Ubuntu 20.04 LTS – SSH login fails with 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 is blocked due to key authentication failure.

Environment & Reproduction

Typically after key rotation, user changes, or restrictive sshd settings.

Root Cause Analysis

Authorized keys, file permissions, or SSH config mismatch prevents auth.

Quick Triage

Verify account status, key presence, and sshd authentication directives.

Step-by-Step Diagnosis

Trace auth attempt in SSH logs and confirm key fingerprint alignment.

Solution – Primary Fix

Correct key deployment, ownership, and permission model for SSH paths.

Solution – Alternative Approaches

Use temporary console access to create break-glass admin key.

Verification & Acceptance Criteria

SSH login succeeds with expected key and without password fallback.

Rollback Plan

Restore prior sshd config and authorized_keys backup if lockout risk increases.

Prevention & Hardening

Automate key rollout validation and enforce secure permission baselines.

Related Errors & Cross-Refs

Permission denied (publickey), Authentication refused, and bad ownership errors.

Related tutorial: View the step-by-step tutorial for Ubuntu 20.04 LTS.

View all Ubuntu 20.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

OpenSSH server hardening and Ubuntu authentication troubleshooting docs.