Debian 9 SSH Public Key Authentication 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

Users cannot log in with keys and are denied or prompted for passwords.

Environment & Reproduction

Often follows key rotation, permission changes, or hardened sshd configuration updates.

Root Cause Analysis

Authorized key validation fails due to permissions, format issues, or policy restrictions.

Quick Triage

Review sshd logs, account shell status, and user home directory ownership.

Step-by-Step Diagnosis

Trace authentication decision points in verbose client and server logs.

Solution – Primary Fix

Correct file permissions, key placement, and sshd auth directives, then reload service.

Solution – Alternative Approaches

Use temporary break-glass account controls while key auth is restored.

Verification & Acceptance Criteria

Public key login succeeds and unauthorized keys remain blocked.

Rollback Plan

Reapply prior sshd configuration and key set if new policy blocks valid access.

Prevention & Hardening

Automate key deployment validation and enforce secure file-permission baselines.

Related Errors & Cross-Refs

Related to PAM account restrictions and locked user states.

Related tutorial: View the step-by-step tutorial for Debian 9.

View all Debian 9 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

OpenSSH and Debian secure remote-access documentation.