Personal notes on hardening an new ubuntu 24.04 LTS ssh daemon setup for incoming ssh traffic.

Port <12345>

PasswordAuthentication no
KbdInteractiveAuthentication no
UsePAM yes
X11Forwarding no
PrintMotd no
UseDNS no

KexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256

HostKeyAlgorithms ssh-ed25519-cert-v01@openssh.com,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,ssh-ed25519,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com,rsa-sha2-512,rsa-sha2-256

MACs umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512

AcceptEnv LANG LC_*
AllowUsers <username>

Subsystem       sftp    /usr/lib/openssh/sftp-server

testing with https://sshcheck.com/

2FA SSH aka OpenSSH OATH, Two-Factor Authentication

prologue

Good security is based on layers of protection. At some point the usability gets in the way. My thread model on accessing systems is to create a different ssh pair of keys (private/public) and only use them instead of a login password. I try to keep my digital assets separated and not all of them under the same basket. My laptop is encrypted and I dont run any services on it, but even then a bad actor can always find a way.

Back in the days, I was looking on Barada/Gort. Barada is an implementation of HOTP: An HMAC-Based One-Time Password Algorithm and Gort is the android app you can install to your mobile and connect to barada. Both of these application have not been updated since 2013/2014 and Gort is even removed from f-droid!

Talking with friends on our upcoming trip to 34C4, discussing some security subjects, I thought it was time to review my previous inquiry on ssh-2FA. Most of my friends are using yubikeys. I would love to try some, but at this time I dont have the time to order/test/apply to my machines. To reduce any risk, the idea of combining a bastion/jump-host server with 2FA seemed to be an easy and affordable solution.

OpenSSH with OATH

As ssh login is based on PAM (Pluggable Authentication Modules), we can use Gnu OATH Toolkit. OATH stands for Open Authentication and it is an open standard. In a nutshell, we add a new authorization step that we can verify our login via our mobile device.

Below are my personal notes on how to setup oath-toolkit, oath-pam and how to synchronize it against your android device. These are based on centos 6.9

EPEL

We need to install the epel repository:

# yum -y install https://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm

Searching packages

Searching for oath

# yum search oath

the results are similar to these:

liboath.x86_64       : Library for OATH handling
liboath-devel.x86_64 : Development files for liboath
liboath-doc.noarch   : Documentation files for liboath

pam_oath.x86_64      : A PAM module for pluggable login authentication for OATH
gen-oath-safe.noarch : Script for generating HOTP/TOTP keys (and QR code)
oathtool.x86_64      : A command line tool for generating and validating OTPs

Installing packages

We need to install three packages:

• pam_oath is the PAM for OATH
• oathtool is the gnu oath-toolkit
• gen-oath-safe is the program that we will use to sync our mobile device with our system

# yum -y install pam_oath oathtool gen-oath-safe

FreeOTP

Before we continue with our setup, I believe now is the time to install FreeOTP

FreeOTP looks like:

HOTP

Now, it is time to generate and sync our 2FA, using HOTP

Generate

You should replace username with your USER_NAME !

# gen-oath-safe username HOTP

Sync

and scan the QR with FreeOTP

You can see in the top a new entry!

Save

Do not forget to save your HOTP key (hex) to the gnu oath-toolkit user file.

eg.

Key in Hex: e9379dd63ec367ee5c378a7c6515af01cf650c89

# echo "HOTP username - e9379dd63ec367ee5c378a7c6515af01cf650c89" > /etc/liboath/oathuserfile

verify:

# cat /etc/liboath/oathuserfile

HOTP username - e9379dd63ec367ee5c378a7c6515af01cf650c89

OpenSSH

The penultimate step is to setup our ssh login with the PAM oath library.

Verify PAM

# ls -l /usr/lib64/security/pam_oath.so

-rwxr-xr-x 1 root root 11304 Nov 11  2014 /usr/lib64/security/pam_oath.so

SSHD-PAM

# cat /etc/pam.d/sshd

In modern systems, the sshd pam configuration file seems:

#%PAM-1.0
auth       required pam_sepermit.so
auth       include      password-auth
account    required     pam_nologin.so
account    include      password-auth
password   include      password-auth
# pam_selinux.so close should be the first session rule
session    required     pam_selinux.so close
session    required     pam_loginuid.so
# pam_selinux.so open should only be followed by sessions to be executed in the user context
session    required     pam_selinux.so open env_params
session    required     pam_namespace.so
session    optional     pam_keyinit.so force revoke
session    include      password-auth

We need one line in the top of the file.

I use something like this:

auth       sufficient /usr/lib64/security/pam_oath.so  debug   usersfile=/etc/liboath/oathuserfile window=5 digits=6

Depending on your policy and thread model, you can switch sufficient to requisite , you can remove debug option. In the third field, you can try typing just the pam_path.so without the full path and you can change the window to something else:

eg.

auth requisite pam_oath.so usersfile=/etc/liboath/oathuserfile window=10 digits=6

Restart sshd

In every change/test remember to restart your ssh daemon:

# service sshd restart

Stopping sshd:                                             [  OK  ]
Starting sshd:                                             [  OK  ]

SELINUX

If you are getting some weird messages, try to change the status of selinux to permissive and try again. If the selinux is the issue, you have to review selinux audit logs and add/fix any selinux policies/modules so that your system can work properly.

# getenforce
Enforcing

# setenforce 0

# getenforce
Permissive

Testing

The last and most important thing, is to test it !

Links

• 34C3 - https://events.ccc.de/congress/2017/wiki/index.php/Main_Page
• HOTP - https://tools.ietf.org/html/rfc4226
• OATH - https://openauthentication.org/
• FreeOTP - https://freeotp.github.io
• Gnu OATH ToolKit - http://www.nongnu.org/oath-toolkit/index.html
• PAM - https://www.linux.com/news/understanding-pam

Post Scriptum

The idea of using OATH & FreeOTP can also be applied to login into your laptop as PAM is the basic authentication framework on a linux machine. You can use OATH in every service that can authenticate it self through PAM.