Ubuntu 20.04 LTS SSH Public Key Login 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

Administrators cannot access systems with keys, increasing operational risk during outages.

Environment & Reproduction

Common after user home changes, strict mode violations, or incompatible key algorithms.

Root Cause Analysis

SSH daemon rejects keys when file permissions, ownership, or accepted algorithms are misconfigured.

Quick Triage

Review sshd logs, confirm key presence, and check account shell and home directory validity.

Step-by-Step Diagnosis

Run verbose client authentication and inspect server-side policy to pinpoint rejection reason.

Solution – Primary Fix

Correct key file permissions, update sshd policy, and reload daemon with validated access settings.

Solution – Alternative Approaches

Use certificate-based SSH auth or temporary break-glass methods with strict auditing.

Verification & Acceptance Criteria

Key login succeeds for authorized users and denied access is properly logged for unauthorized attempts.

Rollback Plan

Revert sshd configuration to prior known-safe state if broad access issues emerge.

Prevention & Hardening

Enforce key management policies and periodic permission integrity checks on user homes.

Related Errors & Cross-Refs

Related to PAM account lockouts and host key mismatch warnings.

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

Consult OpenSSH and Ubuntu security hardening documentation for key authentication.