openssh bugs - Jun 2017 - [Bug 2729] New: Can connect with MAC hmac-sha1 even though it's not configured on the server

https://bugzilla.mindrot.org/show_bug.cgi?id=2729

            Bug ID: 2729
           Summary: Can connect with MAC hmac-sha1 even though it's not
                    configured on the server
           Product: Portable OpenSSH
           Version: 7.5p1
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: security
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: nitin.mahendru88 at gmail.com

Created attachment 2995
  --> https://bugzilla.mindrot.org/attachment.cgi?id=2995&action=edit
wireshark trace for key exchange init packet

I have a 7.5p1 server with the following MACs: 
MACs
hmac-sha2-512-etm at openssh.com,hmac-sha2-256-etm at
openssh.com,hmac-sha2-512,hmac-sha2-256
in the sshd_config

I am using a 7.5p1 client as below:
ssh -m hmac-sha1 user@<IP>

It lets me connect. Although as per RFC
https://www.ietf.org/rfc/rfc4253.txt It should just disconnect.

I have attached wireshark screen shots for client and server "Key
Exchange Init" packets which list the relevant contents.



OS: Centos 7
SSH version: 7.5 p1

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--- Comment #1 from Nitin Mahendru <nitin.mahendru88 at gmail.com> ---
Created attachment 2996
  --> https://bugzilla.mindrot.org/attachment.cgi?id=2996&action=edit
server Key Exchange Init Packet

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Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |djm at mindrot.org

--- Comment #2 from Damien Miller <djm at mindrot.org> ---
The PNG files you uploaded are corrupt and I can't view them. Please
attach a debug trace from the client instead, i.e. "ssh -vvv -m
hmac-sha1 user at ip"

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--- Comment #3 from Nitin Mahendru <nitin.mahendru88 at gmail.com> ---
Amazing response time. Thanks !
I would like to mention that we have the below macs configured on the
server(which is also 7.5p1):
MACs
hmac-sha2-512-etm at openssh.com,hmac-sha2-256-etm at
openssh.com,hmac-sha2-512,hmac-sha2-25

I stopped at the password but I can connect. 

-Nitin

**************************
[nitin at localhost ~]$ ssh -vvv -m hmac-sha1 nitin at 15.214.245.237
OpenSSH_7.5p1, OpenSSL 1.0.1e-fips 11 Feb 2013
debug1: Reading configuration data /usr/local/etc/ssh_config
debug2: resolving "15.214.245.237" port 22
debug2: ssh_connect_direct: needpriv 0
debug1: Connecting to 15.214.245.237 [15.214.245.237] port 22.
debug1: Connection established.
debug1: identity file /home/nitin/.ssh/id_rsa type 1
debug1: key_load_public: No such file or directory
debug1: identity file /home/nitin/.ssh/id_rsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/nitin/.ssh/id_dsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/nitin/.ssh/id_dsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/nitin/.ssh/id_ecdsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/nitin/.ssh/id_ecdsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/nitin/.ssh/id_ed25519 type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/nitin/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.5
debug1: Remote protocol version 2.0, remote software version
OpenSSH_7.5
debug1: match: OpenSSH_7.5 pat OpenSSH* compat 0x04000000
debug2: fd 3 setting O_NONBLOCK
debug1: Authenticating to 15.214.245.237:22 as 'nitin'
debug3: hostkeys_foreach: reading file "/home/nitin/.ssh/known_hosts"
debug3: record_hostkey: found key type ECDSA in file
/home/nitin/.ssh/known_hosts:7
debug3: load_hostkeys: loaded 1 keys from 15.214.245.237
debug3: order_hostkeyalgs: prefer hostkeyalgs:
ecdsa-sha2-nistp256-cert-v01 at openssh.com,ecdsa-sha2-nistp384-cert-v01 at
openssh.com,ecdsa-sha2-nistp521-cert-v01 at
openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521
debug3: send packet: type 20
debug1: SSH2_MSG_KEXINIT sent
debug3: receive packet: type 20
debug1: SSH2_MSG_KEXINIT received
debug2: local client KEXINIT proposal
debug2: KEX algorithms:
curve25519-sha256,curve25519-sha256 at
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-group-exchange-sha1,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,ext-info-c
debug2: host key algorithms:
ecdsa-sha2-nistp256-cert-v01 at openssh.com,ecdsa-sha2-nistp384-cert-v01 at
openssh.com,ecdsa-sha2-nistp521-cert-v01 at
openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519-cert-v01
at openssh.com,ssh-rsa-cert-v01 at
openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
debug2: ciphers ctos:
chacha20-poly1305 at openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm at
openssh.com,aes256-gcm at openssh.com,aes128-cbc,aes192-cbc,aes256-cbc
debug2: ciphers stoc:
chacha20-poly1305 at openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm at
openssh.com,aes256-gcm at openssh.com,aes128-cbc,aes192-cbc,aes256-cbc
debug2: MACs ctos: hmac-sha1
debug2: MACs stoc: hmac-sha1
debug2: compression ctos: none,zlib at openssh.com,zlib
debug2: compression stoc: none,zlib at openssh.com,zlib
debug2: languages ctos: 
debug2: languages stoc: 
debug2: first_kex_follows 0 
debug2: reserved 0 
debug2: peer server KEXINIT proposal
debug2: KEX algorithms:
curve25519-sha256,curve25519-sha256 at
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,diffie-hellman-group14-sha1
debug2: host key algorithms:
ssh-rsa,rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256,ssh-ed25519
debug2: ciphers ctos:
chacha20-poly1305 at openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm at
openssh.com,aes256-gcm at openssh.com
debug2: ciphers stoc:
chacha20-poly1305 at openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm at
openssh.com,aes256-gcm at openssh.com
debug2: MACs ctos:
hmac-sha2-512-etm at openssh.com,hmac-sha2-256-etm at
openssh.com,hmac-sha2-512,hmac-sha2-256
debug2: MACs stoc:
hmac-sha2-512-etm at openssh.com,hmac-sha2-256-etm at
openssh.com,hmac-sha2-512,hmac-sha2-256
debug2: compression ctos: none,zlib at openssh.com
debug2: compression stoc: none,zlib at openssh.com
debug2: languages ctos: 
debug2: languages stoc: 
debug2: first_kex_follows 0 
debug2: reserved 0 
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: chacha20-poly1305 at openssh.com MAC:
<implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305 at openssh.com MAC:
<implicit> compression: none
debug3: send packet: type 30
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug3: receive packet: type 31
debug1: Server host key: ecdsa-sha2-nistp256
SHA256:FxFRdAJWsTnDQU0fif9sNEVRVnPbwfZcijrXAz153gk
debug3: hostkeys_foreach: reading file "/home/nitin/.ssh/known_hosts"
debug3: record_hostkey: found key type ECDSA in file
/home/nitin/.ssh/known_hosts:7
debug3: load_hostkeys: loaded 1 keys from 15.214.245.237
debug1: Host '15.214.245.237' is known and matches the ECDSA host key.
debug1: Found key in /home/nitin/.ssh/known_hosts:7
debug3: send packet: type 21
debug2: set_newkeys: mode 1
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug3: receive packet: type 21
debug1: SSH2_MSG_NEWKEYS received
debug2: set_newkeys: mode 0
debug1: rekey after 134217728 blocks
debug2: key: /home/nitin/.ssh/id_rsa (0x7f4a7ce7e110), agent
debug2: key: /home/nitin/.ssh/id_dsa ((nil))
debug2: key: /home/nitin/.ssh/id_ecdsa ((nil))
debug2: key: /home/nitin/.ssh/id_ed25519 ((nil))
debug3: send packet: type 5
debug3: receive packet: type 7
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info:
server-sig-algs=<ssh-ed25519,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521>
debug3: receive packet: type 6
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug3: send packet: type 50
debug3: receive packet: type 51
debug1: Authentications that can continue:
publickey,password,keyboard-interactive
debug3: start over, passed a different list
publickey,password,keyboard-interactive
debug3: preferred publickey,keyboard-interactive,password
debug3: authmethod_lookup publickey
debug3: remaining preferred: keyboard-interactive,password
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /home/nitin/.ssh/id_rsa
debug3: send_pubkey_test
debug3: send packet: type 50
debug2: we sent a publickey packet, wait for reply
debug3: receive packet: type 51
debug1: Authentications that can continue:
publickey,password,keyboard-interactive
debug1: Trying private key: /home/nitin/.ssh/id_dsa
debug3: no such identity: /home/nitin/.ssh/id_dsa: No such file or
directory
debug1: Trying private key: /home/nitin/.ssh/id_ecdsa
debug3: no such identity: /home/nitin/.ssh/id_ecdsa: No such file or
directory
debug1: Trying private key: /home/nitin/.ssh/id_ed25519
debug3: no such identity: /home/nitin/.ssh/id_ed25519: No such file or
directory
debug2: we did not send a packet, disable method
debug3: authmethod_lookup keyboard-interactive
debug3: remaining preferred: password
debug3: authmethod_is_enabled keyboard-interactive
debug1: Next authentication method: keyboard-interactive
debug2: userauth_kbdint
debug3: send packet: type 50
debug2: we sent a keyboard-interactive packet, wait for reply
debug3: receive packet: type 51
debug1: Authentications that can continue:
publickey,password,keyboard-interactive
debug3: userauth_kbdint: disable: no info_req_seen
debug2: we did not send a packet, disable method
debug3: authmethod_lookup password
debug3: remaining preferred: 
debug3: authmethod_is_enabled password
debug1: Next authentication method: password

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Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WORKSFORME

--- Comment #4 from Damien Miller <djm at mindrot.org> ---
In future, please attach debug traces using the attachments feature.

This is why your connection succeeds:

debug1: kex: server->client cipher: chacha20-poly1305 at openssh.com MAC:
<implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305 at openssh.com MAC:
<implicit> compression: none

chacha20-poly1305 and AES-GCM are AEAD ciphers that effectively include
their own MAC. As such, the MAC you specified is ignored when they are
selected. If you specify a plain confidentiality cipher (e.g. -c
aes128-ctr) then your connection will fail as you expect.

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--- Comment #5 from Nitin Mahendru <nitin.mahendru88 at gmail.com> ---
cool. got ya. 
In file kex.c, below is the relevant code right ?

authlen = cipher_authlen(newkeys->enc.cipher); 
/* ignore mac for authenticated encryption */
if (authlen == 0 &&
   (r = choose_mac(ssh, &newkeys->mac, cprop[nmac],
   sprop[nmac])) != 0) {      }

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--- Comment #6 from Damien Miller <djm at mindrot.org> ---
Yes, see also the PROTOCOL file:
> AES-GCM is only negotiated as the cipher algorithms
> "aes128-gcm at openssh.com" or "aes256-gcm at
openssh.com" and never as
> an MAC algorithm. Additionally, if AES-GCM is selected as the cipher
> the exchanged MAC algorithms are ignored and there doesn't have to be
> a matching MAC.
and PROTOCOL.chacha20poly1305:
> The chacha20-poly1305 at openssh.com offers both encryption and
> authentication. As such, no separate MAC is required. If the
> chacha20-poly1305 at openssh.com cipher is selected in key exchange,
> the offered MAC algorithms are ignored and no MAC is required to be
> negotiated.
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Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |CLOSED

--- Comment #7 from Damien Miller <djm at mindrot.org> ---
Close all resolved bugs after release of OpenSSH 7.7.

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