Ubuntu 14.04 LTS SSH Public Key Login Suddenly 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

Automated SSH access breaks, impacting deployments, backups, and operational response.

Environment & Reproduction

Frequently occurs after permission changes, home directory migrations, or sshd hardening edits.

Root Cause Analysis

StrictModes validation rejects keys when file ownership or permissions violate SSH policy.

Quick Triage

Run ssh -vvv from client and inspect server auth logs for rejection reason.

Step-by-Step Diagnosis

Validate authorized_keys path, permissions, SELinux/AppArmor context where applicable, and sshd settings.

Solution – Primary Fix

Correct ownership and mode bits, confirm key format, reload sshd, and retest authentication.

Solution – Alternative Approaches

Temporarily allow password fallback or deploy new key pair with controlled rotation.

Verification & Acceptance Criteria

Key-based login succeeds non-interactively and logs confirm accepted public key method.

Rollback Plan

Restore prior sshd_config and user SSH directory from backup.

Prevention & Hardening

Automate permission checks and enforce baseline SSH file modes in configuration management.

Related Errors & Cross-Refs

Connected to host key mismatch, PAM auth errors, and user shell misconfiguration.

Related tutorial: View the step-by-step tutorial for Ubuntu 14.04 LTS.

View all Ubuntu 14.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

OpenSSH manual pages, Ubuntu SSH security guides, and key management practices.