Ubuntu 24.04 LTS – SSH public key authentication fails – Fix & Prevention

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

SSH login fails with permission denied despite valid keypair.

Environment & Reproduction

Often occurs after home directory permission changes or sshd config edits.

ssh -vvv user@host
sudo systemctl status ssh --no-pager

Root Cause Analysis

Incorrect file permissions, wrong username, or sshd settings disable key auth.

Quick Triage

Check server logs for exact auth rejection reason.

sudo journalctl -u ssh -n 100 --no-pager
sudo sshd -T | grep -Ei 'pubkeyauthentication|authorizedkeysfile'

Step-by-Step Diagnosis

Validate key files and ownership on target account.

ls -ld ~/.ssh
ls -l ~/.ssh/authorized_keys
namei -l ~/.ssh/authorized_keys

Solution – Primary Fix

Restore canonical SSH permissions and ensure PubkeyAuthentication is enabled.

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
chown -R $USER:$USER ~/.ssh
sudo systemctl reload ssh

Solution – Alternative Approaches

Use Match blocks for specific users if global policy conflicts exist.

sudoedit /etc/ssh/sshd_config.d/99-local.conf
sudo sshd -t
sudo systemctl reload ssh

Verification & Acceptance Criteria

Key-based login succeeds without password fallback.

ssh -o PreferredAuthentications=publickey user@host 'id'

Rollback Plan

Restore previous sshd snippets if authentication behavior regresses.

sudo rm -f /etc/ssh/sshd_config.d/99-local.conf
sudo systemctl reload ssh

Prevention & Hardening

Manage sshd config in drop-in files and test with sshd -t before reload.

sudo sshd -t

Related Errors & Cross-Refs

Related messages include Authentication refused: bad ownership or modes.

Related tutorial: View the step-by-step tutorial for Ubuntu 24.04 LTS.

View all Ubuntu 24.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

OpenSSH and Ubuntu server hardening documentation.