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.

ssh-audit.py

@@ -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))
+            return '[exception] {}'.format(errm)
-            sys.exit(PROGRAM_RETVAL_CONNECTION_ERROR)
 
         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: