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Kata guest escape: runtime-rs guest-root to host-root escape via virtiofs

High severity GitHub Reviewed Published May 21, 2026 in kata-containers/kata-containers • Updated May 27, 2026

Package

gomod github.com/kata-containers/kata-containers (Go)

Affected versions

< 0.0.0-20260519062212-ffa59ce3aa78

Patched versions

0.0.0-20260519062212-ffa59ce3aa78

Description

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:

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 /proc/<pid>/root/... for a live Kata runtime process whose mount namespace can see the guest-created payload. One matching runtime PID is enough.

When the host cron reads /etc/cron.d, it follows the root-owned symlink, loads the guest-created crontab payload, and executes it as host root.

PoC

sudo timeout --foreground --kill-after=10s 600s ctr run --rm \
  --runtime /opt/kata/runtime-rs/bin/containerd-shim-kata-v2 \
  --runtime-config-path /opt/kata/share/defaults/kata-containers/runtime-rs/configuration-qemu-runtime-rs.toml \
  --privileged \
  --privileged-without-host-devices \
  docker.io/library/kata-go-escape:local \
  "$run_id"

The container is privileged only to model the post-escape condition where the attacker already has guest-root capabilities. It is not the vulnerability by itself.

Inside the guest, the PoC:

  1. Writes a cron payload to guest /tmp/kata-go-escape-payload.
  2. Finds the virtio-fs PCI device in guest /sys.
  3. Takes over a virtio-fs queue from userspace.
  4. Sends FUSE_INIT.
  5. Discovers the current runtime-rs sandbox under passthrough/.
  6. Looks up passthrough/<sid>/rootfs/tmp/kata-go-escape-payload.
  7. Sends raw FUSE_SYMLINK requests where the new symlink names are absolute host paths under /etc/cron.d.
  8. Keeps the guest alive while host cron scans.

Example log lines:

[guest] virtio-fs PCI device: /sys/devices/pci0000:00/0000:00:05.0
[res] sandbox_id=kata-go-escape-test-1778522686-1539
[res] lookup_path_error=0 path=passthrough/kata-go-escape-test-1778522686-1539/rootfs/tmp/kata-go-escape-payload nodeid=21
[spray] pid=1 err=-2 created_candidates=1

err=-2 is expected for the symlink spray. virtiofsd can return ENOENT after the side effect because its follow-up lookup is still relative to the export root. The host symlink creation has already happened.

Impact

The PoC proves guest-root to host-root command execution.

Verified host proof:

/run/kata-go-escape.proof

uid=0(root) gid=0(root) groups=0(root)
Mon May 11 18:05:01 UTC 2026

The proof file is written in host /run by host cron. It is not written by the guest process and not written by virtiofsd.

An attacker who reaches guest root can therefore cross the Kata isolation boundary and execute commands as host root on affected runtime-rs virtio-fs deployments.

References

@sprt sprt published to kata-containers/kata-containers May 21, 2026
Published to the GitHub Advisory Database May 27, 2026
Reviewed May 27, 2026
Last updated May 27, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required High
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity High
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:H/VI:H/VA:N/SC:H/SI:H/SA:N/E:P

EPSS score

Weaknesses

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. Learn more on MITRE.

Absolute Path Traversal

The product uses external input to construct a pathname that should be within a restricted directory, but it does not properly neutralize absolute path sequences such as /abs/path that can resolve to a location that is outside of that directory. Learn more on MITRE.

CVE ID

CVE-2026-47243

GHSA ID

GHSA-2gv2-cffp-j227

Source code

Credits

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