Sysdig Falco is an open source behavioral activity monitoring tool for better container security. It can detect anomalous activities occurring on a Linux host as well as inside of a running container and supports Docker orchestration tools like Kubernetes and Mesos.

Intrusion prevention vs. detection

Let us first understand two different aspects of security in software systems i.e. intrusion prevention and intrusion detection. Intrusion prevention is to stop an intruder from gaining an unauthorized access to your environment. For example, role-based access control, two-factor authentication, firewall rules etc. help in preventing intrusion.

But, what if in spite of all the security in place an intruder has successfully breached the walls of container security? This is where intrusion detection techniques come to the rescue. Sysdig Falco is one of the tools which helps in intrusion detection. Falco detects suspicious, anomalous activities that may occur inside a running container on intrusion (for ex. spawning of a shell in a container deployed on production environment; opening of sensitive files like “/etc/shadow” in read/write mode; overwriting of binary files placed in “/bin” or /usr/bin directories; mounting sensitive files/directories inside the containers; unexpected tcp port has been opened/accessed and much more)

Container Security

Sysdig Falco is an auditing and monitoring tool, and it does not enforce any actions that will fix a problem. As such it can detect anomalous behaviors and notify it to the users in different ways such as by logging, notifications etc., but it won’t take any action for you. If we have to draw an analogy the CCTV security systems would be pretty close which helps detect intrusion but would not itself take any action. The action has to be taken by a human who is watching the stream. Or in today’s world, machine learning will aid you in by showing you important frames from the stream but the action has to be taken by someone.

Detecting Anomalous Behavior!

Falco lets you define highly granular rules in a standard format. These rules are checks for activities involving various things such as files, containers network etc. For each of the checks, Falco uses a kernel module which captures the stream of calls made and compares them with the rules defined by the user (The module itself is installed with Falco container image as explained here). What kind of rules you want to define might vary based on your needs, but Falco also comes with a rich set of rules for many commonly used applications.

How does Falco work?

Falco runs in user space, using a kernel module (named sysdig_probe) to intercept system calls and these calls are pushed into user space. The user context (PID, thread ID, session ID etc.) is added to the system calls to create events which are then compared against the rules defined (in /etc/falco_rules.local.yaml). In case of a violation of a rule, Falco notifies the user via the configured way (logging, email, slack). One of the benefits of a user space implementation is being able to integrate with external systems like Docker orchestration tools like Kubernetes and Mesos.

Install Falco

Falco needs kernel headers installed on the host operating system.

Debian: apt-get -y install linux-headers-$(uname -r)
RHEL  : yum -y install kernel-devel-$(uname -r)

Now, Falco image can be pulled and executed:

 docker pull sysdig/falco
 docker run -i -t --name falco --privileged -v /var/run/docker.sock:/host/var/run/docker.sock -v /dev:/host/dev -v /proc:/host/proc:ro -v /boot:/host/boot:ro -v /lib/modules:/host/lib/modules:ro -v /usr:/host/usr:ro sysdig/falco

Detecting anomalous activities:

While running you will notice that the Falco container itself downloads a set of rules and run it within the container. We will use this to try out the rules locally without needing a separate/dedicated setup.

Tue Nov 27 17:59:16 2018: Falco initialized with configuration file /etc/falco/falco.yaml
Tue Nov 27 17:59:16 2018: Loading rules from file /etc/falco/falco_rules.yaml
Tue Nov 27 17:59:17 2018: Loading rules from file /etc/falco/falco_rules.local.yaml

In this small demo we will run the Falco container and do some operations in the same container which we will notice in the logs of the container. In a way, we are checking the container security from the same container which is running Falco!

Assuming you have run the Falco locally, spawn a shell inside a container. You may also want to keep the logs of the container being tailed in another window so that we can watch events in real time.

` $docker exec -it falco sh `

` `

If you try to open a sensitive file in read/write mode: vim /etc/shadow

` `

Move a binary inside the bin directory: mv /bin/ls /bin/ls.hack

As you can notice for every operation which is not valid as defined by the rule, it is logging in the logs.

Further reading

What we have covered is barely the surface of what Falco can do. Falco has the concept of a macro which is similar to a function definition and can be called from any rules file. While Falco comes with a bunch of rules built in, you can also write a custom macro and define your logic in it. Without going too deep into the technical areas, here are some pointers to explore Falco further with some specific use case where Falco is used.

• If you are wondering do you really need yet another additional tool for container security or how does it compare with the likes of Selinux or Seccomp and such similar tools then check out this great post from Sysdig : https://sysdig.com/blog/selinux-seccomp-falco-technical-discussion
• Falco can be used as an auditing tool for Kubernetes by using the events of objects in Kubernetes. This can be very powerful for auditing specific events in Kubernetes objects. The documentation details this with an example: https://github.com/falcosecurity/falco/wiki/K8s-Audit-Event-Support
• A set of default Falco ruleset for various applications: https://github.com/draios/falco-extras
• Tne guide shows how to secure GKE cluster with Sysdig Falco: https://cloud.google.com/blog/products/containers-kubernetes/securing-kubernetes-with-gke-and-sysdig-falco

Hope this blog was informational to you. You will enjoy Freddy’s recent post on Introduction to Kubernetes security using Falco.

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