Ubuntu 18.04 SSH Login Fails with Public Key Authentication

📖 ~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

Key-based SSH logins are denied, forcing fallback methods and risking delayed operational response.

Environment & Reproduction

Happens after user migration, permission drift, or sshd policy hardening on Ubuntu 18.04.

Root Cause Analysis

Incorrect ownership, mode bits, key format mismatch, or disallowed algorithm settings block authentication.

Quick Triage

Use verbose SSH client output and check server auth logs for exact rejection reason.

Step-by-Step Diagnosis

Validate home directory permissions, authorized_keys content, and sshd configuration directives.

Solution – Primary Fix

Correct file permissions, normalize key formats, reload sshd, and confirm algorithm compatibility.

Solution – Alternative Approaches

Enable temporary secondary key, use certificate-based auth, or controlled password fallback with MFA.

Verification & Acceptance Criteria

Public key auth succeeds from approved clients and failed-attempt logs no longer show policy mismatch.

Rollback Plan

Revert sshd changes and authorized key updates if access control becomes too permissive or unstable.

Prevention & Hardening

Automate permission checks and rotate keys with testing gates before enforcing stricter SSH policies.

Related Errors & Cross-Refs

Connect to PAM account lockouts and host key mismatch issues with linked Ubuntu SSH tutorial.

Related tutorial: View the step-by-step tutorial for Ubuntu 18.04 LTS.

View all Ubuntu 18.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Read OpenSSH manuals, Ubuntu hardening guides, and enterprise access policy recommendations.