SSH configuration is not being honored

Security
1

User override in SSH config file is no longer being honored. Problem occurs on AlmaLinux 8.8 and AlmaLinux 9.2. User can login in using any method despite only enable PasswordAuthentication or PubkeyAuthentication.

Here’s my custom SSHD file /etc/ssh/sshd_config.d/sshd_custom_rules.conf

## Custom SSH rules
PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication no

Match User repository-readonly,repository-admin,iso-readonly
    PasswordAuthentication no
    PubkeyAuthentication yes

Match User yubi
    PasswordAuthentication yes
    PubkeyAuthentication no

Match Address 192.168.67.0/22
    AllowUsers repository-admin repository-readonly iso-readonly yubi

Match Address *
    AllowUsers repository-readonly iso-readonly

Here the /etc/ssh/sshd_config.d/50-redhat.conf file

Include /etc/crypto-policies/back-ends/opensshserver.config

SyslogFacility AUTHPRIV

ChallengeResponseAuthentication yes

GSSAPIAuthentication yes
GSSAPICleanupCredentials no

UsePAM yes

X11Forwarding yes

# It is recommended to use pam_motd in /etc/pam.d/sshd instead of PrintMotd,
# as it is more configurable and versatile than the built-in version.
PrintMotd no

#       $OpenBSD: sshd_config,v 1.104 2021/07/02 05:11:21 dtucker Exp $

# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.

# This sshd was compiled with PATH=/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin

# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options override the
# default value.

# To modify the system-wide sshd configuration, create a  *.conf  file under
#  /etc/ssh/sshd_config.d/  which will be automatically included below
Include /etc/ssh/sshd_config.d/*.conf

# If you want to change the port on a SELinux system, you have to tell
# SELinux about this change.
# semanage port -a -t ssh_port_t -p tcp #PORTNUMBER
#
Port 22
#Port 8822
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

#HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
#HostKey /etc/ssh/ssh_host_ed25519_key

# Ciphers and keying
#RekeyLimit default none

# Logging
#SyslogFacility AUTH
#LogLevel INFO

# Authentication:

#LoginGraceTime 2m
#PermitRootLogin prohibit-password
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10
#MaxAuthTries 6
#MaxSessions 10

#PubkeyAuthentication yes

# The default is to check both .ssh/authorized_keys and .ssh/authorized_keys2
# but this is overridden so installations will only check .ssh/authorized_keys
AuthorizedKeysFile      .ssh/authorized_keys

#AuthorizedPrincipalsFile none

#AuthorizedKeysCommand none
#AuthorizedKeysCommandUser nobody

# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for
# HostbasedAuthentication
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
#IgnoreRhosts yes

# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication no
#PermitEmptyPasswords no

# Change to no to disable s/key passwords
#KbdInteractiveAuthentication yes

# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no
#KerberosUseKuserok yes

# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes
#GSSAPIStrictAcceptorCheck yes
#GSSAPIKeyExchange no
#GSSAPIEnablek5users no

# Set this to 'yes' to enable PAM authentication, account processing,
# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the KbdInteractiveAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via KbdInteractiveAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and KbdInteractiveAuthentication to 'no'.
# WARNING: 'UsePAM no' is not supported in RHEL and may cause several
# problems.
#UsePAM no

#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
#X11Forwarding no
#X11DisplayOffset 10
#X11UseLocalhost yes
#PermitTTY yes
#PrintMotd yes
#PrintLastLog yes
#TCPKeepAlive yes
#PermitUserEnvironment no
#Compression delayed
#ClientAliveInterval 0
#ClientAliveCountMax 3
#UseDNS no
#PidFile /var/run/sshd.pid
#MaxStartups 10:30:100
#PermitTunnel no
#ChrootDirectory none
#VersionAddendum none

# no default banner path
#Banner none

# override default of no subsystems
Subsystem       sftp    /usr/libexec/openssh/sftp-server

# Example of overriding settings on a per-user basis
#Match User anoncvs
#       X11Forwarding no
# Example of overriding settings on a per-user basis
#Match User anoncvs
#       X11Forwarding no
#       AllowTcpForwarding no
#       PermitTTY no
#       ForceCommand cvs server
2

AFAIK, the el8 does not have that by default. Red Hat did add it in el9. (The el8 does have /etc/ssh/ssh_config.d/ for client-side.)

While el8 and el9 results might differ due to that, that does not explain the issue for both systems.

What do you get if you run sshd -T? (See man sshd)

3

you’ve got to be very careful with the ordering, its backwards compared to other drop-in files in that the earlier numbered files take precedence, so you should put your config in 49_sshd_custom_rules.conf (note it should have a number).

also note that EL9 (and debian) put the Include /etc/ssh/sshd_config.d/*.conf directive near the top of sshd_config so included files will override duplicated settings in sshd_config.

also be aware of commented out defaults, sshd -T is the only way to really get a clear picture of your actual config - you may need a few -C arguments for user/host etc. to test your Matches - which you’ve also got to be mindful of as they may negate each other and they don’t support all keywords.

i’d strip it down to the bare minimum, get rid of the Matches, remove duplicate configs and add things back in as you get them working - maybe even get rid of the include files and put them in sshd_config to start with.

4

In order to disable password authentication, I had to set ChallengeResponseAuthentication to no.

I have been advised that ChallengeResponseAuthentication has ben deprecated
and I am now using KbdInteractiveAuthentication instead.