Ubuntu 16.04 LTS – SSH public key authentication fails

📖 ~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

Users cannot log in with keys, reducing secure automation and admin access.

Environment & Reproduction

Seen after home directory permission changes or sshd config hardening updates.

Root Cause Analysis

Key files, ownership, or authentication directives violate OpenSSH requirements.

Quick Triage

Verify sshd settings, authorized_keys presence, and file permissions quickly.

Step-by-Step Diagnosis

Use verbose SSH client output and server logs to pinpoint auth rejection reason.

Solution – Primary Fix

Correct permissions, key format, and sshd policy to restore public key login.

Solution – Alternative Approaches

Temporarily enable controlled fallback auth while key policy is repaired.

Verification & Acceptance Criteria

Public key login succeeds and password fallback is restricted per policy.

Rollback Plan

Restore previous sshd config if hardened settings block legitimate access.

Prevention & Hardening

Automate SSH config validation and key permission audits during deployments.

Related Errors & Cross-Refs

Permission denied publickey and authentication refused are principal clues.

Related tutorial: View the step-by-step tutorial for Ubuntu 16.04 LTS.

View all Ubuntu 16.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Review OpenSSH server hardening and key management best-practice references.