Ubuntu 16.04 LTS SSH permission denied (publickey)

📖 ~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

Remote administration fails and operational access is interrupted.

Environment & Reproduction

Ubuntu 16.04 server hardened with custom sshd settings.

Root Cause Analysis

Invalid key permissions or conflicting sshd directives block authentication.

Quick Triage

Verify sshd service, listener port, and authentication method negotiation.

Step-by-Step Diagnosis

Use verbose SSH client logs and server auth logs for mismatch clues.

Solution – Primary Fix

Correct permissions, authorized_keys ownership, and sshd configuration.

Solution – Alternative Approaches

Use console access to add emergency admin key temporarily.

Verification & Acceptance Criteria

Public key login succeeds while unauthorized methods stay blocked.

Rollback Plan

Reinstate prior sshd_config snapshot and restart service.

Prevention & Hardening

Test SSH policy changes in staged rollout with fallback access.

Related Errors & Cross-Refs

Compare with fail2ban blocks and PAM account lockouts.

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

OpenSSH hardening guides and Ubuntu server security docs.