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CVE-2026-87038

Time-of-check/time-of-use double fetch in the Zephyr i3c_transfer() syscall handler allows kernel-mode control-flow hijack from user mode

No exploitation reported by the sources we read.

  • Published2026-11-04
  • WeaknessCWE-367

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

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.

Timeline

dates in UTC
  1. 2026-11-0406:09CVE publishedCNA

Observed activity

isMalicious corpus · updated 2026-11-04 03:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2026-87038?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-87038?format=csv
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Versions per branch

Products declared by the CNA

As zephyr, the CNA that assigned this CVE, declared them in the CVE record; versions as written. NVD lists no product for this CVE yet: this is the only product data so far.

Source: the CNA record (CVE Program), © The MITRE Corporation,CVE Terms of UseCVE recordupdated 2026-11-04

Neither OSV.dev nor the vendor advisories we read list affected versions for CVE-2026-87038 yet.

Full record on NVD

Fix

No fix published by the sources we read yet. NVD record

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub
Metasploit modules
none listed
Metasploit Framework

Sources

  • NVDCVE record2026-11-04
  • CNACVSS 3.172026-11-04
  • CNAweakness (CWE)CWE-3672026-11-04

Technical details

CVSS v3 Vector
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
Published
2026-11-04

Frequently asked questions

What is CVE-2026-87038?

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.

Is CVE-2026-87038 actively exploited?

Active exploitation of CVE-2026-87038 has not been confirmed.

What is the CVSS score for CVE-2026-87038?

CVE-2026-87038 has a CVSS v3 base score of 7 (HIGH severity), with vector string CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H.

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