Linux

Secure Shell (SSH) enables two computers to establish an encrypted and direct connection within a possibly insecure network on the standard port TCP 22.

OpenSSH has six different authentication methods:

Password authentication
Public-key authentication
Host-based authentication
Keyboard authentication
Challenge-response authentication
GSSAPI authentication

More info about authentication methods

Default Configuration

The sshd_config file, responsible for the OpenSSH server, has only a few of the settings configured by default.

neutron@kali[/kali]$ cat /etc/ssh/sshd_config  | grep -v "#" | sed -r '/^\s*$/d'

Include /etc/ssh/sshd_config.d/*.conf
ChallengeResponseAuthentication no
UsePAM yes
X11Forwarding yes
PrintMotd no
AcceptEnv LANG LC_*
Subsystem       sftp    /usr/lib/openssh/sftp-server

Dangerous Settings

Despite the SSH protocol being one of the most secure protocols available today, some misconfigurations can still make the SSH server vulnerable to easy-to-execute attacks.

Setting	Description
`PasswordAuthentication yes`	Allows password-based authentication.
`PermitEmptyPasswords yes`	Allows the use of empty passwords.
`PermitRootLogin yes`	Allows to log in as the root user.
`Protocol 1`	Uses an outdated version of encryption.
`X11Forwarding yes`	Allows X11 forwarding for GUI applications.
`AllowTcpForwarding yes`	Allows forwarding of TCP ports.
`PermitTunnel`	Allows tunneling.
`DebianBanner yes`	Displays a specific banner when logging in.

Footprinting the Service

One of the tools we can use to fingerprint the SSH server is ssh-audit. It checks the client-side and server-side configuration and shows some general information and which encryption algorithms are still used by the client and server.

neutron@kali[/kali]$ git clone https://github.com/jtesta/ssh-audit.git && cd ssh-audit
neutron@kali[/kali]$ ./ssh-audit.py 10.129.14.132

# general
(gen) banner: SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.3
(gen) software: OpenSSH 8.2p1
(gen) compatibility: OpenSSH 7.4+, Dropbear SSH 2018.76+
(gen) compression: enabled ([email protected])                                   

# key exchange algorithms
(kex) curve25519-sha256                     -- [info] available since OpenSSH 7.4, Dropbear SSH 2018.76                            
(kex) [email protected]          -- [info] available since OpenSSH 6.5, Dropbear SSH 2013.62
(kex) ecdh-sha2-nistp256                    -- [fail] using weak elliptic curves
                                            `- [info] available since OpenSSH 5.7, Dropbear SSH 2013.62
(kex) ecdh-sha2-nistp384                    -- [fail] using weak elliptic curves
                                            `- [info] available since OpenSSH 5.7, Dropbear SSH 2013.62
(kex) ecdh-sha2-nistp521                    -- [fail] using weak elliptic curves
                                            `- [info] available since OpenSSH 5.7, Dropbear SSH 2013.62
(kex) diffie-hellman-group-exchange-sha256 (2048-bit) -- [info] available since OpenSSH 4.4
(kex) diffie-hellman-group16-sha512         -- [info] available since OpenSSH 7.3, Dropbear SSH 2016.73
(kex) diffie-hellman-group18-sha512         -- [info] available since OpenSSH 7.3
(kex) diffie-hellman-group14-sha256         -- [info] available since OpenSSH 7.3, Dropbear SSH 2016.73

# host-key algorithms
(key) rsa-sha2-512 (3072-bit)               -- [info] available since OpenSSH 7.2
(key) rsa-sha2-256 (3072-bit)               -- [info] available since OpenSSH 7.2
(key) ssh-rsa (3072-bit)                    -- [fail] using weak hashing algorithm
                                            `- [info] available since OpenSSH 2.5.0, Dropbear SSH 0.28
                                            `- [info] a future deprecation notice has been issued in OpenSSH 8.2: https://www.openssh.com/txt/release-8.2
(key) ecdsa-sha2-nistp256                   -- [fail] using weak elliptic curves
                                            `- [warn] using weak random number generator could reveal the key
                                            `- [info] available since OpenSSH 5.7, Dropbear SSH 2013.62
(key) ssh-ed25519                           -- [info] available since OpenSSH 6.5
...SNIP...

The first thing we can see in the first few lines of the output is the banner that reveals the version of the OpenSSH server. The previous versions had some vulnerabilities, such as CVE-2020-14145, which allowed the attacker the capability to Man-In-The-Middle and attack the initial connection attempt. The detailed output of the connection setup with the OpenSSH server can also often provide important information, such as which authentication methods the server can use.

Change Authentication Method

neutron@kali[/kali]$ ssh -v [email protected]

OpenSSH_8.2p1 Ubuntu-4ubuntu0.3, OpenSSL 1.1.1f  31 Mar 2020
debug1: Reading configuration data /etc/ssh/ssh_config 
...SNIP...
debug1: Authentications that can continue: publickey,password,keyboard-interactive

For potential brute-force attacks, we can specify the authentication method with the SSH client option PreferredAuthentications.

neutron@kali[/kali]$ ssh -v [email protected] -o PreferredAuthentications=password

OpenSSH_8.2p1 Ubuntu-4ubuntu0.3, OpenSSL 1.1.1f  31 Mar 2020
debug1: Reading configuration data /etc/ssh/ssh_config
...SNIP...
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Next authentication method: password
[email protected]'s password: