How to Secure and Harden Cloud VMs (Ubuntu & CentOS) – A Comprehensive Guide


Hey there!

Securing your cloud VMs is no simple task. As a fellow system administrator, I know how tough it can be to lock things down in a dynamic environment like the public cloud.

In this comprehensive guide, we‘ll tackle proven techniques to harden Ubuntu and CentOS VMs deployed in clouds like AWS, Google Cloud, and Azure.

I‘ll share insights from my experience as both a cloud architect and security analyst to help you meet your security requirements. You‘ll find actionable steps here to protect your cloud VMs from common threats and vulnerabilities.

Let‘s dig in!

Why Proper Cloud VM Security Matters

First, it‘s important to understand why diligently securing your cloud VMs matters. Migrating workloads to the cloud doesn‘t make them inherently more secure.

Cloud providers secure the underlying infrastructure and abstract resources to you through VMs. But properly configuring the VMs themselves is entirely your responsibility according to the shared responsibility model.

Without proper security hardening, your cloud VMs are vulnerable to an array of attacks, including:

• Brute force attacks – Attackers try endless combinations of usernames and passwords to break into VMs.
• DDoS attacks – Floods of malicious traffic can overwhelm your VMs and cause outages.
• Malware infections – Malicious software is installed to steal data or computing resources from compromised VMs.
• System exploits – Unpatched vulnerabilities are leveraged to gain control of VMs.

The impact of successful attacks includes data breaches, service disruptions, regulatory non-compliance penalties, and more. That‘s why hardening your cloud VMs should be a top priority.

The good news is that core security principles apply whether you use Ubuntu, CentOS, or any mainstream Linux distribution. Now let‘s check out the key steps you should take to lock down your cloud VMs.

10 Essential Steps for Hardening Cloud VMs

These best practices form the foundation of a secure cloud VM deployment:

1. Isolate your VMs in a private cloud

Public cloud providers like AWS, GCP, and Azure offer isolated virtual private clouds (VPCs) to limit exposure.

AWS VPCs, GCP Virtual Private Clouds, and Azure VNets provide you a private virtual networking environment in the public cloud. This segregates your VMs and resources from the wider Internet.

You maintain full control over the virtual "walls" of your VPC. For example, you can:

• Selectively assign public IP addresses to connect VMs to the Internet
• Define granular security group rules to restrict traffic
• Use private IP addressing between VMs and resources inside the VPC

This private cloud model limits your potential attack surface. Baking in these network security controls is a good foundational step for your cloud VMs.

2. Define strict firewall policies

Once you have a VPC in place, take advantage of managed cloud firewall services like AWS Security Groups, Azure Firewall, and GCP VPC Firewall.

These built-in firewalls make it easy to restrict access to your VMs at a network traffic level. Rules typically provide control based on:

• Source IP address or CIDR range
• Destination port and protocol
• Direction of traffic (ingress vs. egress)

With properly configured firewall policies, you can limit traffic to only the ports and protocols required for approved functions. For example, you may permit only SSH on port 22 from your corporate headquarters IP range to administer internal Linux VMs.

The cloud firewall acts as an extra layer of network filtering to better secure VMs. Consult your cloud provider‘s documentation to implement appropriate firewall rules.

3. Disable password-based SSH and use keys

Leaving port 22 open to the Internet for password-based SSH access is asking for brute force attacks. A better approach is to disable password authentication altogether and require SSH public key authentication.

Here are the steps:

1. On your local computer, generate an SSH key pair:

   ssh-keygen -t rsa

2. Copy your new public key to the cloud VM:

   ssh-copy-id username@server-ip

3. Edit /etc/ssh/sshd_config on the VM and set:

   PasswordAuthentication no
   PubkeyAuthentication yes

4. Restart the ssh service to apply the changes:

   sudo systemctl restart sshd

With password-based access disabled, attackers can‘t brute force SSH credentials on your VM. Requiring an SSH key enhances security.

4. Change the default SSH port

Many attackers hunt for cloud VMs listening on the standard SSH port 22. Changing the port makes your VM harder to find.

Edit /etc/ssh/sshd_config and update to a custom port like 2222. Just be sure to permit access for that port in your cloud firewall rules.

After updating the SSH daemon configuration, restart it to bind it to your new non-standard port. Going forward, you‘ll connect over your custom port instead of 22.

5. Use allow lists over deny lists

When defining firewall policies, add rules to explicitly allow required access while denying everything else by default.

This "allow list" approach is considered more secure than a "deny list" that permits all traffic but blocks certain ports and protocols.

Allow lists force you to intentionally enable only the traffic types you need to operate. The default-deny posture blocks any unapproved connectivity attempts to your VMs.

6. Install host-based firewalls

In addition to cloud provider firewalls, installing host-based firewall software like iptables or firewalld directly on VMs adds defense in depth.

Host firewalls create granular rules to selectively filter traffic to your VM itself, independent of the external cloud firewall.

This allows you to implement fine-grained access controls while also obscuring your actual VM infrastructure from port scans.

7. Leverage VM security groups

Cloud platforms like AWS and GCP provide security groups as a logical construct to control VM access. With security groups, you can:

• Logically group VMs by role like "web servers" or "databases"
• Define allow rules for communication between security groups
• Set default deny to isolate groups and restrict access

For example, 3 sample groups:

• Web – Contains your web server VMs
• App – Contains backend application VMs
• DB – Contains database VMs

You could allow the web group to talk to app, and app to talk to DB. But block web direct to DB for added security.

8. Centralize and analyze VM logs

Your cloud VMs generate valuable security event log data that provides visibility into activity across your infrastructure.

Make sure you have VM logging enabled, then centralize logs from all VMs into a system like Elasticsearch or Splunk for analysis.

Reviewing aggregated security logs allows you to detect anomalies, quickly track down incidents, and gain operational insights.

9. Install, update, and run antivirus tools

Installing antivirus software like ClamAV helps detect malware and protect critical VM file systems. Make sure to:

• Schedule regular scans to check for known attack payloads and malware signatures
• Keep antivirus tools updated with the latest definitions for new threats

While not foolproof, antivirus provides valuable visibility into abnormalities that could indicate nefarious activity.

10. Never stop patching and upgrading

Diligently applying the latest OS security patches and updates is non-negotiable. Cloud VMs still require patching to fix vulnerabilities just like physical servers.

Set up a formal OS patch management process if you haven‘t already. Monitor announcements and test and deploy patches in a timely manner.

For Ubuntu:

apt update && apt upgrade -y

For CentOS:

yum update -y

Don‘t let your cloud VMs lag on patching, as that exposes significant windows for exploitation and compromises security.

Deeper OS Hardening for Cloud VMs

Beyond the top 10, expanding your OS hardening improves security and reduces attack surface further. Here are some additional areas to address:

Remove unnecessary packages – Uninstall any default OS packages not needed that could pose additional risk like old web servers or remote desktop services.

Disable root SSH login – Only permit root login via sudo after authenticating with a normal user account for better privilege control.

Set resource limits – Use ulimit to constrain things like max processes and open files per user to help guard against denial of service.

Disable core dumps – Don‘t allow process memory core dumps that could leak otherwise inaccessible data.

Carefully permit sudo – Only allow required users to escalate privileges with sudo according to principle of least privilege.

Disable USB storage – Cloud VMs shouldn‘t need USB media, so disable USB storage in VM configuration to prevent sneakernet data extraction or introduction of malware.

These steps make the VM more purpose-built and secure by reducing potential vectors. Combine them with the top 10 best practices to harden your cloud VMs.

Leverage Encryption for Cloud VM Security

Encrypting VM disks, volumes, data transfers, and communication channels is vital for security:

• Storage encryption – Encrypt both VM boot/root disks and data volumes. AWS, DigitalOcean, Azure, GCP and other major cloud providers offer storage encryption options to protect data at rest.

• File encryption – Leverage encrypted containers like crypt or eCryptfs to encrypt sensitive files and data at the OS level.

• Network encryption – Use IPSec VPNs or TLS to encrypt traffic between VMs and externally. TLS certificates also encrypt the web traffic from external visitors.

Proper encryption protects your VM data and communications even if other defenses fail. Make sure to deploy encryption both in transit and at rest.

Guard Access to VM Metadata Services

Cloud VMs rely on an internal metadata service to query dynamic instance information and temporary credentials.

But attackers may attempt to compromise the metadata API and gather insider infrastructure knowledge and access keys.

Use permissions, network rules, and other access controls to limit metadata service access only to your own authorized VMs. Never expose the metadata service to public IP ranges.

Monitor VM Network Traffic for Threat Detection

Constantly monitoring network traffic provides visibility into connections and can quickly detect anomalies that indicate malicious activity:

• Tools like Suricata installed on VMs can automatically analyze traffic against known bad signatures.

• Centralizing logs in a SIEM lets you correlate insights across VMs to identify broader issues.

• Machine learning techniques can establish traffic baselines and flag deviations.

Combining traffic analysis with other system logs gives you greater detection capabilities.

Frequently Perform Security Audits and Pen Testing

Conduct recurring security reviews consisting of audits and penetration testing against your VMs:

• Audits validate compliance with CIS and other security benchmarks for OS hardening.

• Penetration testing goes further by actively probing your actual production infrastructure for vulnerabilities using techniques real attackers employ. White hat hackers simulate attacks against your deployed systems and report on vulnerabilitiesdetected.

Together these assessments validate controls and reveal gaps. Quickly remediate any findings to further enhance security.

Configuration Management Secures the VM Fleet

Manually hardening and updating configurations for more than a few VMs just isn‘t practical. That‘s where configuration management (CM) tools like Ansible, Puppet, and Chef shine.

CM tools allow you to codify and automate security:

• Enforce consistent configs and compliance at scale
• Rapidly orchestrate updates across 1000s of VMs
• Gain visibility into overall system state

For example, you can define Ansible roles that encompass tasks like:

• Installing and configuring OS firewalls
• Deploying logging and monitoring agents
• Removing unneeded packages
• Running antivirus and vulnerability scans

Then with just one command, immediately apply that hardened configuration across your entire VM estate.

CM brings order and oversight to cloud security. Learn to leverage these tools to optimize hardening.

The Importance of a Holistic Approach

Comprehensively securing your cloud VMs requires looking at the big picture. VM hardening is crucial but works hand in hand with other measures:

• Employee security training – Your team is your first line of defense. Regular training ensures they remain vigilant against social engineering andunauthorized system access.

• Identity and access management (IAM) – Properly scoping identity permissions and limiting account access prevents abuse of credentials to infiltrate cloud environments.

• Logging and monitoring – Comprehensive visibility into system activity and events allows quick threat detection and response.

• Vulnerability management – Continuously scanning for and remediating vulnerabilities prevents exploitation.

• Data encryption – Encrypting sensitive data both at rest and in transit protects your assets if other defenses fail.

• Network segmentation – Placing tighter network controls between VM groups limits lateral movement during incidents.

No single method will fully secure your cloud VMs. Combine strong VM hardening with other critical controls for defense in depth.

Key Takeaways for Cloud VM Hardening

Let‘s recap the key takeaways:

• Properly configure cloud VMs to avoid data breaches and outages – don‘t rely on the cloud provider alone.
• Isolate VMs in private VPCs and implement cloud firewalls.
• Require SSH keys over passwords and change the default port.
• Monitor traffic, events, and system changes for incident response.
• Automate security configuration through infrastructure as code tools like Ansible.
• Combine VM hardening with encryption, access controls, and other processes.

I hope these practical tips provide you a blueprint for securely operating Ubuntu, CentOS, and other Linux distros in the public cloud. Feel free to reach out if you have any additional questions! I‘m always happy to chat cloud security.

Stay safe out there!