Zephyr Project up to 4.3.1/4.4.x I3C i3c_handlers.c z_vrfy_i3c_transfer bus toctou

Summaryinfo

A vulnerability was found in Zephyr Project Zephyr up to 4.3.1/4.4.x. It has been classified as very critical. This issue affects the function z_vrfy_i3c_transfer of the file drivers/i3c/i3c_handlers.c of the component I3C. Performing a manipulation of the argument bus results in toctou. This vulnerability is known as CVE-2026-87038. Attacking locally is a requirement. No exploit is available. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability, which was classified as very critical, was found in Zephyr Project Zephyr up to 4.3.1/4.4.x. This affects the function z_vrfy_i3c_transfer of the file drivers/i3c/i3c_handlers.c of the component I3C. The manipulation of the argument bus with an unknown input leads to a toctou vulnerability. CWE is classifying the issue as CWE-367. The product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. This can cause the product to perform invalid actions when the resource is in an unexpected state. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

The user-mode verification handler for the i3c_transfer() system call, z_vrfy_i3c_transfer() in drivers/i3c/i3c_handlers.c, validated the caller-supplied struct i3c_device_desc in place instead of snapshotting it. It confirmed only that the descriptor was readable by the calling thread (K_SYSCALL_MEMORY_READ()), read target->bus once to check it with K_SYSCALL_OBJ(), and then passed the caller's live pointer down. The implementation z_impl_i3c_transfer() re-reads target->bus from that user memory and dispatches through it as DEVICE_API_GET(i3c, target->bus)->i3c_xfers(...), so the bus that was validated is not necessarily the bus that is used. DEVICE_API_GET() performs no runtime type check outside CONFIG_DEVICE_API_ASSERT builds. On a build with CONFIG_USERSPACE and an I3C controller driver, a user thread that has been granted an I3C device object can exploit this with a second thread in the same memory domain: one thread calls i3c_transfer() while the other rewrites the bus field of the descriptor. The window between the check and the use spans the num_msgs validation, the message-array access check, a stack VLA allocation and a memcpy() of up to 32 struct i3c_msg, so the race is repeatable and can simply be retried until it lands. When the substitution succeeds, the kernel dereferences an attacker-chosen pointer as a struct device, loads a function pointer from the attacker-controlled api table and calls it in supervisor mode with attacker-influenced arguments. This yields kernel-mode code execution from an unprivileged user thread — a complete escape of the userspace memory-protection sandbox; lesser outcomes are an arbitrary two-level kernel read or a kernel fault. Builds without CONFIG_USERSPACE, or without an I3C driver, are unaffected. The fix copies the descriptor into a kernel-stack local with k_usermode_from_copy(), validates the bus in that copy, and dispatches on the copy, so the validated value is provably the value used.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-87038 since 09/08/2026. The exploitability is told to be easy. Attacking locally is a requirement. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 11/04/2026).

Upgrading to version 4.3.2 or 4.5.0 eliminates this vulnerability.

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Productinfo

Type

• Project Management Software

Vendor

• Zephyr Project

Name

• Zephyr

Version

• 4.0
• 4.1
• 4.2
• 4.3
• 4.3.0
• 4.3.1
• 4.4

CPE 2.3info

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CPE 2.2info

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