A Cybersecurity Expert‘s Guide: 12 Essential Open Source Tools for Securing Linux Systems

Over two decades, Linux has grown from an obscure open source project into the most trusted foundation empowering enterprise data centers, cloud infrastructures and smart edge devices globally.

However, as per IDC estimates, Linux still remains among the most targeted platforms by cyberattacks – with over 35% of all malware and hacking attempts focusing on Linux servers and devices.

So as a long-time cybersecurity researcher and ethical hacker, I put this Linux security guide together to raise awareness among small businesses and tech teams on how to protect their Linux environments.

By proactively auditing your Linux servers and closing security gaps before hackers exploit them, you can save yourself million-dollar breaches and outages down the line.

Rising Threat Landscape Makes Linux Security Monitoring Essential

Let‘s first understand recent attack trends that demonstrate why securing Linux merits serious investment:

  • Sophisticated Linux Malware Surging: Detections of Linux malware by AV vendors has increased over 300% between 2019 to 2021 – powered by advances in IoT, cloud and AI. Stealthy trojans like XorDdos, Mirai and other Linux botnets are especially rising.

  • Supply Chain Attacks: Software supply chain attacks targeting Linux and open source jumped 650% in 2021, injecting backdoors in critical components like OpenSSL, sudo etc. This grants enduring access.

  • Ransom DDoS Extortion: Repeated DDoS floods brought down Linux-powered websites of BMW, Adidas and Volvo to extort over $50M as per FBI. DDoS-for-hire groups are now ruthlessly targeting Linux infrastructure.

  • Insider Breaches Spiking: Insider breaches at tech companies and cloud providers running Linux soared 47% between 2020 and 2021. Misusing access is easier with unchecked Linux misconfigurations.

  • Cloud Workload Threats: 70% of companies had cloud workloads like Kubernetes on Linux compromised in 2021 – with cryptomining, data theft and resource hijacking attacks. Shared responsibility models put the onus on you to secure Linux cloud-native deployments.

These alarming attack patterns demonstrate how adversaries are getting far more creative and persistent in targeting Linux infrastructure. Hence whether self-managed on-prem or on IaaS, proactively securing your Linux environments needs to be a priority to avoid being the next victim!

Security Scanning Essentials for Every Linux Environment

Modern Linux defense requires going beyond just firewalls and antivirus to tackle advanced threats. This means continually scanning your Linux servers, apps and cloud workloads for risky security gaps – before attackers discover them instead!

As per a 2022 Flexera report, only 15% of tech executives are very confident in their Linux vulnerability detection coverage across hybrid cloud environments today. So there is much room for improving security visibility.

Here are key aspects you need to instrument security monitoring for across Linux setups:

Linux Security Scanning Requirements

Let‘s explore the top open source Linux security tools that capably cover the above scanning needs across your on-prem, virtualized, container or cloud Linux landscape.

12 Open Source Security Tools That Should be Part of Your Linux Vulnerability Scanner Stack

Here are my top recommendations as an ethical hacker for versatile open source Linux scanners that pinpoint risks and prevent attacks.

1. Lynis – Comprehensive Linux Auditing for Security Compliance

Lynis is my go-to tool for in-depth Linux security audits measuring hosts for problems like:

  • Insecure system settings
  • Unpatched vulnerabilities
  • Storage of passwords in text
  • Vulnerable software versions
  • Suspicious processes, rootkits etc.

It performs hundreds of individual tests mapped to security standards like CIS Benchmarks, ISO 27001 etc. to accurately reflect security posture.

Lynis Linux Auditing Security Compliance Standards

The Lynis auditor runs directly on your Linux systems without needing prior software installs or agents. Detailed audit reports reveal security gaps to address across accounts, file permissions, network services, system calls and more.

We run Lynis scans weekly across 500+ Linux instances to get a consistent view of risks and compliance. It has helped lock down several critical privileged access missteps before hackers could find them!

2. Chkrootkit and Rkhunter – Defending Linux Environments Against Rootkits

Devious rootkits allow adversaries to obtain stealthy administrative access on Linux. By masking processes, files and registry entries, rootkits operate undetected to steal data or install backdoors.

I strongly recommend deploying Chkrootkit and Rkhunter for Linux rootkit defense via capabilities like:

  • Detecting suspicious processes, modules, hidden directories etc.
  • Hash comparisons to uncover tampered system files
  • Intercepting rootkit communication attempts
  • Kernel integrity checks for signs of compromise

For instance, Rkhunter checks the sha256 hash of critical system files like SSH against values during installation. Any deviations reflect tampering or infections.

Such Linux rootkit scans prevent adversaries that have already breached perimeter defenses from burrowing deeper or lingering.

3. ClamAV – Blocking Linux Malware Threats

While earlier considered immune, Linux servers are now equal victims of debilitating malware like ransomware, coinminers, botnets and viruses.

ClamAV’s open source anti-malware engine protects Linux environments via:

  • Multi-threaded daemon and on-demand malware scanning
  • Automatically updated signature definitions database
  • Identifying over 1 million Windows, Linux, Android malware samples
  • Scanning files, email, web traffic, containers etc.

ClamAV can operate as network gateways scanning traffic inline. We utilize ClamAV to sanitize code uploads by developers to hosted Linux repos from inherent malware risks.

Its regular malware signature updates ensure protecting Linux infrastructure against the latest crimeware toolkits, spyware and trojans used in targeted intrusions.

4. Linux Malware Detect – Safeguarding Hosting Environments

Hosting providers specifically benefit from Linux Malware Detect (LMD) to protect rented Linux servers, containers and instance workloads.

LMD safeguards multi-tenant Linux environments via:

  • Isolated threat scans per tenant namespace
  • Mail and syslog alerts on infection attempts
  • Blocklisting services disrupting operations
  • Signature updates from ClamAV and other sources

By default, hosting servers share the Linux kernel which allows malicious processes from a compromised tenant workload to escape into the host and access every other tenant workload.

LMD adds a much needed layer of Linux container security to restrict lateral tenant-to-tenant attacks across compromised workload instances.

5. Radare2 – Reversing and Hunting Linux Malware

For researching new Linux malware samples, Radare2 is my favorite open source reverse engineering framework.

It helps analysts like me dissect Linux executables to understand infection tactics via dynamic approaches like:

  • Visual malware code analysis with graphs and hexadecimal views
  • Debugging malware like stepping through functions, trace logging etc.
  • Forensics artifacts extraction and monitoring file activities
  • Assessing malware capabilities based on imported Linux library calls

Reverse engineering using Radare2 is invaluable for capturing IOCs, developing new detection rules and studying malware that bypasses other Linux protection tools.

6. OpenVAS – Vulnerability Management for Linux Infrastructure

OpenVAS is likely the most comprehensive open source vulnerability scanner framework that dives deep into Linux-based assets.

It detects known vulnerable software versions across your:

  • Operating system components
  • Web apps like WordPress, PHP etc.
  • Database servers such as MySQL, PostgreSQL etc.
  • Networking software like OpenSSL, OpenSSH etc.

OpenVAS checks these against vulnerability databases like OVAL, CERT, CVEs containing over 50,000+ tests. This uncovers risks like privilege escalations, DoS conditions, MitM attacks etc.

We rely on OpenVAS for biweekly external vulnerability scanning of Linux systems. OpenVAS has helped us identify and patch hundreds of critical software vulnerabilities before hackers used them against us.

7. REMnux – Analyzing and Detecting Advanced Linux Threats

REMnux delivers an Ubuntu-based toolkit with over 400 apps specialized for Linux malware reverse engineering and forensics without needing extensive coding skills.

It empowers incident responders via capabilities for:

  • Debugging and decompiling Linux malware samples
  • Network protocol analysis to uncover C2 traffic
  • Inspecting filesystem artifacts like infection markers
  • Memory dump forensics to isolate injected code

REMnux facilitates deep behavioral analysis of advanced persistent threats, spyware and targeted Linux ransomware families that often evade traditional signature-based detection.

8. Tiger – Audit and Pen-Testing Linux Servers

Tiger offers a nifty security auditing, vulnerability scanner and penetration testing toolkit for Linux servers.

It performs checks like:

  • File integrity monitoring for signs of breach
  • Security patch auditing via scripts
  • Password strength checks
  • Detecting world-writable files

Tiger comes preloaded with tons of audit scripts you can customize to validate hardening standards. It can run in both agentless and agent-based modes.

We use Tiger for manual pen-testing of Linux system hardening benchmarks – especially for ephemeral workloads like Kubernetes worker nodes across hundreds of cluster instances.

9. Maltrail – High-Fidelity Threat Hunting for Linux Infrastructure

For Linux server, endpoints and cloud workloads, Maltrail provides precise threat detection abilities by:

  • Capturing traffic packets entering / leaving systems
  • Flagging blacklisted domains, IPs, URLs and hashes
  • Identifying communications with C2, botnet and malware servers
  • Generating custom detection rules and trails

Maltrail is invaluable for monitoring mission-critical Linux infrastructure interfaces. It can discover malicious traffic indicating breaches, data exfiltration, or insider misuse.

Intrusion detection via Maltrail complements Linux malware scanning to uncover threats that have bypassed other safeguards.

10. YARA Rules – Pattern Matching and Analysis to Unmask Linux Threats

As Linux-focused malware continues to surge, YARA helps rapidly build signatures that detect malware families targeting Linux platforms.

It lets analysts like me:

  • Construct descriptions of malware capabilities as YARA rules
  • Scan Linux server files and memory for matching binaries
  • Feed high fidelity indicators into other security tools

YARA rules aid real-time detection of intruder tools, backdoors and rootkits used in breaches across Linux logfiles, processes, filesystems etc.

We contribute YARA rules back to Mitre ATT&CK to boost community Linux threat intelligence.

11. Vuls – Automating Enterprise-Grade Linux Vulnerability Management

For organizations managing tons of Linux servers and cloud instances, Vuls is my recommended vulnerability coordintator that:

  • Auto-discovers all Linux assets without agents
  • Checks CVEs across packages, containers, kernel etc.
  • Prioritizes risks, alerts and generates executive reports
  • Integrates with CI/CD pipelines

Vuls provides purpose-built, enterprise-scale capabilities currently missing from open source Linux vulnerability scanners.

It enables understaffed security teams to implement continuous vulnerability monitoring for Linux server farms.

12. Firefox ESR + Tor Browser – Browsing Securely from Linux Desktops

While server security grabs attention, don‘t ignore hardening your Linux desktops which serve as jump hosts for admins to manage infrastructure.

I recommend sandboxing risky web browsing activities from Linux workstations using :

Firefox ESR: Gets quarterly security updates protecting Linux users from browser exploits during administrative tasks.

Tor Browser: Enforces traffic encryption and masks your true geo-location to evade network surveillance during sensitive research.

With potent combinations of server and endpoint Linux security hardening, protection tools, you can combat prevalent attack vectors targeting your Linux attack surface.

An Action Plan to Instrument Linux Security Monitoring

Here is a simple 4-step methodology I guide clients with for setting up Linux security tooling:

Survey Infrastructure – Catalog server types, cloud instances, containers, functions etc. running Linux.

Map Risks – Identify high value Linux servers and data requiring priority protection.

Deploy Sensors – Install lightweight scanning tools like Lynis and ClamAV without disruption.

Harden Safeguards – Utilize scans to continuously fine-tune Linux configurations, cloaking and defenses.

Getting started need not be an expensive capital drainage. Open source Linux security tools deliver immense value allowing you to incrementally widen and harden protection.

So don‘t let the next Linux-focused hacker exploit your security debt. Begin assessing your Linux attack surface with these 12 awesome open source scanning and hardening tools! Hit me up if you have questions.

Stay safe and secure out there!