Container runtime scanning is the continuous inspection of containers while they are running, so you catch vulnerabilities, configuration drift, and active exploitation that a build-time scan cannot see. A scan at build time tells you what was true the moment you shipped an image. Container runtime scanning tells you what is true right now, which is a different and often more urgent question. This guide covers what runtime scanning actually inspects, how it works under the hood, and how it complements the scanning you already do in CI.
Why build-time scanning is not enough
Scanning an image before it ships is necessary and cheap, and you should do it. But it has three blind spots that only runtime visibility closes.
First, new CVEs. An image that was clean when you built it in January can be full of critical findings by March, because vulnerabilities are disclosed against packages that were previously considered safe. Nothing about the running container changed, but its risk did.
Second, drift. A container is supposed to be immutable, but people exec in and install tools, processes download payloads, and configurations get patched live during incidents. What is running can diverge from what you built, and only runtime observation sees that gap.
Third, behavior. A build scan sees packages; it cannot see a container spawning a shell, opening an unexpected outbound connection, or reading files it never touched in testing. Those behaviors are the signal that a known vulnerability has been turned into an active compromise.
What runtime scanning inspects
Container runtime scanning spans a few distinct layers, and good tools cover more than one.
- Live vulnerability posture. Continuously matching the packages in running containers against updated vulnerability feeds, so a newly disclosed CVE surfaces against workloads already in production without waiting for a rebuild.
- Configuration and drift. Detecting when a running container's filesystem, privileges, or mounted secrets differ from the image or the intended policy.
- Behavioral detection. Watching syscalls and process activity for patterns that indicate exploitation: unexpected shells, privilege escalation, crypto-mining, or connections to known-bad hosts.
The first layer connects directly to your software composition data. Knowing which running pods contain a vulnerable version of a library, and which are actually reachable, is the difference between a triage list of thousands and a work queue of ten. An SCA tool that tracks the same package inventory across build and runtime makes that correlation possible instead of guesswork.
How it works under the hood
Most modern runtime scanners get their behavioral visibility from eBPF, a Linux kernel technology that lets a program safely observe syscalls, network events, and process activity without modifying the kernel or instrumenting each container. eBPF-based sensors run once per node and see every container on that node, which is far lighter than a sidecar per pod.
Falco is the widely used open-source example. It loads rules that describe suspicious behavior and raises alerts when running containers match them. A simple rule reads much like a policy statement:
- rule: Shell spawned in container
desc: Detect a shell running inside a container
condition: >
spawned_process and container
and proc.name in (bash, sh, zsh)
output: >
Shell spawned (container=%container.name
proc=%proc.name user=%user.name)
priority: WARNING
The vulnerability side works differently. A node agent inventories the packages in each running container and matches them against feeds like the NVD and ecosystem advisories, re-evaluating as new advisories land. Because it runs continuously, a CVE disclosed today shows up against yesterday's deployment automatically.
Where it fits in Kubernetes
In a Kubernetes cluster, runtime scanning usually deploys as a DaemonSet so every node runs a sensor. That gives cluster-wide coverage without touching application manifests. The sensor feeds two outputs: alerts for behavioral detections, and an updated inventory for vulnerability matching.
Runtime scanning pairs naturally with admission control. An admission controller stops a bad image or misconfigured pod from ever being scheduled, which prevents known-bad workloads at the gate. Runtime scanning covers what gets through and what changes afterward. One is prevention, the other is detection, and you want both. Our guide on Kubernetes security risks walks through the cluster-level threats these controls are meant to address.
Turning findings into response
Detection without response is just expensive logging. Wire runtime findings into a workflow:
- Route behavioral alerts to your incident channel with enough context to triage, including the pod, namespace, and the process that fired the rule.
- Correlate live vulnerability findings with reachability and exposure so you patch the exploitable ones first, not the alphabetically first ones.
- For high-confidence exploitation signals, automate containment where you can, such as cordoning a node or killing a pod, with a human in the loop for anything ambiguous.
The tuning cost is real. Behavioral rules generate false positives until you baseline what normal looks like in your environment, and skipping that tuning is how teams end up ignoring the alerts entirely. Budget a few weeks of tuning before you treat the alerts as actionable.
The honest limitations
Runtime scanning sees behavior, not intent, so it will flag legitimate operations that happen to look suspicious, like a debugging session or a scheduled maintenance job. It also adds a node-level agent that consumes resources and must itself be kept patched and access-controlled, because a sensor with kernel visibility is a high-value target. Treat it as one layer in a defense-in-depth setup, most effective when it sits alongside solid build-time scanning and enforced admission policy rather than replacing them.
FAQ
How is container runtime scanning different from image scanning?
Image scanning inspects a container before it ships and reflects a single point in time. Runtime scanning inspects containers while they run, catching newly disclosed CVEs, configuration drift, and active exploitation that a build-time scan cannot see.
Does runtime scanning slow down my containers?
Modern eBPF-based sensors run once per node and observe the kernel directly, so overhead is low compared to per-pod sidecars. You should still measure it in your own environment, especially on high-throughput nodes.
What is eBPF's role in runtime scanning?
eBPF lets a sensor safely observe syscalls, network activity, and process events in the kernel without modifying it. It provides the behavioral visibility that vulnerability feeds alone cannot, such as detecting an unexpected shell in a container.
Do I still need build-time scanning if I scan at runtime?
Yes. Build-time scanning is cheaper to remediate and stops known-bad images before they ship. Runtime scanning covers what changes after deployment. They solve different problems and work best together.