Hardening and secure access case study

Linux Hardening & Secure Access Control

Implemented a secure Linux access baseline using UFW default-deny firewall rules, SSH restricted to a trusted administrative host, ED25519 key-based access, validation checks, and documented rollback procedures.

LinuxUFWOpenSSHED25519 keysRollback planningSecure administration

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Objective

Reduce unnecessary inbound exposure and harden administrative SSH access.

Firewall

UFW default deny inbound, allow outbound, and SSH limited to a trusted admin host.

Access

Password authentication and root login disabled; ED25519 key access validated.

Outcome

A documented secure baseline with validation screenshots and recovery steps.

Case-study summary

Security problem

Default Linux access patterns can leave administrative services broader than they need to be. The goal was to reduce exposure, enforce key-based access, and keep a safe rollback path in case the change locked out legitimate administration.

Environment

Ubuntu host protected by UFW, Kali trusted administrative host, OpenSSH service, ED25519 SSH key pair, and documented command-line validation artifacts.

Build / implementation

Reset and configured UFW, applied default-deny inbound policy, allowed outbound traffic, restricted SSH to the trusted admin host, generated and installed ED25519 keys, hardened sshd_config, restarted services, and tested the result.

Validation

Validated firewall status, SSH service status, key-only login, and rollback/recovery steps before treating the control as complete.

Analyst takeaway

What this proves to a hiring manager

This project shows security control implementation with operational discipline: plan the control, apply it carefully, validate access, document evidence, and prepare rollback so the system remains recoverable.

Skills demonstrated

Role-aligned capabilities

Linux system hardening
Firewall policy implementation
SSH secure administration
Change validation and rollback planning
Security documentation that a team can repeat

Evidence gallery

Screenshots and artifacts

Diagram showing Kali admin host connecting over SSH to the Ubuntu host, with UFW default-deny inbound policy and SSH key-only authentication. — Secure-access topology showing trusted admin access to the hardened Ubuntu host.

UFW implementation sequence showing firewall baseline commands and final status output. — Firewall baseline sequence: reset, default deny inbound, allow outbound, trusted SSH rule, logging, enable, and status.

Terminal output showing generation of an ED25519 SSH key pair. — ED25519 key-pair generation for SSH key-only access.

Terminal output showing ssh-copy-id installing the public key on the Ubuntu host. — Public key installation on the Ubuntu host.

Excerpt from sshd_config showing PasswordAuthentication no, ChallengeResponseAuthentication no, PubkeyAuthentication yes, and PermitRootLogin no. — sshd_config hardening choices for key-only, non-root administrative access.

Terminal output showing successful SSH login to the Ubuntu host after key-only hardening. — Successful key-based SSH login validation after hardening.

Composite image showing numbered UFW rollback and SSH recovery steps. — Documented rollback and recovery commands for safe change management.

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