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Security in containerized environments is no longer optional. With the growing complexity of cloud-native systems, the need for dynamic, context-aware runtime protection has become vital. Enter Falco, the open-source runtime security tool that is purpose-built for modern container ecosystems like Kubernetes, Docker, and Cloud Native platforms.
In this comprehensive guide, we’ll go beyond the basics. We’ll break down Falco rules, explain how to write your own custom detection logic, and demonstrate how developers can integrate and operationalize these rules to gain deep visibility into system behavior. Whether you're just adopting Falco for container security, or trying to fine-tune existing rule sets for production, this blog gives you a complete developer-centric breakdown of Falco’s rule engine, benefits, and real-world application.
Falco is the de facto standard for container runtime security. Backed by the Cloud Native Computing Foundation (CNCF), it gives developers real-time alerts when something suspicious happens inside your containers, nodes, or orchestrated workloads. Unlike traditional antivirus or host intrusion detection systems (HIDS) that rely on signatures or static policies, Falco detects behaviors, making it a perfect fit for dynamic container environments.
Some of the primary features that make Falco stand out include:
The keyword here is "runtime detection for containers", and Falco excels at this with its powerful rules engine.
At its core, Falco functions as a rule-based engine for analyzing system calls and kernel-level activity. These rules are written in YAML format, making them accessible, readable, and easy to modify. Each rule specifies a condition under which an event is considered suspicious and triggers an alert.
A Falco rule consists of:
While YAML might seem trivial, Falco uses this declarative format to enable complex behavior modeling. By leveraging macros and condition chains, you can define rules that look for:
All of this happens at runtime, giving developers a powerful container threat detection capability without heavy system agents or kernel modules.
Out-of-the-box, Falco comes with a comprehensive default rule set. These default Falco rules cover a wide range of suspicious activity: shell execution inside containers, privilege escalation, modifying sensitive files like /etc/shadow, and more. But they are intentionally generic and may produce either too many alerts or miss specific edge cases in your applications.
That’s where custom Falco rules come in.
Custom rules allow you to:
In short, writing your own Falco rules puts the power of behavioral security into the hands of developers and platform engineers. It enables security as code, reducing reliance on external monitoring platforms and helping shift security left in the DevSecOps pipeline.
A macro is like a function in programming, use it to store a condition you plan to reuse. Macros help keep your rules clean, readable, and maintainable. For example:
- macro: container_exec
condition: evt.type = execve and container.id != host
Here, container_exec acts as a shortcut for checking whether a process was executed within a container. You can use this in multiple rules.
A list lets you define a group of items like suspicious commands, IP addresses, or paths. It makes your rules modular.
- list: suspicious_binaries
items: [nc, ncat, curl, wget]
Using lists simplifies conditions and makes updating easier.
Here’s a full example of a custom rule that detects the use of a shell within a container:
- rule: Shell launched in container
desc: Detect when an interactive shell is launched in a container
condition: container_exec and proc.name in (sh, bash, zsh)
output: Shell command %proc.cmdline executed by user %user.name inside container=%container.id
priority: WARNING
tags: [container, shell, security, runtime-detection]
This rule checks if a shell is run in a container (which often indicates debugging, or worse, malicious activity).
Avoid false alarms by setting exceptions. For instance, if your CI/CD system regularly executes bash, you can exempt it:
exceptions:
- name: known_safe_ci
fields: [container.image.repository]
comps: [in]
values: [["ci-runner-image", "trusted-image"]]
What abnormal behavior are you trying to catch? Some examples:
Use macros for syscall filtering, lists for matching binaries or file paths, and conditions for event logic. Modularize as much as possible.
Context is king. Exceptions reduce noise and increase signal fidelity. Use user.name, container.image, or proc.exepath to filter known-good behavior.
Customize your output to be descriptive. Include container IDs, user names, process details, etc. Set appropriate priority to distinguish between INFO and CRITICAL events.
Want to ensure your containers don’t change at runtime? Use this:
- rule: Detect new executable file in container
desc: Alert when a new executable is written inside a running container
condition: evt.type in (open, creat) and evt.is_open_exec=true and container
output: Executable file %evt.arg.filename written in container=%container.id
priority: ERROR
tags: [drift, container, runtime-security]
Drift detection is key to ensuring immutable infrastructure and catching zero-day persistence mechanisms.
Outbound calls to non-whitelisted domains or IPs? This could be data exfiltration:
- rule: Suspicious outbound network
desc: Detect outbound connections to unknown destinations from containers
condition: evt.type = connect and fd.sip != 127.0.0.1 and container.id != host
output: Connection attempt to %fd.sip:%fd.sport from process=%proc.name
priority: WARNING
tags: [network, container, threat-detection]
Use this to monitor containers that may be communicating with C2 servers.
Falco uses efficient syscall tracing or eBPF hooks, making it ideal for clusters with limited resources. Unlike traditional endpoint security agents, it won’t bloat your containers or VMs.
Falco is built for cloud-native. It understands container IDs, namespaces, pod metadata, and even supports Kubernetes audit log ingestion via plugins. You can filter events using pod names, image labels, or namespace boundaries, perfect for multi-tenant Kubernetes clusters.
Each alert includes rich metadata: user info, container image, command executed, system call used, file path, and more. This allows developers to triage incidents faster, write better postmortems, and automate mitigation.
Rules are YAML. You can commit them to Git, use CI/CD to validate rule changes, and manage security like application code.
You can export Falco alerts as:
Connect Falco with SIEMs, Slack bots, dashboards, or even custom response handlers.
Falco is not a replacement for all security tools, but it fills a crucial gap that traditional tools miss.
Traditional tools often:
Falco, in contrast:
Split complex logic into macros and lists. It helps with rule reusability and version control.
Use exceptions and environment-specific filters. Add tags and clear descriptions for readability and searchability.
Run in dry-run mode or log-only mode for new rules. Observe behavior before triggering alerts in production.
CI/CD pipelines can run test events against your rules. Falco supports unit testing for rule validation.
Custom Falco rules transform your runtime security posture. They let developers:
Falco’s power lies in its flexibility. With a solid understanding of how to write and tune custom rules, your development team becomes the first line of defense in runtime security.
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