Imported from mukul975/anthropic-cybersecurity-skills (skills/detecting-container-drift-at-runtime/SKILL.md). Install upstream with npx skills add mukul975/anthropic-cybersecurity-skills --skill detecting-container-drift-at-runtime. Copyright stays with the author (Apache-2.0).

Detecting Container Drift at Runtime

Overview

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

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

1. Binary drift: Execution of binaries not present in the original image (downloaded malware, compiled tools)
2. File drift: Creation, modification, or deletion of files in the container filesystem
3. Configuration drift: Changes to environment variables, mounted secrets, or runtime parameters
4. Package drift: Installation of new packages via apt, yum, pip, or npm at runtime
5. 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: Drift Detected (Container Image Modified Binary)
  desc: Detect execution of a binary not present in the original container image
  condition: >
    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: WARNING
  tags: [container, drift]

- rule: Container Shell Spawned
  desc: Detect interactive shell in a container that should be immutable
  condition: >
    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: WARNING
  tags: [container, drift, shell]

Detecting Package Manager Usage

- rule: Package Manager Execution in Container
  desc: Detect use of package managers indicating drift
  condition: >
    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: ERROR
  tags: [container, drift, package-manager]

Detecting File System Modifications

- rule: Container File System Write
  desc: Detect writes to container upper layer filesystem
  condition: >
    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: NOTICE
  tags: [container, drift, filesystem]

Implementation with Kubernetes Enforcement

Read-Only Root Filesystem

Prevent drift by making container filesystems immutable:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: immutable-app
spec:
  template:
    spec:
      containers:
        - name: app
          image: app:v1.0@sha256:abc123...
          securityContext:
            readOnlyRootFilesystem: true
            allowPrivilegeEscalation: false
            runAsNonRoot: true
          volumeMounts:
            - name: tmp
              mountPath: /tmp
            - name: cache
              mountPath: /var/cache
      volumes:
        - name: tmp
          emptyDir:
            sizeLimit: 100Mi
        - name: cache
          emptyDir:
            sizeLimit: 50Mi

Pod Security Standards Enforcement

apiVersion: v1
kind: Namespace
metadata:
  name: production
  labels:
    pod-security.kubernetes.io/enforce: restricted
    pod-security.kubernetes.io/audit: restricted
    pod-security.kubernetes.io/warn: restricted

Image Digest Verification

Continuous Digest Monitoring

#!/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)"
' | while read 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:"* ]]; then
    echo "[WARN] Container using mutable tag: $IMAGE"
  fi
done

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

1. Detect: Alert fires on drift event (Falco, Defender, Sysdig)
2. Validate: Confirm the drift is not from an approved process (init containers, config reloads)
3. Isolate: Apply a deny-all NetworkPolicy to the affected pod
4. Investigate: Capture container filesystem diff and process list
5. Evict: Delete the drifted pod (ReplicaSet will recreate from clean image)
6. Remediate: Fix the root cause (patch vulnerability, update image, tighten RBAC)

References

• Container Drift Detection with Falco - Sysdig
• Microsoft Defender for Containers Drift Detection
• Ensure Immutability of Containers at Runtime
• Falco Runtime Security