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To reduce SSH password-guessing risk, first confirm that an administrator can log in with a public key, then disable password-based authentication if every required user and recovery route supports it. Also restrict which accounts and networks can connect, limit unauthenticated attempts, and test the new policy from a fresh session before closing your existing one.
1. Preserve a working way back in
Before changing SSH settings, open a second session using the key and account you intend to keep. Leave the known-good session open while you work, and make sure you have a separate console or provider recovery path. An incorrect authentication setting or allow-list can lock out legitimate administrators.
Check the installed OpenSSH manual and your distribution’s configuration layout before editing. Included files and configuration ordering can affect which value takes effect; a fragment copied from another server may not produce the policy you expect.
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Public-key authentication is usually the strongest practical baseline for reducing password guessing. OpenSSH documents PubkeyAuthentication and PasswordAuthentication; its manual lists both as enabled by default in the documented configuration context, but packaged configuration can change the effective settings. OpenSSH sshd_config(5)
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If every administrator, automation account, and required recovery path can authenticate by key, set PasswordAuthentication no. Do not make this change until a separate key-based login has succeeded. The Fail2Ban OpenSSH filter guidance likewise advises establishing public-key access before disabling password authentication in its brute-force protection context.
Check keyboard-interactive and PAM too
Turning off PasswordAuthentication alone may not remove every password-like route. Review KbdInteractiveAuthentication, PAM-related settings, and any Match blocks. OpenSSH notes that PAM keyboard-interactive authentication can serve a role equivalent to password authentication, so confirm how the installed server handles these methods before assuming the server is key-only.
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Consider whether FIDO-backed keys fit
OpenSSH documents FIDO security-key authentication and options such as touch-required and verify-required. These options apply to FIDO key types and require compatible client, server, and deployment support; a FIDO key is optional, not a prerequisite for disabling passwords.
3. Limit direct root access and permitted accounts
Choose a root-login policy that matches your administrative and recovery model. OpenSSH supports PermitRootLogin yes, prohibit-password, forced-commands-only, and no. The prohibit-password policy disables password and keyboard-interactive authentication for root, but still permits other configured methods. For many systems, named administrator accounts with controlled privilege escalation are preferable to routine direct root login.
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Limit SSH to the accounts or groups that need it with settings such as AllowUsers or AllowGroups. Red Hat’s examples cover these controls and firewall configuration for Red Hat Enterprise Linux 8. An overly narrow allow-list can deny necessary access, so verify the intended users and group membership before applying it.
4. Reduce which systems can reach SSH
When administrators connect from known networks, restrict SSH at both the cloud security group or provider firewall and the host firewall. These are separate control layers: changing one does not automatically configure the other. Keep approved management sources and recovery access allowed.
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In suitable deployments, OpenSSH authorized-key options can also constrain a key to specified source addresses. See the OpenSSH sshd(8) manual for authorized-key behavior. Firewall commands and interface details vary by distribution and provider; Red Hat’s cited instructions apply specifically to RHEL 8.
5. Add attempt limits and log-based blocking
Use OpenSSH connection controls
MaxAuthTries limits authentication attempts permitted on one connection. The current Linux man-pages OpenSSH manual documents a default of 6; this is a configuration default, not a measure of attack frequency or control effectiveness. MaxStartups controls concurrent unauthenticated connections. Tune both with legitimate client behavior in mind rather than treating lower values as a complete defense.
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Use Fail2Ban as an additional layer
Fail2Ban can react to matching SSH log entries and temporarily ban source IP addresses. Confirm that its installed SSH filter matches the host’s log format, and check that trusted administrators or automation will not be blocked. It complements authentication and network restrictions; it does not replace them.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.6. Apply changes and verify a fresh login
- Review the effective policy. Inspect the main SSH daemon configuration, included snippets, and any matching rules for the accounts or addresses affected. Consult the manual for the installed OpenSSH build rather than relying on a generic default.
- Validate syntax. Use the configuration-validation method supported by your installed OpenSSH version before reloading or restarting the daemon.
- Apply the change using your distribution’s service procedure. Service names and reload behavior vary, so follow the local system’s documentation.
- Test from a new client session. Confirm that an approved administrator can connect using the intended method and that disallowed authentication paths are unavailable.
- Check logs and retain recovery access. Look for unexpected failures or bans, and keep the existing session or console route until the new access policy is confirmed.
How the controls fit together
| Control | What it addresses | Important limit |
|---|---|---|
| Public-key-only authentication | Removes password login as a guessing path when all required access uses keys. | Requires working keys for users, automation, and recovery; inspect keyboard-interactive and PAM behavior too. |
| Account and root restrictions | Reduces the set of accounts that can log in directly. | An incorrect policy or allow-list can block required administrators. |
| Firewall and source restrictions | Reduces which network sources can reach SSH at all. | Host and provider firewalls are separate; authorized sources and recovery routes must remain reachable. |
MaxAuthTries and MaxStartups |
Limit attempts per connection and concurrent unauthenticated connections. | They do not eliminate password exposure or secure authentication by themselves. |
| Fail2Ban | Uses log patterns to trigger temporary source-IP bans. | Depends on correct log/filter matching and can block trusted sources if misconfigured. |
CISA’s communications-infrastructure guidance also recommends disabling SSH version 1 and using version 2. That is broader protocol hardening guidance rather than a specific brute-force control; follow the supported protocol behavior of the OpenSSH release you run. CISA Enhanced Visibility and Hardening Guidance for Communications Infrastructure
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