# Hardening SSH ### LinuxPa Spain, Nov 09, 2018</h3> <div class="presenter"> <p>E. Marcos</p> <a style="color: black" href="https://www.marcoslar.com">www.marcoslar.com</a> </div>

### A bit of context - **<span style="color: #F2CA30">Open</span><span style="color: #CF4229">SSH</span>** 7.9 - Ubuntu 18.04 - We focus on hardening the `sshd` daemon (i.e., `/etc/ssh/sshd_config`) - Not covered here: SELinux, MFA, cryptographic devices to store keys, brute-force protection ### Disclaimer - This is just an introduction to SSH hardening, not a reference guide <div class="image"> <img src="/static/talks/hardening-ssh/ssh.png" height="150px" alt="OpenSSH logo" style="filter: grayscale(75%);"> </div>

## Hardening `/etc/ssh/sshd_config`

### Before you start - Don't get yourself locked out of your server while hardening (!) - Have open two SSH sessions - Use one of the sessions to update the SSH daemon configuration file

### Strict modes ``` StrictModes yes ``` - Force `sshd` to check the permissions of important files and directories - Owned by root or the account owner. Group and world write permissions disabled - `~/.ssh/authorized_keys` - `~/.ssh` - The user's home directory - The user and system "known hosts" files

### Port number and network interface ``` Port 605 ``` - "Security through obscurity"... but it usually keeps away *script kiddies* and regular scanners - Do not use an unprivileged port: any user, local or remote, can DDoS it until it crashes. Then any non-root user can start its own `sshd` daemon on the same port ``` ListenAddress 172.16.20.20 ``` - By default, the port is bound to all network interfaces in your host (`0.0.0.0`). You may not always want that

### Keep alive messages ``` TCPKeepAlive yes ``` - Prevents half-dead connections from building up over time - Operates on the TCP layer. `sshd` does not set the timers (they are properties of the host's TCP stack) ``` ClientAliveInterval 300 ClientAliveCountMax 3 ``` - The server will send "alive?" messages to the client every 5 minutes until it receives a response or gives up and then terminates the connection

### Failed logins ``` LoginGraceTime 30 MaxAuthTries 3 ``` - The server disconnects the client after a number of failed authentication requests (forces SSH brute force attackers to initiate new connections) - Make sure you use `IdentityFile` in the client side to associate particular keys with particular hosts

### Maximum number of simultaneous connections ``` MaxStartups 5:50:10 ``` - Refuses connections based on probabilities - To conserve resources on your host - To reduce the risk of denial-of-service attacks

### Ciphers, Macs and key exchange algorithms ``` Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com, aes256-ctr,aes192-ctr,aes128-ctr Macs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com, umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp521, ecdh-sha2-nistp384,ecdh-sha2-nistp256 ``` - Remove AES, MD5 and SHA-1 support

### Authentication ``` PasswordAuthentication no PubKeyAuthentication yes HostbasedAuthentication no ChallengeResponseAuthentication no GSSAPIAuthentication no PermitRootLogin no AuthenticationMethods publickey ``` - Disable password-based logins

### Key generation ``` ssh-keygen -t ed25519 -f ssh_host_ed25519_$(date +%Y-%m-%d) -C "Server X key" ``` - Unique for each server - Favor ED25519 keys over RSA keys if backward compatibility is not required - Favor `ProxyJump` over agent forwarding if you need to jump between hosts

### References - SSH, The Secure Shell: The Definitive Guide by Daniel J. Barrett and Richard Silverman