Infosec Journey

Modern cloud-native environments increasingly rely on containerized workloads—from microservices to business applications. Containers, designed for speed and portability, are ephemeral but not inherently immutable at runtime. This means files inside containers—including application code, configs, and even critical system files—can be modified during their lifecycle. For security teams, FIM is no longer optional. It is a core detection and compliance control. Standards like PCI DSS 4.0 and NIST SP 800-53 mandate it. It is essential for a ‘true’ cloud defense-in-depth environment.

Containers might be designed to be temporary, but they’re not locked down while running. Attackers can find ways to get inside. They exploit a software bug, misconfigure a setting, or have too much access. Once they gain access, they can change important files like your app code (JARs). They might also alter config files or even system files such as /etc/passwd. If the container allows file changes, and your app can reload files “live”, those changes can take effect instantly. This is sometimes called dynamic reload. This happens without restarting anything. This makes it much harder to catch or stop the attack before damage is done.

FIM is now a must-have, not optional. Security rules like PCI DSS 4.0 specifically state that you must constantly monitor your containers for any unauthorized changes to important files. If you don’t, you risk audit failures. Real-world breaches can also occur.

Other standards like NIST 800-53 also require this kind of monitoring, and experts agree that tools like endpoint protection and network firewalls are not enough on their own—FIM fills a critical gap by showing you when files are tampered with, even if all other controls are working

Example: Linux Critical File That Must Be Monitored

/etc/passwd inside each Linux container is a critical security file that must be monitored by FIM.

• How it works: Each Linux container has its own /etc/passwd. It is independent of the host’s. Explicitly mounting from the host is risky and rare in production.
• Security impact: If an attacker gains shell inside the container, they could:
  • Add a new user (e.g., echo "malicioususer:x:0:0::/:/bin/sh" >> /etc/passwd), creating a backdoor.
  • Escalate privileges within the container, potentially enabling further attacks—including escape to the host if additional vulnerabilities exist.
• Detection: FIM must alert on any modification to /etc/passwd inside any container. This provides an early warning of privilege escalation. It also warns of persistence attempts.
• PCI DSS & NIST relevance: This is a “critical system file”—explicitly in scope for compliance monitoring.

Java/Spring Boot Application Example: Runtime File Mutability

Let’s take a Spring Boot application with mutable filesystem, externalized config, and (for illustration) live reload enabled.

• application.properties: Contains DB URLs, feature flags, secrets (if misconfigured), etc.
  If the app supports Actuator /refresh, changes to this file can be applied instantly without restart.
  Attack: Modify spring.datasource.url to a malicious DB, exfiltrating data in real time.
• JAR file: The compiled application. If an attacker replaces the JAR, the next restart (or, in rare hot-reload scenarios, immediately) executes attacker code.
  Attack: Overwrite /app/app.jar with a malicious version; if the app restarts (or hot-reloads), the attacker’s logic runs.
• Templates: If live-reloaded (not typical in production), changes can inject malicious content (XSS, etc.).

All of these—if writable at runtime—are prime targets for FIM.

Defense-in-Depth: Why FIM Works Best with Other Tools

FIM is most powerful when paired with:

• Endpoint Detection and Response (EDR): Detects malicious processes, anomalous behavior. Refer this blog for additional insights.
• Secrets Management: Prevents secrets from being stored in config files.
• Immutable Infrastructure: Enforce read-only root filesystems where possible.
• RBAC and Network Policies: Limit who can exec into containers or modify runtime resources.

Together, these controls create a resilient security posture—but FIM is the only control that catches unauthorized file changes, even if other layers are bypassed

Strict access controls aren’t enough—mistakes, hidden threats, and insider risks mean FIM still has a role. Containers are only truly immutable at build, not always while running; attackers can change files unless FIM is watching. 

PCI DSS 4.0 and NIST: The Letter of the Law

PCI DSS 4.0 Requirement 11.5.2:

“Deploy a change-detection mechanism (for example, file-integrity monitoring tools) to alert personnel to unauthorized modification of critical system files, configuration files, or content files; and configure the software to perform critical file comparisons at least weekly.”
This now explicitly (refer scope section of PCI) includes containers—not just traditional host

NIST 800-53

“The organization employs automated mechanisms to determine whether software, firmware, or information has been altered without authorization.”
This is directly applicable to containerized workloads

FIM is the unsung hero of container security—catching what other defenses miss by spotting unauthorized file changes in running apps. As containers become the go-to for critical workloads, FIM should be in every team’s toolkit, not just an afterthought.Top-tier security teams lock down their environment and watch for tampering in real time. Plus, standards like PCI DSS 4.0 and NIST have made it crystal clear: 

FIM for containers isn’t just smart—it’s non-negotiable.