CVE-2026-47243

Kata Containers is an open source project focusing on a standard implementation of lightweight Virtual Machines (VMs) that perform like containers. Prior to 3.31.0, the runtime-rs standalone virtio-fs path is vulnerable to a guest-root to host-root escape. In this configuration, Kata runs the host virtiofsd as root with --sandbox none --seccomp none, so an attacker with root-equivalent access inside the guest can bypass the guest virtio-fs client entirely by taking over the virtio-fs PCI device and building a virtqueue in userspace to submit raw FUSE requests directly to the host virtiofsd. A crafted FUSE_SYMLINK request whose new symlink name is an absolute host path is honored outside the configured shared directory, allowing guest root to create root-owned symlinks in sensitive host locations such as /etc/cron.d. By pointing such a symlink at a guest-controlled crontab payload reachable through a live runtime process's mount namespace, the attacker causes the host cron daemon to execute that payload as host root, crossing the Kata isolation boundary. This issue is fixed in version 3.31.0.

Kata Containers is an open source project focusing on a standard implementation of lightweight Virtual Machines (VMs) that perform like containers. Prior to 3.31.0, the runtime-rs standalone virtio-fs path is vulnerable to a guest-root to host-root escape. In this configuration, Kata runs the host virtiofsd as root with --sandbox none --seccomp none, so an attacker with root-equivalent access inside the guest can bypass the guest virtio-fs client entirely by taking over the virtio-fs PCI device and building a virtqueue in userspace to submit raw FUSE requests directly to the host virtiofsd. A crafted FUSE_SYMLINK request whose new symlink name is an absolute host path is honored outside the configured shared directory, allowing guest root to create root-owned symlinks in sensitive host locations such as /etc/cron.d. By pointing such a symlink at a guest-controlled crontab payload reachable through a live runtime process's mount namespace, the attacker causes the host cron daemon to execute that payload as host root, crossing the Kata isolation boundary. This issue is fixed in version 3.31.0.

Summary

In the runtime-rs standalone virtio-fs path, verified here with QEMU (and verified with Cloud Hypervisor too), Kata Containers runs host virtiofsd as root with:

--sandbox none --seccomp none

If an attacker has root-equivalent execution inside the Kata guest VM, they can send raw FUSE requests directly to the host virtiofsd. With the tested runtime-rs virtio-fs configuration, a raw FUSE_SYMLINK request whose new symlink name is an absolute host path is honored outside the virtio-fs shared directory.

This lets guest root create host-root owned symlinks in sensitive host paths. The PoC created here will create symlinks in the host /etc/cron.d directory, causing host cron to execute a guest-controlled payload as host root.

Impact: guest root can execute code as host root.

Affected configuration

The verified host used:

/opt/kata/share/defaults/kata-containers/runtime-rs/configuration-qemu-runtime-rs.toml

rootless = false
shared_fs = "virtio-fs"
virtio_fs_daemon = "/opt/kata/libexec/virtiofsd"
hypervisor_name = "qemu"
debug_console_enabled = false

Pinned upstream references, using Kata Containers main commit 2ffd1538a296cff93a357bfba0dfca747480a1f8:

• runtime-rs standalone virtio-fs adds --sandbox none --seccomp none to the virtiofsd command line.
• runtime-rs QEMU leaves rootless mode disabled by default: rootless = false.
• The QEMU runtime-rs config template generates an installed config that uses standalone virtio-fs and points runtime-rs at the host virtiofsd binary: shared_fs and virtio_fs_daemon.
• The runtime-rs Makefile resolves those placeholders to virtio-fs and $(LIBEXECDIR)/virtiofsd.
• runtime-rs selects the same standalone virtio-fs implementation whenever shared_fs = "virtio-fs": ShareVirtioFsStandalone.

Details

The guest kernel normally owns the virtio-fs client. A normal guest process will use filesystem syscalls, and the guest kernel will validate the paths, and only then does the kernel send FUSE messages to the host backend.

An attacker with root-equivalent access inside the guest can bypass that guest virtio-fs client. They can access the virtio-fs PCI device, mmap the virtio PCI BAR, recover guest physical addresses from /proc/self/pagemap, and build their own virtqueue from userspace. That queue can submit attacker-built FUSE messages directly to host virtiofsd.

The relevant primitive is FUSE_SYMLINK. An attacker can send a request whose body contains:

new symlink name: /etc/cron.d/kata-go-escape-cron-<pid>
symlink target: /proc/<pid>/root/run/kata-containers/shared/sandboxes/<sid>/ro/passthrough/<sid>/rootfs/tmp/kata-go-escape-payload

The new symlink name is an absolute host path. virtiofsd should reject that request or force it to resolve below the configured --shared-dir. In the tested runtime-rs path, host-root unsandboxed virtiofsd accepts the absolute name, creating a real host symlink under /etc/cron.d.

The attacker can make the symlink target resolve through…

Kata Containers is an open source project focusing on a standard implementation of lightweight Virtual Machines (VMs) that perform like containers. Prior to 3.31.0, the runtime-rs standalone virtio-fs path is vulnerable to a guest-root to host-root escape. In this configuration, Kata runs the host virtiofsd as root with --sandbox none --seccomp none, so an attacker with root-equivalent access inside the guest can bypass the guest virtio-fs client entirely by taking over the virtio-fs PCI device and building a virtqueue in userspace to submit raw FUSE requests directly to the host virtiofsd. A crafted FUSE_SYMLINK request whose new symlink name is an absolute host path is honored outside the configured shared directory, allowing guest root to create root-owned symlinks in sensitive host locations such as /etc/cron.d. By pointing such a symlink at a guest-controlled crontab payload reachable through a live runtime process's mount namespace, the attacker causes the host cron daemon to execute that payload as host root, crossing the Kata isolation boundary. This issue is fixed in version 3.31.0.

Kata Containers is an open source project focusing on a standard implementation of lightweight Virtual Machines (VMs) that perform like containers. Prior to 3.31.0, the runtime-rs standalone virtio-fs path is vulnerable to a guest-root to host-root escape. In this configuration, Kata runs the host virtiofsd as root with --sandbox none --seccomp none, so an attacker with root-equivalent access inside the guest can bypass the guest virtio-fs client entirely by taking over the virtio-fs PCI device and building a virtqueue in userspace to submit raw FUSE requests directly to the host virtiofsd. A crafted FUSE_SYMLINK request whose new symlink name is an absolute host path is honored outside the configured shared directory, allowing guest root to create root-owned symlinks in sensitive host locations such as /etc/cron.d. By pointing such a symlink at a guest-controlled crontab payload reachable through a live runtime process's mount namespace, the attacker causes the host cron daemon to execute that payload as host root, crossing the Kata isolation boundary. This issue is fixed in version 3.31.0.

Summary

In the runtime-rs standalone virtio-fs path, verified here with QEMU (and verified with Cloud Hypervisor too), Kata Containers runs host virtiofsd as root with:

--sandbox none --seccomp none

If an attacker has root-equivalent execution inside the Kata guest VM, they can send raw FUSE requests directly to the host virtiofsd. With the tested runtime-rs virtio-fs configuration, a raw FUSE_SYMLINK request whose new symlink name is an absolute host path is honored outside the virtio-fs shared directory.

This lets guest root create host-root owned symlinks in sensitive host paths. The PoC created here will create symlinks in the host /etc/cron.d directory, causing host cron to execute a guest-controlled payload as host root.

Impact: guest root can execute code as host root.

Affected configuration

The verified host used:

/opt/kata/share/defaults/kata-containers/runtime-rs/configuration-qemu-runtime-rs.toml

rootless = false
shared_fs = "virtio-fs"
virtio_fs_daemon = "/opt/kata/libexec/virtiofsd"
hypervisor_name = "qemu"
debug_console_enabled = false

Pinned upstream references, using Kata Containers main commit 2ffd1538a296cff93a357bfba0dfca747480a1f8:

• runtime-rs standalone virtio-fs adds --sandbox none --seccomp none to the virtiofsd command line.
• runtime-rs QEMU leaves rootless mode disabled by default: rootless = false.
• The QEMU runtime-rs config template generates an installed config that uses standalone virtio-fs and points runtime-rs at the host virtiofsd binary: shared_fs and virtio_fs_daemon.
• The runtime-rs Makefile resolves those placeholders to virtio-fs and $(LIBEXECDIR)/virtiofsd.
• runtime-rs selects the same standalone virtio-fs implementation whenever shared_fs = "virtio-fs": ShareVirtioFsStandalone.

Details

The guest kernel normally owns the virtio-fs client. A normal guest process will use filesystem syscalls, and the guest kernel will validate the paths, and only then does the kernel send FUSE messages to the host backend.

An attacker with root-equivalent access inside the guest can bypass that guest virtio-fs client. They can access the virtio-fs PCI device, mmap the virtio PCI BAR, recover guest physical addresses from /proc/self/pagemap, and build their own virtqueue from userspace. That queue can submit attacker-built FUSE messages directly to host virtiofsd.

The relevant primitive is FUSE_SYMLINK. An attacker can send a request whose body contains:

new symlink name: /etc/cron.d/kata-go-escape-cron-<pid>
symlink target: /proc/<pid>/root/run/kata-containers/shared/sandboxes/<sid>/ro/passthrough/<sid>/rootfs/tmp/kata-go-escape-payload

The new symlink name is an absolute host path. virtiofsd should reject that request or force it to resolve below the configured --shared-dir. In the tested runtime-rs path, host-root unsandboxed virtiofsd accepts the absolute name, creating a real host symlink under /etc/cron.d.

The attacker can make the symlink target resolve through…