How to secure SSH?

Management interfaces only

While reading dedicated instructions and resources, like Security Technical Implementation Guidelines, you'll notice that they always mark access management as the most crucial part of the process, and top priority. And they are right. It is very, very important to bind the server only to the interface that is trusted.

• In Enterprise infrastructures, virtual machines can have their own management interfaces, with proper firewalld rules enacted.
• For virtual private servers, we can align this rule to the trusted subset of public addresses that are whitelisted in the firewall.
• In the Cloud environments, we can either build a specific security group that defines the access to the server, or use other cloud services, like DigitalOcean Firewall, Cloudflare, or any other.

firewall-cmd --set-default-zone=drop
firewall-cmd --zone=trusted --change-interface=eth1 --permanent
firewall-cmd --zone=trusted --add-service=ssh --permanent

Mandatory Access Control - SELinux

I am the guy, that would do setenforce 1 even if it was already set to 1. Just in case. To be sure. Being absolutely honest, I strongly believe SELinux is one of the most helpful services in the Linux world, when you want to improve and harden your operating system.

I've read a lot of documentation and tutorials that recommend the end-user to disable SELinux or AppArmor. I completely understand their authors. They told you to do that, because it simplifies the process of software installation and fades out the burden of hardening. On the other hand, I fully believe it is like getting your house alarm wrongly configured, getting upset because of the false-positives, then disabling it, just to feel fine with yourself.

# sestatus
SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          enforcing
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Memory protection checking:     actual (secure)
Max kernel policy version:      31
#

# netstat -pnltuZ | grep ":22"
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      696/sshd             system_u:system_r:sshd_t:s0-s0:c0.c1023
tcp6       0      0 :::22                   :::*                    LISTEN      696/sshd             system_u:system_r:sshd_t:s0-s0:c0.c1023
#

Use Protocol 2

To be honest, I have never seen any secure shell server with enabled Protocol 1, or at least, never seen such one after i've done my work on it. To maintain the state of the art and comprehensiveness of this article, it is important to note it here, that it is insecure and it is not recommended to use Protocol 1. It has a set of vulnerabilities and problems with security. Protocol 1 support was removed in RHEL7.4

Protocol 2

Use tcp/22

I have seen a lot of write-ups that recommend using a different port than tcp/22

Port 22

# ss -pnltu | grep 22
tcp     LISTEN   0        128              0.0.0.0:22            0.0.0.0:*       users:(("sshd",pid=696,fd=4))
tcp     LISTEN   0        128                 [::]:22               [::]:*       users:(("sshd",pid=696,fd=6))
#

Do not use tcpwrapperd

A set of known Linux websites still recommend using tcpwrapperd as another security layer for sshd

The very first argument for that, is that tcpwrapperd is mostly deprecated technology

# ldd /sbin/sshd
  linux-vdso.so.1 (0x00007fff47b1a000)
  libfipscheck.so.1 => /lib64/libfipscheck.so.1 (0x00007f107113a000)
  libaudit.so.1 => /lib64/libaudit.so.1 (0x00007f1070f10000)
  libpam.so.0 => /lib64/libpam.so.0 (0x00007f1070d00000)
  libsystemd.so.0 => /lib64/libsystemd.so.0 (0x00007f10709bc000)
  libcrypto.so.1.1 => /lib64/libcrypto.so.1.1 (0x00007f10704d9000)
  libdl.so.2 => /lib64/libdl.so.2 (0x00007f10702d5000)
  libutil.so.1 => /lib64/libutil.so.1 (0x00007f10700d1000)
  libz.so.1 => /lib64/libz.so.1 (0x00007f106feba000)
  libcrypt.so.1 => /lib64/libcrypt.so.1 (0x00007f106fc91000)
  libresolv.so.2 => /lib64/libresolv.so.2 (0x00007f106fa7a000)
  libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f106f84f000)
  libgssapi_krb5.so.2 => /lib64/libgssapi_krb5.so.2 (0x00007f106f5ff000)
  libkrb5.so.3 => /lib64/libkrb5.so.3 (0x00007f106f30f000)
  libk5crypto.so.3 => /lib64/libk5crypto.so.3 (0x00007f106f0f3000)
  libcom_err.so.2 => /lib64/libcom_err.so.2 (0x00007f106eeef000)
  libc.so.6 => /lib64/libc.so.6 (0x00007f106eb2d000)
  libcap-ng.so.0 => /lib64/libcap-ng.so.0 (0x00007f106e927000)
  libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f106e707000)
  librt.so.1 => /lib64/librt.so.1 (0x00007f106e4fe000)
  liblzma.so.5 => /lib64/liblzma.so.5 (0x00007f106e2d7000)
  liblz4.so.1 => /lib64/liblz4.so.1 (0x00007f106e0c0000)
  libcap.so.2 => /lib64/libcap.so.2 (0x00007f106deba000)
  libmount.so.1 => /lib64/libmount.so.1 (0x00007f106dc60000)
  libgcrypt.so.20 => /lib64/libgcrypt.so.20 (0x00007f106d944000)
  libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007f106d72c000)
  /lib64/ld-linux-x86-64.so.2 (0x00007f1071613000)
  libpcre2-8.so.0 => /lib64/libpcre2-8.so.0 (0x00007f106d4a8000)
  libkrb5support.so.0 => /lib64/libkrb5support.so.0 (0x00007f106d297000)
  libkeyutils.so.1 => /lib64/libkeyutils.so.1 (0x00007f106d093000)
  libblkid.so.1 => /lib64/libblkid.so.1 (0x00007f106ce41000)
  libuuid.so.1 => /lib64/libuuid.so.1 (0x00007f106cc39000)
  libgpg-error.so.0 => /lib64/libgpg-error.so.0 (0x00007f106ca18000)
#

Do not accept password logins

The only allowed authentication method should be Public-Key authentication

AuthenticationMethods publickey
PasswordAuthentication no
PermitEmptyPasswords no

Do not accept forwarding

X11Forwarding is a Linux mechanism created to expose X11 into the remote shell. It is very difficult to imagine reasons for using graphical software on Linux server, per se. That being said, I do only see enabling this as another vector of possible attack, and risk to omit firewalls. Port forwarding is the functionality that allows the client to bind local-port to a remote address. It is a very straight and easy way to omit and skip firewall-rules and break compliance.

X11Forwarding no
AllowTcpForwarding no

Minimize MaxSessions and MaxAuthTries

The authorized user shouldn't have any difficulties with logging in successfully... Of course, there are exceptions to that statement, but in general, it is safe to assume it should be completely sufficient to have two, max three tries during one connection session. In contrast, the attacker trying to apply brute-force attacks needs as much login tries as possible. Therefore, I think we can all agree, minimizing MaxAuthTries is a good way of hardening. Another useful hardening trick, is to limit possible multiplexed sshd_sessions with MaxSessions.

MaxAuthTries 2
MaxSessions 10

Add metadata information

This part combines the usage of two separate files: motd and banner. Both should be used accordingly. motd is the file that is displayed by Unix after a successful login, yet before executing the shell. Banner is a file that is visible for each user trying to connect before the authentication process.

#Unauthorized access to this device is prohibited!#

You are accessing a U.S. Government (USG) Information System (IS) that is provided for USG-authorized use only.
By using this IS (which includes any device attached to this IS), you consent to the following conditions:

- The USG routinely intercepts and monitors communications on this IS for purposes including, but not limited to, penetration testing, COMSEC monitoring, network operations and defense, personnel misconduct (PM), law enforcement (LE), and counterintelligence (CI) investigations.
- At any time, the USG may inspect and seize data stored on this IS.
- Communications using, or data stored on, this IS are not private, are subject to routine monitoring, interception, and search, and may be disclosed or used for any USG-authorized purpose.
- This IS includes security measures (e.g. authentication and access controls) to protect USG interests — not for your personal benefit or privacy.
- Notwithstanding the above, using this IS does not constitute consent to PM, LE or CI investigative searching or monitoring of the content of privileged communications, or work product, related to personal representation or services by attorneys, psychotherapists, or clergy, and their assistants. Such communications and work product are private and confidential. See User Agreement for details.

Do not use HostbasedAuthentication, rhosts

rhosts

hamlet dewey

HostbasedAuthentication no
IgnoreRhosts yes

Do not accept root login

It should not be permitted for the end-user to login to root directly. There are many reasons for that. For example, one is that it limits the auditing possibilities. It is way easier to audit and do the forensics when the user has to log in and then escalate his privileges to the superuser from there using tools like sudo.

PermitRootLogin no

Configure KeepAliveSession

Inactive sessions should be terminated. It limits persistence, therefore at least partially rendering the "persistence part" of ATT&CK

ClientAliveInterval 5m
ClientAliveCountMax 2

Use two-factor authentication

Since OpenSSH 6.3 it is possible to use two-factor authentication, and I would strongly recommend you to use it. There are many providers available, both software, like Google Authenticator, or hardware keys, like Yubikey or Google Security Key.

Use a strong set of algorithms

The default setting for the current encryption and hash algorithms is not really the best option out there. We can refer to the various standards like PCI DSS, FIPS-140-3. It is important to remember that the secure shell server encrypts the connection through the standard public-key-infrastructure. Said that, the same problems with Logjam attack still apply to this service

KexAlgorithms [email protected],ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
Ciphers [email protected],[email protected],[email protected],aes256-ctr,aes192-ctr,aes128-ctr
MACs [email protected],[email protected],[email protected],hmac-sha2-512,hmac-sha2-256,[email protected]

Protect server keys

Host keys should be accessible by the root user only. Once deployed, it is recommended that Host keys should be registered in an automated inventory system. Server keys should also be monitored by host intrusion detection systems, like Auditbeat

Use port knocking

The paranoics can use port knocking

Build a Certificate Authority

Even more paranoic... paranoics can create a full certificate authority for the whole inventory.

TrustedUserCAKeys /etc/ssh/ca.pub
AuthorizedPrincipalsFile /etc/ssh/auth_principals/%u

Hash known_hosts file

Hashing the known_hosts file prevents leakage and information disclosure. The following examples present non-hashed and hashed known_hosts files.

255.255.255.255 ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ==

|1|iAsVNM5h7KXr9LM59l6UXSdjLpk=|IWK/TOxygz+vQNt9R600PNG4/ts= ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBMH96GPuCQuE6JIWqmuzClWiecxwc7orVmisNPQyN0eW7K0Jj3kJ/GsuxPr3AujjiJPQHO3M8m4Oqd+/0p254/Q=

Protect private keys with passphrase

Private keys should be protected with a strong passphrase; they should never be copied from one system to another using disks, USB drives or tokens. They should have very, very strict permissions applied as well.

umask 077
chmod 0400 ~/.ssh/*

Agent forwarding

Disable agent forwarding. Since OpenSSH 7.3 it is possible to jump through servers much, much more comfortably and in a way better automated manner using ProxyJump.

Host *
    ForwardAgent no