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Send KEX before reading server's KEX during host key and GEX tests; this prevents deadlock against certain server implementations.

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This commit is contained in:
Joe Testa
2021-01-20 15:27:38 -05:00
parent 338ffc5adb
commit 1ba4c7c7ca
3 changed files with 20 additions and 25 deletions
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22
src/ssh_audit/gextest.py
View File

@@ -1,7 +1,7 @@
"""
The MIT License (MIT)
Copyright (C) 2017-2020 Joe Testa (jtesta@positronsecurity.com)
Copyright (C) 2017-2021 Joe Testa (jtesta@positronsecurity.com)
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
@@ -40,7 +40,7 @@ class GEXTest:
# Creates a new connection to the server. Returns True on success, or False.
@staticmethod
def reconnect(s: 'SSH_Socket', gex_alg: str) -> bool:
def reconnect(s: 'SSH_Socket', kex: 'SSH2_Kex', gex_alg: str) -> bool:
if s.is_connected():
return True
@@ -48,24 +48,22 @@ class GEXTest:
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()
_, _, err = s.get_banner()
if err is not None:
s.close()
return False
# Parse the server's initial KEX.
packet_type = 0 # pylint: disable=unused-variable
packet_type, payload = s.read_packet(2)
kex = SSH2_Kex.parse(payload)
# Send our KEX using the specified group-exchange and most of the
# server's own values.
client_kex = SSH2_Kex(os.urandom(16), [gex_alg], kex.key_algorithms, kex.client, kex.server, False, 0)
s.write_byte(Protocol.MSG_KEXINIT)
client_kex.write(s)
s.send_packet()
# Parse the server's KEX.
_, payload = s.read_packet(2)
SSH2_Kex.parse(payload)
return True
# Runs the DH moduli test against the specified target.
@@ -87,7 +85,7 @@ class GEXTest:
for gex_alg in GEX_ALGS:
if gex_alg in kex.kex_algorithms:
if GEXTest.reconnect(s, gex_alg) is False:
if GEXTest.reconnect(s, kex, gex_alg) is False:
break
kex_group = GEX_ALGS[gex_alg]()
@@ -117,7 +115,7 @@ class GEXTest:
if bits >= smallest_modulus > 0:
break
if GEXTest.reconnect(s, gex_alg) is False:
if GEXTest.reconnect(s, kex, gex_alg) is False:
reconnect_failed = True
break

22
src/ssh_audit/hostkeytest.py
View File

@@ -1,7 +1,7 @@
"""
The MIT License (MIT)
Copyright (C) 2017-2020 Joe Testa (jtesta@positronsecurity.com)
Copyright (C) 2017-2021 Joe Testa (jtesta@positronsecurity.com)
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
@@ -117,20 +117,16 @@ class HostKeyTest:
s.close()
return
# Parse the server's initial KEX.
packet_type = 0 # pylint: disable=unused-variable
packet_type, payload = s.read_packet()
# Send our KEX using the specified group-exchange and most of the server's own values.
client_kex = SSH2_Kex(os.urandom(16), [kex_str], [host_key_type], server_kex.client, server_kex.server, False, 0)
s.write_byte(Protocol.MSG_KEXINIT)
client_kex.write(s)
s.send_packet()
# Parse the server's KEX.
_, payload = s.read_packet()
SSH2_Kex.parse(payload)
# Send the server our KEXINIT message, using only our
# selected kex and host key 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], [host_key_type], server_kex.client, server_kex.server, False, 0)
s.write_byte(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. We also get back the host key fingerprint.