Ubuntu 20.04 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

SSH key authentication fails, preventing remote administration access.

Environment & Reproduction

Ubuntu 20.04 LTS host with modified home directory or sshd settings.

Root Cause Analysis

Incorrect file permissions, wrong authorized_keys content, or disabled pubkey auth.

Quick Triage

Use verbose SSH client mode and inspect auth logs for rejection reason.

Step-by-Step Diagnosis

Validate ownership, file modes, and relevant sshd_config directives.

Solution – Primary Fix

Correct permissions, reload sshd, and redeploy valid authorized_keys entries.

Solution – Alternative Approaches

Temporarily enable password authentication only during controlled recovery windows.

Verification & Acceptance Criteria

Public key login succeeds and password login policy matches security baseline.

Rollback Plan

Restore prior sshd_config and key files from backup if lockout risk rises.

Prevention & Hardening

Automate SSH baseline checks for permissions and auth method consistency.

Related Errors & Cross-Refs

Related to cloud-init key injection and fail2ban lockout scenarios.

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

Refer to sshd_config manual and Ubuntu OpenSSH server documentation.