...
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smallest_modulus = kex_group.get_dh_modulus_size()
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out.d('Modulus size returned by server: ' + str(smallest_modulus) + ' bits', write_now=True)
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except Exception:
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except Exception as e:
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out.d('[exception] ' + str(e), write_now=True)
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pass
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finally:
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s.close()
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@ -120,7 +194,7 @@ class GEXTest:
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if bits >= smallest_modulus > 0:
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break
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out.d('Preparing to perform DH group exchange using ' + gex_alg + ' with modulus size ' + str(bits) + '...', write_now=True)
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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)
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if GEXTest.reconnect(out, s, kex, gex_alg) is False:
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reconnect_failed = True
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@ -130,7 +204,9 @@ class GEXTest:
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kex_group.send_init_gex(s, bits, bits, bits)
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kex_group.recv_reply(s, False)
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smallest_modulus = kex_group.get_dh_modulus_size()
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except Exception:
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out.d('Modulus size returned by server: ' + str(smallest_modulus) + ' bits', write_now=True)
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except Exception as e:
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out.d('[exception] ' + str(e), write_now=True)
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# import traceback
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# print(traceback.format_exc())
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pass
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@ -87,6 +87,7 @@ def usage(err: Optional[str] = None) -> None:
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uout.info(' -b, --batch batch output')
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uout.info(' -c, --client-audit starts a server on port 2222 to audit client\n software config (use -p to change port;\n use -t to change timeout)')
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uout.info(' -d, --debug debug output')
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uout.info(' -g, --gex-test=<n[,n,...] | min:pref:max[,min:pref:max,...] | n-n[:step]> dh gex modulus size test')
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uout.info(' -j, --json JSON output (use -jj to enable indents)')
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uout.info(' -l, --level=<level> minimum output level (info|warn|fail)')
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uout.info(' -L, --list-policies list all the official, built-in policies')
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@ -589,8 +590,8 @@ def process_commandline(out: OutputBuffer, args: List[str], usage_cb: Callable[.
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# pylint: disable=too-many-branches
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aconf = AuditConf()
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try:
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sopts = 'h1246M:p:P:jbcnvl:t:T:Lmd'
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lopts = ['help', 'ssh1', 'ssh2', 'ipv4', 'ipv6', 'make-policy=', 'port=', 'policy=', 'json', 'batch', 'client-audit', 'no-colors', 'verbose', 'level=', 'timeout=', 'targets=', 'list-policies', 'lookup=', 'threads=', 'manual', 'debug']
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sopts = 'h1246M:p:P:jbcnvl:t:T:Lmdg:'
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lopts = ['help', 'ssh1', 'ssh2', 'ipv4', 'ipv6', 'make-policy=', 'port=', 'policy=', 'json', 'batch', 'client-audit', 'no-colors', 'verbose', 'level=', 'timeout=', 'targets=', 'list-policies', 'lookup=', 'threads=', 'manual', 'debug', 'gex-test=']
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opts, args = getopt.gnu_getopt(args, sopts, lopts)
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except getopt.GetoptError as err:
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usage_cb(str(err))
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@ -652,6 +653,10 @@ def process_commandline(out: OutputBuffer, args: List[str], usage_cb: Callable[.
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elif o in ('-d', '--debug'):
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aconf.debug = True
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out.debug = True
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elif o in ('-g', '--gex-test'):
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if not((any(re.search(regex_str, a) for regex_str in get_permitted_syntax_for_gex_test().values()))):
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usage_cb('{} {} is not valid'.format(o, a))
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aconf.gex_test = a
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if len(args) == 0 and aconf.client_audit is False and aconf.target_file is None and aconf.list_policies is False and aconf.lookup == '' and aconf.manual is False:
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usage_cb()
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@ -922,7 +927,11 @@ def audit(out: OutputBuffer, aconf: AuditConf, sshv: Optional[int] = None, print
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if aconf.client_audit is False:
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HostKeyTest.run(out, s, kex)
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GEXTest.run(out, s, kex)
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if aconf.gex_test != '':
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program_retval = invoke_modulus_size_test(out, s, kex, aconf)
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return program_retval
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else:
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GEXTest.run(out, s, kex)
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# This is a standard audit scan.
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if (aconf.policy is None) and (aconf.make_policy is False):
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@ -1061,6 +1070,91 @@ def windows_manual(out: OutputBuffer) -> int:
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return retval
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def get_permitted_syntax_for_gex_test() -> Dict[str, str]:
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syntax = {
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'RANGE': r'^\d+-\d+(:\d+)?$',
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'LIST_WITHOUT_MIN_PREF_MAX': r'^\d+(,\d+)*$',
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'LIST_WITH_MIN_PREF_MAX': r'^\d+:\d+:\d+(,\d+:\d+:\d+)*$'
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}
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return syntax
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def invoke_modulus_size_test(out: OutputBuffer, s: 'SSH_Socket', kex: 'SSH2_Kex', aconf: AuditConf) -> int:
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'''Extracts the user specified modulus sizes and submits them for testing against the target target. Returns an exitcodes.* flag.'''
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permitted_syntax = get_permitted_syntax_for_gex_test()
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if not((any(re.search(regex_str, aconf.gex_test) for regex_str in permitted_syntax.values()))):
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out.fail("Invalid syntax.")
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return exitcodes.FAILURE
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mod_dict: Dict[str, List[int]] = {}
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# Range syntax.
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if re.search(permitted_syntax['RANGE'], aconf.gex_test):
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extracted_digits = re.findall(r'\d+', aconf.gex_test)
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bits_left_bound = int(extracted_digits[0])
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bits_right_bound = int(extracted_digits[1])
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if (len(extracted_digits)) == 3:
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bits_step = int(extracted_digits[2])
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else:
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bits_step = 1
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if bits_step <= 0:
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out.fail("Step value must be greater than zero.")
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return exitcodes.FAILURE
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if all(x < 0 for x in (bits_left_bound, bits_right_bound)):
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out.fail("Start and end values cannot be negative.")
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return exitcodes.FAILURE
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# If the left value is greater than the right value, then the sequence
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# operates from right to left.
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if bits_left_bound <= bits_right_bound:
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bits_in_range_to_test = range(bits_left_bound, bits_right_bound + 1, bits_step)
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else:
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bits_in_range_to_test = range(bits_left_bound, bits_right_bound - 1, -abs(bits_step))
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out.v("A separate test will be performed against each of the following modulus sizes: " + ", ".join([str(x) for x in bits_in_range_to_test]) + ".", write_now=True)
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for i_bits in bits_in_range_to_test:
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program_retval = GEXTest.modulus_size_test(out, s, kex, i_bits, i_bits, i_bits, mod_dict)
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if program_retval != exitcodes.GOOD:
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return program_retval
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# Two variations of list syntax.
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if re.search(permitted_syntax['LIST_WITHOUT_MIN_PREF_MAX'], aconf.gex_test):
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bits_in_list_to_test = aconf.gex_test.split(',')
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out.v("A separate test will be performed against each of the following modulus sizes: " + ", ".join([str(x) for x in bits_in_list_to_test]) + ".", write_now=True)
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for s_bits in bits_in_list_to_test:
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program_retval = GEXTest.modulus_size_test(out, s, kex, int(s_bits), int(s_bits), int(s_bits), mod_dict)
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if program_retval != exitcodes.GOOD:
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return program_retval
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if re.search(permitted_syntax['LIST_WITH_MIN_PREF_MAX'], aconf.gex_test):
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sets_of_min_pref_max = aconf.gex_test.split(',')
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out.v("A separate test will be performed against each of the following sets of 'min:pref:max' modulus sizes: " + ', '.join(sets_of_min_pref_max), write_now=True)
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for set_of_min_pref_max in sets_of_min_pref_max:
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bits_in_list_to_test = set_of_min_pref_max.split(':')
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program_retval = GEXTest.modulus_size_test(out, s, kex, int(bits_in_list_to_test[0]), int(bits_in_list_to_test[1]), int(bits_in_list_to_test[2]), mod_dict)
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if program_retval != exitcodes.GOOD:
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return program_retval
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if mod_dict:
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if aconf.json:
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json_struct = {'dh-gex-modulus-size': mod_dict}
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out.info(json.dumps(json_struct, indent=4 if aconf.json_print_indent else None, sort_keys=True))
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else:
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out.head('# diffie-hellman group exchange modulus size')
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max_key_len = len(max(mod_dict, key=len))
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for key, value in mod_dict.items():
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padding = (max_key_len - len(key)) + 1
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out.info(key + " " * padding + '--> ' + ', '.join(map(str, value)))
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return program_retval
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def main() -> int:
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out = OutputBuffer()
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aconf = process_commandline(out, sys.argv[1:], usage)
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65
ssh-audit.1
65
ssh-audit.1
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@ -1,4 +1,4 @@
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.TH SSH-AUDIT 1 "March 2, 2021"
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.TH SSH-AUDIT 1 "February 13, 2022"
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.SH NAME
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\fBssh-audit\fP \- SSH server & client configuration auditor
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.SH SYNOPSIS
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@ -51,6 +51,43 @@ Starts a server on port 2222 to audit client software configuration. Use -p/--p
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.br
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Enable debug output.
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.TP
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.B -g, \-\-gex-test=<n[,n,...] | min:pref:max[,min:pref:max,...] | n-n[:step]>
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.br
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Runs a Diffie-Hellman Group Exchange modulus size test against a server.
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Diffie-Hellman requires the client and server to agree on a generator value and
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a modulus value. In the "Group Exchange" implementation of Diffie-Hellman, the
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client specifies the size of the modulus in bits by providing the server with
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minimum, preferred and maximum values. The server then finds a group that best
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matches the client's request, returning the corresponding generator and modulus.
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For a full explanation of this process see RFC 4419 and its successors.
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This test acts as a client by providing an SSH server with the size of a modulus
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and then obtains the size of the modulus returned by the server.
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Three types of syntax are supported:
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1. <n[,n,...]>
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A comma delimited list of modulus sizes.
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A test is performed against each value in the list where it acts as the minimum, preferred and maximum modulus size.
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2. <min:pref:max[,min:pref:max,...]>
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A set of three colon delimited values denoting minimum, preferred and maximum modulus size.
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A test is performed against each set.
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Multiple sets can specified as a comma separated list.
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3. <n-n[:step]>
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A range of modulus sizes with an optional step value. Step defaults to 1 if omitted.
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If the left value is greater than the right value, then the sequence operates from right to left.
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A test is performed against each value in the range where it acts as the minimum, preferred and maximum modulus size.
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Duplicates are excluded from the return value.
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.TP
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.B -j, \-\-json
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.br
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@ -130,7 +167,7 @@ When the -P/--policy option is used, \fBssh-audit\fP performs a policy audit. T
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Policy auditing is helpful for ensuring a group of related servers are properly hardened to an exact specification.
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.PP
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The set of official built-in policies can be viewed with -L/--list-policies. Multiple servers can be audited with -T/--targets=<servers.txt>. Custom policies can be made from an ideal target server with -M/--make-policy=<custom_policy.txt>.
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The set of official built-in policies can be viewed with -L/--list-policies. Multiple servers can be audited with -T/--targets=<servers.txt>. Custom policies can be made from an preferred target server with -M/--make-policy=<custom_policy.txt>.
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.SH EXAMPLES
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@ -219,6 +256,30 @@ ssh-audit -M new_policy.txt targetserver
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.fi
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.RE
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.LP
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To run a Diffie-Hellman Group Exchange modulus size test using the values 2000 bits, 3000 bits, 4000 bits and 5000 bits:
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.RS
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.nf
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ssh-audit targetserver --gex-test=2000,3000,4000,5000
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.fi
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.RE
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.LP
|
||||
To run a Diffie-Hellman Group Exchange modulus size test where 2048 bits is the minimum, 3072 bits is the preferred and 5000 bits is the maximum:
|
||||
.RS
|
||||
.nf
|
||||
ssh-audit targetserver --gex-test=2048:3072:5000
|
||||
.fi
|
||||
.RE
|
||||
|
||||
.LP
|
||||
To run a Diffie-Hellman Group Exchange modulus size test from 0 bits to 5120 bits in increments of 1024 bits:
|
||||
.RS
|
||||
.nf
|
||||
ssh-audit targetserver --gex-test=0-5120:1024
|
||||
.fi
|
||||
.RE
|
||||
|
||||
.SH RETURN VALUES
|
||||
When a successful connection is made and all algorithms are rated as "good", \fBssh-audit\fP returns 0. Other possible return values are:
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue
Block a user