Introduction
In today’s threat landscape, cloud based VM hardening is a best practice that’s actively recommended by industry frameworks and security tools, including CIS Benchmarks, vulnerability scanners like Nessus, cloud-native tools/frameworks such as AWS Inspector, Azure’s Defender for Cloud and more!
Just to clarify, virtual machine hardening is essentially the process of securing virtualized environments by reducing their attack surface through configuration changes, removing unnecessary services, and implementing strict access controls.
However, speaking from experience, VM hardening presents challenges that security teams often find out the hard way.
The pursuit of maximum security can create operational nightmares.
While closing all gaps might seem like the safest approach, overly aggressive hardening can really break important applications, degrade system performance, and create maintenance headaches that can actually increase your overall risk!
I believe the question isn’t really about whether to harden your VM’s, it’s about how to do it effectively without shooting yourself in the foot.
The Case for VM Hardening
VM hardening really can deliver tangible security benefits that make it an essential practice for any organization serious about cybersecurity and its importance. It’s something that should always be promoted.
Attack Surface Reduction. It’s forms the very foundation of VM hardening. By disabling unnecessary services, uninstalling packages that have vulnerabilities associated to them, closing unneeded network ports (think 3389/22 etc.), you eliminate potential entry points that attackers could theoretically exploit. This can be classed as reducing your overall attack surface.
Threat Mitigation. In the context of VM hardening, threat mitigation refers to the rewards of hardening efforts. When virtual machines are properly configured, they’re much more resistant to common MITRE ATT&CK vectors like privilege escalation, lateral movement and data exfiltration. If an attacker was to gain initial access, a hardened system limits their ability to expand their foothold and conduct any sort of reconnaissance.
Compliance Requirements. For an organization to fully meet many compliance requirements, it means hardening is a non-negotiable. There are standards like PCI DSS (financial) and HIPAA (health) + more that have specific requirements around hardening that have to be met.
There are frameworks such as NIST and ISO 270001 which heavily emphasise secure system configuration. Therefore, failure to meet such requirements could technically leads to audit failures, the loss of certifications and more. Ultimately, the organization would be impacted in more than one way.
The above case explains why security security professionals advocate for VM hardening but it also sets the stage for understanding why more isn’t always better!
When Hardening Goes Too Far
In this section, I will be providing *some* scenarios where excessive hardening can/has caused headaches (to say the least) within security teams.
The Time Bomb Effect. Aggressive file system permissions, unmounting seemingly unused drives, or removing user accounts might seem harmless during implementation, but speaking from personal experience, these changes can come back to HAUNT you down the line, if you’re not careful!
Application Compatibility Issues. Disabling Windows services that may seem “unnecessary” can actually break applications that may have documented/undocumented dependencies. For example, if one was to disable Windows RPC service on a DC, there’s a serious possibility that some processes and procedures will not work. It’s best practice NOT to disable the RPC service.
Operational Difficulties. Extreme hardening makes routine maintenance tasks virtually impossible. For example, removing administrative tools, restricting certain access methods, implementing over the top authentication procedures. This sort of stuff can really turn simple package updates into day-long ordeals which isn’t ideal for anyone especially system admins.
The real risk isn’t just “something breaks” but it’s that things fail silently, undetected, or during a critical moment like an incident or even a patch cycle.
Finding Your Balance
Speaking from experience and learning the hard way, effective virtual machine hardening isn’t really about trying to implement every single possible security control. It should be about making informed, thoughtful decisions that strengthen security without necessary crippling critical operations.
A security team should adopt a strategic approach that considers both immediate security needs and also long term operational realities.
Risk-Based hardening strategies should be driving your decision making process. Not every VM needs the same level of hardening. For example, a web server that is publicly accessible would require a more aggressive hardening approach compared to a internal dev VM/environment. There should be a documentation around each systems exposure levels, critically and data sensitivity which will then help you/your team decide the appropriate hardening levels. I believe that this sort of targeted approach ensure that your investing security efforts where it matters the most.
Incremental Implementation and Testing prevents the shock personal reaction when you realise that you’ve broken many of your VM’s core functionalities, by disabling critical dependencies. One way you could go about this, is by deploying hardening measure in phases. Possibly using the ‘Crawl-Walk-Run’ approach. This is where, ‘Crawl’ represents applying low-risk hardening measure first (e.g disabling guest accounts). ‘Walk’ represents implementing moderate controls (e.g removing unused services) while validating compatibility and finally ‘Run’ which represents applying stricter policy changes (e.g Windows registry changes, Linux AppArmor changes etc).
Final thing I’d like to say here, is that regular validation of hardened configuration changes ensures you’re able to strike the balance between security and functionality as your environment evolves.
Maintaining Rollback Capabilities is basically a safety net when hardening goes wrong. Making sure that you document every single change is CRITICAL! In the event of a problem, you would then be able to go back to that document and identity which one of those changes could’ve caused the problem. You should also be looking to maintain configuration backups and establish clear rollback procedures. When something breaks in the early hours of the morning, you will really appreciate having clear path back to a working state.
Conclusion
To round this off, I’d like to say again that goal shouldn’t be to implement every possible security control, but to thoughtfully apply the right measures that strengthen your security posture without creating utter chaos.
I believe that the most secure system is essentially useless if it can’t perform its intended purposes and the most functional system is worthless if it can be easily compromised.
Therefore, success lies in not treating security and usability as opposing forces but understanding that they are both key aspects of a well designed system.
