Description
The virtio PCI driver (drivers/virtio/virtio_pci.c) parses a device's PCI capability list during driver initialization. In virtio_pci_read_cap() the device-supplied capability length byte cap_len (read from PCI config space via pcie_conf_read()) was only checked with assert(tmp.cap_len == cap_struct_size). That assert resolves to __ASSERT_NO_MSG(), gated by CONFIG_ASSERT, which defaults off in production builds, so the value reached the copy logic completely unvalidated.

The length then drives a loop that copies extra capability dwords into a fixed-size stack buffer supplied by the caller. A cap_len below the 24-byte base struct virtio_pci_cap underflows the unsigned extra_data_words count to a near-SIZE_MAX value, producing an effectively unbounded stack write; a cap_len above the caller's buffer (up to 255) writes up to roughly 228 bytes of device-controlled data past the buffer. Both are out-of-bounds writes of attacker-controlled content executed in kernel mode during boot-time device probe.

The input originates from the virtio device. In the common deployment where Zephyr runs as a guest under a hypervisor, the device backend is the host, which already fully outranks the guest, so the bug yields no privilege escalation. The exploitable case is a virtio device that is untrusted relative to the Zephyr kernel — an untrusted or physical/passthrough virtio PCIe device on a bare-metal system, or a confidential-computing posture where the guest must defend against the host — where a malicious device can corrupt the kernel stack and potentially achieve code execution or a crash.

The fix replaces the compiled-out assert with a runtime range check rejecting cap_len outside [sizeof(struct virtio_pci_cap), cap_struct_size] before any arithmetic or copy.
Published: 2026-08-25
Score: 6.1 Medium
EPSS: n/a
KEV: No
Impact: n/a
Action: n/a
AI Analysis

Impact

A length field supplied by a virtio PCI device is read into a buffer without proper validation, allowing an attacker to craft a small capability length that underflows the count of data words or a large length that overflows the caller’s buffer. This causes an unbounded write into the kernel stack while the device is probed at boot time. The overwritten stack content can be arbitrary device‑controlled data, providing a path to corrupt control flow or execute arbitrary code in kernel mode. The weakness is a classic out‑of‑bounds write (CWE‑787).

Affected Systems

All Zephyr releases that employ the virtio PCI driver without the runtime range check introduced in commit d98dacee24ad10c972d3b7281c9009d82ed351c9. The vulnerability is present wherever the driver is compiled with the default CONFIG_ASSERT disabled, which is the default for production builds.

Risk and Exploitability

The CVSS score of 6.1 indicates a moderate severity. No EPSS score is available, and the vulnerability is not listed in CISA's KEV catalog, suggesting no known widespread exploitation. The attack requires an attacker-controlled or untrusted virtio PCI device that the Zephyr kernel will probe during boot. In typical guest‑under‑hypervisor configurations the host already has full privilege, so the flaw does not provide escalation. The real risk arises on bare‑metal systems or confidential‑computing scenarios where the guest must protect itself against untrusted or pass‑through PCI devices; in those contexts the flaw can lead to kernel code execution or a system crash.

Generated by OpenCVE AI on August 25, 2026 at 17:30 UTC.

Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

  • Upgrade Zephyr to the latest release that incorporates the runtime range check for virtio PCI capability lengths, as provided in commit d98dacee24ad10c972d3b7281c9009d82ed351c9.
  • If an immediate upgrade is not possible, disable or remove any untrusted virtio PCI devices from the system, ensuring that only devices controlled by trusted firmware or host can be probed during kernel initialization.
  • Configure the kernel build to enable CONFIG_ASSERT or to enforce strict capability‑length validation at runtime, thereby preventing the unchecked copy logic from executing in the future.

Generated by OpenCVE AI on August 25, 2026 at 17:30 UTC.

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History

Tue, 25 Aug 2026 17:45:00 +0000

Type Values Removed Values Added
First Time appeared Zephyrproject
Zephyrproject zephyr
Vendors & Products Zephyrproject
Zephyrproject zephyr

Tue, 25 Aug 2026 16:30:00 +0000

Type Values Removed Values Added
Description The virtio PCI driver (drivers/virtio/virtio_pci.c) parses a device's PCI capability list during driver initialization. In virtio_pci_read_cap() the device-supplied capability length byte cap_len (read from PCI config space via pcie_conf_read()) was only checked with assert(tmp.cap_len == cap_struct_size). That assert resolves to __ASSERT_NO_MSG(), gated by CONFIG_ASSERT, which defaults off in production builds, so the value reached the copy logic completely unvalidated. The length then drives a loop that copies extra capability dwords into a fixed-size stack buffer supplied by the caller. A cap_len below the 24-byte base struct virtio_pci_cap underflows the unsigned extra_data_words count to a near-SIZE_MAX value, producing an effectively unbounded stack write; a cap_len above the caller's buffer (up to 255) writes up to roughly 228 bytes of device-controlled data past the buffer. Both are out-of-bounds writes of attacker-controlled content executed in kernel mode during boot-time device probe. The input originates from the virtio device. In the common deployment where Zephyr runs as a guest under a hypervisor, the device backend is the host, which already fully outranks the guest, so the bug yields no privilege escalation. The exploitable case is a virtio device that is untrusted relative to the Zephyr kernel — an untrusted or physical/passthrough virtio PCIe device on a bare-metal system, or a confidential-computing posture where the guest must defend against the host — where a malicious device can corrupt the kernel stack and potentially achieve code execution or a crash. The fix replaces the compiled-out assert with a runtime range check rejecting cap_len outside [sizeof(struct virtio_pci_cap), cap_struct_size] before any arithmetic or copy.
Title Out-of-bounds stack write in Zephyr virtio PCI driver from unvalidated device-supplied capability length
Weaknesses CWE-787
References
Metrics cvssV3_1

{'score': 6.1, 'vector': 'CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H'}

Subscriptions

Zephyrproject Zephyr
cve-icon MITRE

Status: PUBLISHED

Assigner: zephyr

Published:

Updated: 2026-08-25T16:05:36.964Z

Reserved: 2026-06-24T15:29:25.534Z

Link: CVE-2026-13216

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Received

Published: 2026-08-25T17:17:04.993

Modified: 2026-08-25T17:17:04.993

Link: CVE-2026-13216

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-08-25T17:30:07Z

Weaknesses