[Bug 3823] New: SSH on same device ignores MAC restrictions

bugzilla-daemon at mindrot.org bugzilla-daemon at mindrot.org
Tue May 13 19:37:44 AEST 2025 

https://bugzilla.mindrot.org/show_bug.cgi?id=3823

            Bug ID: 3823
           Summary: SSH on same device ignores MAC restrictions
           Product: Portable OpenSSH
           Version: 10.0p2
          Hardware: Other
                OS: Other
            Status: NEW
          Severity: major
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: adrian.jarc at aviatnet.com

If I disable certain mac algorithms on sshd and then try to connect (on
same device) to ssh with thiose disabled algorithms, SSH allows me to
do this.

Scenario:

I have disabled hmac-sha2-256 in sshd config on device. Then I try
establishing connection via ssh client on same device with '-m
hmac-sha2-256' flag. Instead of connection being rejected because mac
algorithm is not supported on server, Client just ignores this flag and
connects.

Logs of what happens:
"
ssh -m hmac-sha2-256 admin@::
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts' ->
'/home/root/.ssh/known_hosts'
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts2' ->
'/home/root/.ssh/known_hosts2'
debug1: Authenticator provider $SSH_SK_PROVIDER did not resolve;
disabling
debug3: channel_clear_timeouts: clearing
debug3: ssh_connect_direct: entering
debug1: Connecting to :: [::] port 22.
debug3: set_sock_tos: set socket 3 IPV6_TCLASS 0x48
debug1: Connection established.
debug1: identity file /home/root/.ssh/id_rsa type -1
debug1: identity file /home/root/.ssh/id_rsa-cert type -1
debug1: identity file /home/root/.ssh/id_ecdsa type -1
debug1: identity file /home/root/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/root/.ssh/id_ecdsa_sk type -1
debug1: identity file /home/root/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /home/root/.ssh/id_ed25519 type -1
debug1: identity file /home/root/.ssh/id_ed25519-cert type -1
debug1: identity file /home/root/.ssh/id_ed25519_sk type -1
debug1: identity file /home/root/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /home/root/.ssh/id_xmss type -1
debug1: identity file /home/root/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_10.0
debug1: Remote protocol version 2.0, remote software version
OpenSSH_10.0
debug1: compat_banner: match: OpenSSH_10.0 pat OpenSSH* compat
0x04000000
debug2: fd 3 setting O_NONBLOCK
debug1: Authenticating to :::22 as 'admin'
debug1: load_hostkeys: fopen /home/root/.ssh/known_hosts: No such file
or directory
debug1: load_hostkeys: fopen /home/root/.ssh/known_hosts2: No such file
or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or
directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or
directory
debug3: order_hostkeyalgs: no algorithms matched; accept original
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:
mlkem768x25519-sha256,sntrup761x25519-sha512,sntrup761x25519-sha512 at openssh.com,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,ext-info-c,kex-strict-c-v00 at openssh.com
debug2: host key algorithms:
ssh-ed25519-cert-v01 at openssh.com,ecdsa-sha2-nistp256-cert-v01 at openssh.com,ecdsa-sha2-nistp384-cert-v01 at openssh.com,ecdsa-sha2-nistp521-cert-v01 at openssh.com,rsa-sha2-512-cert-v01 at openssh.com,rsa-sha2-256-cert-v01 at openssh.com,ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,rsa-sha2-512,rsa-sha2-256
debug2: ciphers ctos:
chacha20-poly1305 at openssh.com,aes128-gcm at openssh.com,aes256-gcm at openssh.com,aes128-ctr,aes192-ctr,aes256-ctr
debug2: ciphers stoc:
chacha20-poly1305 at openssh.com,aes128-gcm at openssh.com,aes256-gcm at openssh.com,aes128-ctr,aes192-ctr,aes256-ctr
debug2: MACs ctos: hmac-sha2-256
debug2: MACs stoc: 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 
debug2: peer server KEXINIT proposal
debug2: KEX algorithms:
curve25519-sha256,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,ext-info-s,kex-strict-s-v00 at openssh.com
debug2: host key algorithms:
rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256,ssh-ed25519
debug2: ciphers ctos:
aes128-ctr,aes128-gcm at openssh.com,aes256-ctr,aes256-gcm at openssh.com
debug2: ciphers stoc:
aes128-ctr,aes128-gcm at openssh.com,aes256-ctr,aes256-gcm at openssh.com
debug2: MACs ctos: hmac-sha2-512
debug2: MACs stoc: hmac-sha2-512
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 
debug3: kex_choose_conf: will use strict KEX ordering
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ssh-ed25519
debug1: kex: server->client cipher: aes128-gcm at openssh.com MAC:
<implicit> compression: none
debug1: kex: client->server cipher: aes128-gcm 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: SSH2_MSG_KEX_ECDH_REPLY received
debug1: Server host key: ssh-ed25519
SHA256:GRAoWefV8I0f3qaObur4WmJ2a70rOgYEePRl2Wo5U5E
debug1: load_hostkeys: fopen /home/root/.ssh/known_hosts: No such file
or directory
debug1: load_hostkeys: fopen /home/root/.ssh/known_hosts2: No such file
or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or
directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or
directory
debug3: hostkeys_find_by_key_hostfile: trying user hostfile
"/home/root/.ssh/known_hosts"
debug1: hostkeys_find_by_key_hostfile: hostkeys file
/home/root/.ssh/known_hosts does not exist
debug3: hostkeys_find_by_key_hostfile: trying user hostfile
"/home/root/.ssh/known_hosts2"
debug1: hostkeys_find_by_key_hostfile: hostkeys file
/home/root/.ssh/known_hosts2 does not exist
debug3: hostkeys_find_by_key_hostfile: trying system hostfile
"/etc/ssh/ssh_known_hosts"
debug1: hostkeys_find_by_key_hostfile: hostkeys file
/etc/ssh/ssh_known_hosts does not exist
debug3: hostkeys_find_by_key_hostfile: trying system hostfile
"/etc/ssh/ssh_known_hosts2"
debug1: hostkeys_find_by_key_hostfile: hostkeys file
/etc/ssh/ssh_known_hosts2 does not exist
The authenticity of host ':: (::)' can't be established.
ED25519 key fingerprint is
SHA256:GRAoWefV8I0f3qaObur4WmJ2a70rOgYEePRl2Wo5U5E.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])?
"

Logs of what should happen:
"
ssh -vvv -m hmac-sha2-256 admin@<server-ip>
OpenSSH_9.6p1 Ubuntu-3ubuntu13.11, OpenSSL 3.0.13 30 Jan 2024
debug1: Reading configuration data /home/ajarc/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: include
/etc/ssh/ssh_config.d/*.conf matched no files
debug1: /etc/ssh/ssh_config line 21: Applying options for *
debug2: resolve_canonicalize: hostname <server-ip> is address
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts' ->
'/home/ajarc/.ssh/known_hosts'
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts2' ->
'/home/ajarc/.ssh/known_hosts2'
debug3: channel_clear_timeouts: clearing
debug3: ssh_connect_direct: entering
debug1: Connecting to <server-ip> [<server-ip>] port 22.
debug3: set_sock_tos: set socket 3 IP_TOS 0x10
debug1: Connection established.
debug1: identity file /home/ajarc/.ssh/id_rsa type 0
debug1: identity file /home/ajarc/.ssh/id_rsa-cert type -1
debug1: identity file /home/ajarc/.ssh/id_ecdsa type -1
debug1: identity file /home/ajarc/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/ajarc/.ssh/id_ecdsa_sk type -1
debug1: identity file /home/ajarc/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /home/ajarc/.ssh/id_ed25519 type 3
debug1: identity file /home/ajarc/.ssh/id_ed25519-cert type -1
debug1: identity file /home/ajarc/.ssh/id_ed25519_sk type -1
debug1: identity file /home/ajarc/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /home/ajarc/.ssh/id_xmss type -1
debug1: identity file /home/ajarc/.ssh/id_xmss-cert type -1
debug1: identity file /home/ajarc/.ssh/id_dsa type -1
debug1: identity file /home/ajarc/.ssh/id_dsa-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_9.6p1 Ubuntu-3ubuntu13.11
debug1: Remote protocol version 2.0, remote software version
OpenSSH_10.0
debug1: compat_banner: match: OpenSSH_10.0 pat OpenSSH* compat
0x04000000
debug2: fd 3 setting O_NONBLOCK
debug1: Authenticating to <server-ip>:22 as 'admin'
debug3: record_hostkey: found key type ED25519 in file
/home/ajarc/.ssh/known_hosts:257
debug3: load_hostkeys_file: loaded 1 keys from <server-ip>
debug1: load_hostkeys: fopen /home/ajarc/.ssh/known_hosts2: No such
file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or
directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or
directory
debug3: order_hostkeyalgs: have matching best-preference key type
ssh-ed25519-cert-v01 at openssh.com, using HostkeyAlgorithms verbatim
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:
sntrup761x25519-sha512 at openssh.com,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,ext-info-c,kex-strict-c-v00 at openssh.com
debug2: host key algorithms:
ssh-ed25519-cert-v01 at openssh.com,ecdsa-sha2-nistp256-cert-v01 at openssh.com,ecdsa-sha2-nistp384-cert-v01 at openssh.com,ecdsa-sha2-nistp521-cert-v01 at openssh.com,sk-ssh-ed25519-cert-v01 at openssh.com,sk-ecdsa-sha2-nistp256-cert-v01 at openssh.com,rsa-sha2-512-cert-v01 at openssh.com,rsa-sha2-256-cert-v01 at openssh.com,ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ssh-ed25519 at openssh.com,sk-ecdsa-sha2-nistp256 at openssh.com,rsa-sha2-512,rsa-sha2-256
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-sha2-256
debug2: MACs stoc: hmac-sha2-256
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,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,ext-info-s,kex-strict-s-v00 at openssh.com
debug2: host key algorithms:
rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256,ssh-ed25519
debug2: ciphers ctos:
aes128-ctr,aes128-gcm at openssh.com,aes256-ctr,aes256-gcm at openssh.com
debug2: ciphers stoc:
aes128-ctr,aes128-gcm at openssh.com,aes256-ctr,aes256-gcm at openssh.com
debug2: MACs ctos: hmac-sha2-512
debug2: MACs stoc: hmac-sha2-512
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 
debug3: kex_choose_conf: will use strict KEX ordering
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ssh-ed25519
Unable to negotiate with 11.214.153.42 port 22: no matching MAC found.
Their offer: hmac-sha2-512
"

In both logs instances sshd was installed from:
https://ftp.openbsd.org/pub/OpenBSD/OpenSSH/portable/openssh-10.0p1.tar.gz

In instance where connection proceeds, ssh client was installed from:
https://ftp.openbsd.org/pub/OpenBSD/OpenSSH/portable/openssh-10.0p1.tar.gz.

In instance where we get an error it is ssh client that comes installed
with Ubuntu 24.04.2

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