CVE-2026-9728 (GCVE-0-2026-9728)
Vulnerability from cvelistv5 – Published: 2026-08-24 14:28 – Updated: 2026-08-24 14:28
VLAI
EPSS
VEX
Title
TOCTOU race in mbox_send syscall verifier allows userspace to leak kernel memory
Summary
The userspace syscall verifier z_vrfy_mbox_send() in drivers/mbox/mbox_handlers.c validated the nested msg->data/msg->size fields by reading them directly out of live userspace memory, and then forwarded the original, still-mutable userspace struct mbox_msg * pointer to z_impl_mbox_send() and the underlying driver. Between the access check and the driver's use of msg->data, the validated pointer could be replaced, leaving a time-of-check/time-of-use window.
On a system built with CONFIG_USERSPACE, any unprivileged userspace thread may invoke the mbox_send() system call. A second thread sharing the caller's address space can race to overwrite msg->data with a supervisor (kernel) address after the verifier's bounds check has passed but before the driver dereferences it. The driver then reads from the attacker-chosen address in supervisor context (for example memcpy(&data32, msg->data, msg->size) in the NXP mailbox driver, whose bytes are subsequently emitted to the peer mailbox endpoint).
The impact is a userspace-to-supervisor access-control bypass: disclosure of kernel memory contents (high confidentiality impact), or, for an invalid/unmapped target address, a faulting kernel read causing denial of service. The fix snapshots the entire struct mbox_msg into a kernel-stack copy with k_usermode_from_copy() and validates and forwards that immutable copy, closing the race.
Severity
6.4 (Medium)
CWE
- CWE-367 - race
Assigner
References
2 references
Impacted products
1 product
| Vendor | Product | Version | CPE status | |
|---|---|---|---|---|
| zephyrproject | zephyr |
Affected:
3.0.0 , < 4.4.2
(semver)
|
guessed |
{
"containers": {
"cna": {
"affected": [
{
"collectionURL": "https://github.com/zephyrproject-rtos/zephyr",
"defaultStatus": "unaffected",
"packageName": "zephyr",
"product": "zephyr",
"programFiles": [
"drivers/mbox/mbox_handlers.c"
],
"vendor": "zephyrproject",
"versions": [
{
"lessThan": "4.4.2",
"status": "affected",
"version": "3.0.0",
"versionType": "semver"
}
]
}
],
"descriptions": [
{
"lang": "en",
"value": "The userspace syscall verifier z_vrfy_mbox_send() in drivers/mbox/mbox_handlers.c validated the nested msg-\u003edata/msg-\u003esize fields by reading them directly out of live userspace memory, and then forwarded the original, still-mutable userspace struct mbox_msg * pointer to z_impl_mbox_send() and the underlying driver. Between the access check and the driver\u0027s use of msg-\u003edata, the validated pointer could be replaced, leaving a time-of-check/time-of-use window.\n\nOn a system built with CONFIG_USERSPACE, any unprivileged userspace thread may invoke the mbox_send() system call. A second thread sharing the caller\u0027s address space can race to overwrite msg-\u003edata with a supervisor (kernel) address after the verifier\u0027s bounds check has passed but before the driver dereferences it. The driver then reads from the attacker-chosen address in supervisor context (for example memcpy(\u0026data32, msg-\u003edata, msg-\u003esize) in the NXP mailbox driver, whose bytes are subsequently emitted to the peer mailbox endpoint).\n\nThe impact is a userspace-to-supervisor access-control bypass: disclosure of kernel memory contents (high confidentiality impact), or, for an invalid/unmapped target address, a faulting kernel read causing denial of service. The fix snapshots the entire struct mbox_msg into a kernel-stack copy with k_usermode_from_copy() and validates and forwards that immutable copy, closing the race."
}
],
"metrics": [
{
"cvssV3_1": {
"baseScore": 6.4,
"baseSeverity": "MEDIUM",
"vectorString": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:L",
"version": "3.1"
},
"format": "CVSS"
}
],
"problemTypes": [
{
"descriptions": [
{
"cweId": "CWE-367",
"description": "race",
"lang": "en",
"type": "CWE"
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-08-24T14:28:46.429Z",
"orgId": "e2e69745-5e70-4e92-8431-deb5529a81ad",
"shortName": "zephyr"
},
"references": [
{
"name": "Fix commit",
"tags": [
"patch"
],
"url": "https://github.com/zephyrproject-rtos/zephyr/commit/ab35eaccec5976f05c196f176d0c32885754496f"
},
{
"name": "GHSA-47q2-w832-7w67",
"url": "https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-47q2-w832-7w67"
}
],
"title": "TOCTOU race in mbox_send syscall verifier allows userspace to leak kernel memory",
"x_generator": {
"engine": "cvelib 1.8.0"
}
}
},
"cveMetadata": {
"assignerOrgId": "e2e69745-5e70-4e92-8431-deb5529a81ad",
"assignerShortName": "zephyr",
"cveId": "CVE-2026-9728",
"datePublished": "2026-08-24T14:28:46.429Z",
"dateReserved": "2026-05-27T17:11:30.502Z",
"dateUpdated": "2026-08-24T14:28:46.429Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2",
"vulnerability-lookup:meta": {
"nvd": "{\"cve\":{\"id\":\"CVE-2026-9728\",\"sourceIdentifier\":\"vulnerabilities@zephyrproject.org\",\"published\":\"2026-08-24T15:16:49.010\",\"lastModified\":\"2026-08-24T15:16:49.010\",\"vulnStatus\":\"Received\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"The userspace syscall verifier z_vrfy_mbox_send() in drivers/mbox/mbox_handlers.c validated the nested msg-\u003edata/msg-\u003esize fields by reading them directly out of live userspace memory, and then forwarded the original, still-mutable userspace struct mbox_msg * pointer to z_impl_mbox_send() and the underlying driver. Between the access check and the driver\u0027s use of msg-\u003edata, the validated pointer could be replaced, leaving a time-of-check/time-of-use window.\\n\\nOn a system built with CONFIG_USERSPACE, any unprivileged userspace thread may invoke the mbox_send() system call. A second thread sharing the caller\u0027s address space can race to overwrite msg-\u003edata with a supervisor (kernel) address after the verifier\u0027s bounds check has passed but before the driver dereferences it. The driver then reads from the attacker-chosen address in supervisor context (for example memcpy(\u0026data32, msg-\u003edata, msg-\u003esize) in the NXP mailbox driver, whose bytes are subsequently emitted to the peer mailbox endpoint).\\n\\nThe impact is a userspace-to-supervisor access-control bypass: disclosure of kernel memory contents (high confidentiality impact), or, for an invalid/unmapped target address, a faulting kernel read causing denial of service. The fix snapshots the entire struct mbox_msg into a kernel-stack copy with k_usermode_from_copy() and validates and forwards that immutable copy, closing the race.\"}],\"affected\":[{\"source\":\"vulnerabilities@zephyrproject.org\",\"affectedData\":[{\"vendor\":\"zephyrproject\",\"product\":\"zephyr\",\"defaultStatus\":\"unaffected\",\"collectionURL\":\"https://github.com/zephyrproject-rtos/zephyr\",\"packageName\":\"zephyr\",\"programFiles\":[\"drivers/mbox/mbox_handlers.c\"],\"versions\":[{\"version\":\"3.0.0\",\"lessThan\":\"4.4.2\",\"versionType\":\"semver\",\"status\":\"affected\"}]}]}],\"metrics\":{\"cvssMetricV31\":[{\"source\":\"vulnerabilities@zephyrproject.org\",\"type\":\"Secondary\",\"cvssData\":{\"version\":\"3.1\",\"vectorString\":\"CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:L\",\"baseScore\":6.4,\"baseSeverity\":\"MEDIUM\",\"attackVector\":\"LOCAL\",\"attackComplexity\":\"HIGH\",\"privilegesRequired\":\"LOW\",\"userInteraction\":\"NONE\",\"scope\":\"CHANGED\",\"confidentialityImpact\":\"HIGH\",\"integrityImpact\":\"NONE\",\"availabilityImpact\":\"LOW\"},\"exploitabilityScore\":1.1,\"impactScore\":4.7}]},\"weaknesses\":[{\"source\":\"vulnerabilities@zephyrproject.org\",\"type\":\"Secondary\",\"description\":[{\"lang\":\"en\",\"value\":\"CWE-367\"}]}],\"references\":[{\"url\":\"https://github.com/zephyrproject-rtos/zephyr/commit/ab35eaccec5976f05c196f176d0c32885754496f\",\"source\":\"vulnerabilities@zephyrproject.org\"},{\"url\":\"https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-47q2-w832-7w67\",\"source\":\"vulnerabilities@zephyrproject.org\"}]}}"
}
}
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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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