Detecting Container Drift At Runtime

What it does

When your agent uses it

• Validating immutable-infrastructure controls
• Hunting for unexpected package installs
• Binaries written inside a running container
• Determining whether a container diverged from the image it was built from

Example prompts

• “Use the detecting-container-drift-at-runtime skill to detect unauthorized runtime drift in containers by monitoring binary execution, filesystem…”
• “/detecting-container-drift-at-runtime”

Requirements

• Python 3

Workflow steps

5 steps, taken from the first numbered list in SKILL.md.

1. 1Binary drift: Execution of binaries not present in the original image (downloaded malware, compiled tools)
2. 2File drift: Creation, modification, or deletion of files in the container filesystem
3. 3Configuration drift: Changes to environment variables, mounted secrets, or runtime parameters
4. 4Package drift: Installation of new packages via apt, yum, pip, or npm at runtime
5. 5Network drift: New listening ports or outbound connections not expected for the workload

What it can do on your machine

Read from SKILL.md and the folder at commit 54a7988. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  Ships 2 files in scripts/ (Python), which the agent can run.

  Shell commands in SKILL.md call:

  • kubectl
  • jq

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  Links to these hosts (documentation or services it may open):

  • sysdig.com
  • techcommunity.microsoft.com
  • notes.kodekloud.com
  • falco.org

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Detecting Container Drift At Runtime loads about 2.1k tokens when it runs, and up to ~3.4k if it reads all its reference files. Until then it costs about 171 tokens; SKILL.md has 425 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~171

When it runs · the whole SKILL.md, loaded when a task matches

~2.1k

With references · SKILL.md plus every file in references/, read only if the agent opens them

~3.4k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); the scripts in this folder are not scanned.

SKILL.md

The full file from mukul975/Anthropic-Cybersecurity-Skills at commit 54a7988, republished under its Apache-2.0 licence (© mukul975). 425 words, ~2,137 tokens.

name

detecting-container-drift-at-runtime

description

Detects unauthorized runtime drift in containers by monitoring binary execution, filesystem changes, and configuration deviation from the original immutable image, using Falco and Microsoft Defender for Containers. Use when validating immutable-infrastructure controls, hunting for unexpected package installs or binaries written inside a running container, or determining whether a container diverged from the image it was built from. Keywords: drift, immutable infrastructure, new binary executed, package install, image mismatch, Falco. Do not use for detecting breakout from the container to the host - use detecting-container-escape-attempts.

domain

cybersecurity

subdomain

container-security

tags

container-drift, runtime-security, immutable-containers, falco, kubernetes, container-security, drift-detection, microsoft-defender

version

1.0

author

mahipal

license

Apache-2.0

nist_csf

PR.PS-01, PR.IR-01, ID.AM-08, DE.CM-01

mitre_attack

T1610, T1611, T1609, T1525

- 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]

- 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]

- 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]

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

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

#!/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

{
  "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"
  ]
}

© mukul975, Apache-2.0. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 7 other files (scripts, references, assets) in skills/detecting-container-drift-at-runtime of mukul975/Anthropic-Cybersecurity-Skills.

• SKILL.md
• LICENSE
• assets/template.md
• references/api-reference.md
• references/standards.md
• references/workflows.md
• scripts/agent.py
• scripts/process.py

Open the folder on GitHubat commit 54a7988