Ubuntu 20.04 LTS SSH Login Rejected With Permission Denied

📖 ~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 lose remote access, delaying incident response and requiring console fallback for routine operational changes.

Environment & Reproduction

Appears after key rotation, user migration, strict security hardening, or accidental ownership changes on home and SSH directories.

Root Cause Analysis

sshd rejects auth when key files, authorized_keys paths, account policy, or daemon settings violate expected security constraints.

Quick Triage

Confirm account status and test with verbose SSH output to isolate key mismatch, policy denial, or server-side configuration issues.

Step-by-Step Diagnosis

Inspect sshd logs, validate ~/.ssh permissions, and test effective daemon config to identify the exact rejection reason.

Solution – Primary Fix

Repair ownership and mode bits, correct sshd_config directives, reload daemon, and revalidate key-based login from trusted clients.

Solution – Alternative Approaches

Use emergency password auth with MFA controls, out-of-band management channels, or temporary bastion relays for controlled recovery.

Verification & Acceptance Criteria

Accept success when key login works consistently, brute-force protections remain active, and audit logs show expected authentication paths.

Rollback Plan

Restore known-good sshd configuration snapshot and prior authorized_keys metadata if new hardening introduces operational lockouts.

Prevention & Hardening

Automate permission checks, enforce config linting pre-deploy, and include SSH reachability tests in post-change validation pipelines.

Related Errors & Cross-Refs

Related issues include PAM account lockouts, fail2ban bans, and host key mismatch warnings covered in companion access tutorials.

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

Read OpenSSH manual pages, Ubuntu SSH hardening recommendations, and incident response guides for secure remote administration.