# CVE-2026-10681: SMP race in `thread_idx_alloc()` lets concurrent `k_object_alloc(K_OBJ_THREAD)` callers share a kernel-object permission slot

Status: published. Published: 2026-07-25. Updated: 2026-11-04.

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).

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: zephyr) — https://github.com/zephyrproject-rtos/zephyr/commit/862ea2fbbeb2ccdf8ff994b03e2e3b4405f2c37d
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: vulnerabilities@zephyrproject.org) — https://github.com/zephyrproject-rtos/zephyr/commit/862ea2fbbeb2ccdf8ff994b03e2e3b4405f2c37d
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: vulnerabilities@zephyrproject.org) — https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-j693-5rh5-8g8h

## Affected products and versions

- zephyrproject / zephyr (zephyr): ≥ 2.0.0; < 4.3.2 (affected, semver); ≥ 4.4.0; < 4.4.2 (affected, semver). Source: CNA: zephyr

## NVD configuration conditions

Affected CPE matches depend on the following AND/OR and negation conditions:

```json
[
  {
    "negate": false,
    "nodes": [
      {
        "cpeMatch": [
          {
            "criteria": "cpe:2.3:o:zephyrproject:zephyr:*:*:*:*:*:*:*:*",
            "versionEndExcluding": "4.5.0",
            "versionStartIncluding": "2.0.0",
            "vulnerable": true
          }
        ],
        "negate": false,
        "operator": "OR"
      }
    ],
    "operator": "OR"
  }
]
```

## Severity and prioritization

- CVSS 3.1: 6.5 (medium); CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L; source: CNA: zephyr

- CVSS 3.1: 7 (high); CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H; source: NVD: nvd@nist.gov

- CVSS 3.1: 6.5 (medium); CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L; source: NVD: vulnerabilities@zephyrproject.org

- CISA KEV: No listing in collected sources

- EPSS: 0.11% as of 2026-11-03; percentile 1.57%

- SSVC: exploitation none; automatable no; technical impact total; source ADP: CISA-ADP

- Weaknesses: CWE-362

## Exploit evidence

- exploit: https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-j693-5rh5-8g8h (vulnerabilities@zephyrproject.org)

## References

- https://github.com/zephyrproject-rtos/zephyr/commit/862ea2fbbeb2ccdf8ff994b03e2e3b4405f2c37d (CNA: zephyr; patch)

- https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-j693-5rh5-8g8h (CNA: zephyr)

- https://github.com/zephyrproject-rtos/zephyr/commit/862ea2fbbeb2ccdf8ff994b03e2e3b4405f2c37d (vulnerabilities@zephyrproject.org; Patch)

- https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-j693-5rh5-8g8h (vulnerabilities@zephyrproject.org; Exploit, Patch, Vendor Advisory)

## Provenance

- CNA: zephyr: https://www.cve.org/CVERecord?id=CVE-2026-10681; retrieved 2026-11-04; source updated 2026-11-04

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-10681; retrieved 2026-11-04; source updated 2026-11-04

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-10681; retrieved 2026-11-04; source updated Not reported

Revision: 998ca6017e595463f0c9954b846e03273e42f0582b83718dd24b9190aa1d56c8

Canonical: https://y2.dev/cve/CVE-2026-10681

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