SSH Public Key Authentication Fails on Ubuntu 14.04

📖 ~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

Users with valid keys cannot log in and receive Permission denied (publickey), delaying operational access and emergency response.

Environment & Reproduction

Frequent on Ubuntu 14.04 after home directory migration, permission drift, or hardened sshd_config changes without compatibility validation.

Root Cause Analysis

sshd rejects key authentication because file ownership, directory mode, key format, or Match/User policy does not meet security requirements.

Quick Triage

Check auth.log entries, verify ~/.ssh and authorized_keys permissions, and confirm user shell and account state before deeper changes.

Step-by-Step Diagnosis

Run ssh -vvv from client, inspect server-side sshd -T output, and compare offered key type with accepted algorithms on old OpenSSH versions.

Solution – Primary Fix

Correct ownership and mode for user home and .ssh files, place valid public key in authorized_keys, then reload ssh service.

Solution – Alternative Approaches

Temporarily permit password auth for controlled break-glass access, deploy centralized key management, or rotate users to standardized key policy.

Verification & Acceptance Criteria

Key-based login succeeds from authorized clients, auth logs show accepted publickey entries, and password fallback is disabled if policy requires.

Rollback Plan

Restore prior sshd_config and authorized_keys backups if new hardening changes block legitimate access unexpectedly.

Prevention & Hardening

Automate key deployment with strict permissions, monitor auth failures, and enforce SSH configuration linting before rollouts.

Related Errors & Cross-Refs

Related to UFW lockouts, DNS reverse lookup delays, and PAM misconfiguration that presents as SSH auth failure.

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

See sshd_config manual, OpenSSH troubleshooting guides, and your security baseline for SSH access on legacy Linux.