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

SSH hardening depends on valid key format and strict file permissions.

Environment & Reproduction

Users receive Permission denied (publickey) despite correct key pair.

Root Cause Analysis

Servers with updated sshd settings or recently modified home directories.

Quick Triage

Incorrect ownership, mode bits, or disabled authentication options.

Step-by-Step Diagnosis

Capture ssh -vvv output and auth logs from server side.

Solution – Primary Fix

Fix file permissions and validate sshd auth directives.

Solution – Alternative Approaches

Enforce key deployment and permission baselines via automation.

Verification & Acceptance Criteria

Authenticate with key-only login from trusted client.

Rollback Plan

Run periodic compliance checks on SSH configuration and file modes.

Prevention & Hardening

Track failed login spikes and auth method fallback attempts.

Related Errors & Cross-Refs

Restore prior sshd_config from backup and reload service safely.

Related tutorial: View the step-by-step tutorial for debian-10.

View all debian-10 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

OpenSSH server documentation and Debian security hardening guidance.