High

CVE-2026-31706 — ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl()

In the Linux kernel, the following vulnerability has been resolved: ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl() smb_inherit_dacl() trusts the on-disk num_aces value from the parent directory's DACL xattr and uses it to size a heap allocation: aces_base = kmalloc(sizeof(struct smb_ace) * num_aces * 2, ...); num_aces is a u16 read from le16_to_cpu(parent_pdacl->num_aces) without checking that it is consistent with the declared pdacl_size. An authenticated client whose parent directory's security.NTACL is tampered (e.g. via offline xattr corruption or a concurrent path that bypasses parse_dacl()) can present num_aces = 65535 with minimal actual ACE data. This causes a ~8 MB allocation (not kzalloc, so uninitialized) that the subsequent loop only partially populates, and may also overflow the three-way size_t multiply on 32-bit kernels. Additionally, the ACE walk loop uses the weaker offsetof(struct smb_ace, access_req) minimum size check rather than the minimum valid on-wire ACE size, and does not reject ACEs whose declared size is below the minimum. Reproduced on UML + KASAN + LOCKDEP against the real ksmbd code path. A legitimate mount.cifs client creates a parent directory over SMB (ksmbd writes a valid security.NTACL xattr), then the NTACL blob on the backing filesystem is rewritten to set num_aces = 0xFFFF while keeping the posix_acl_hash bytes intact so ksmbd_vfs_get_sd_xattr()'s hash check still passes. A subsequent SMB2 CREATE of a child under that parent drives smb2_open() into smb_inherit_dacl() (share has "vfs objects = acl_xattr" set), which fails the page allocator: WARNING: mm/page_alloc.c:5226 at __alloc_frozen_pages_noprof+0x46c/0x9c0 Workqueue: ksmbd-io handle_ksmbd_work __alloc_frozen_pages_noprof+0x46c/0x9c0 ___kmalloc_large_node+0x68/0x130 __kmalloc_large_node_noprof+0x24/0x70 __kmalloc_noprof+0x4c9/0x690 smb_inherit_dacl+0x394/0x2430 smb2_open+0x595d/0xabe0 handle_ksmbd_work+0x3d3/0x1140 With the patch applied the added guard rejects the tampered value with -EINVAL before any large allocation runs, smb2_open() falls back to smb2_create_sd_buffer(), and the child is created with a default SD. No warning, no splat. Fix by: 1. Validating num_aces against pdacl_size using the same formula applied in parse_dacl(). 2. Replacing the raw kmalloc(sizeof * num_aces * 2) with kmalloc_array(num_aces * 2, sizeof(...)) for overflow-safe allocation. 3. Tightening the per-ACE loop guard to require the minimum valid ACE size (offsetof(smb_ace, sid) + CIFS_SID_BASE_SIZE) and rejecting under-sized ACEs, matching the hardening in smb_check_perm_dacl() and parse_dacl(). v1 -> v2: - Replace the synthetic test-module splat in the changelog with a real-path UML + KASAN reproduction driven through mount.cifs and SMB2 CREATE; Namjae flagged the kcifs3_test_inherit_dacl_old name in v1 since it does not exist in ksmbd. - Drop the commit-hash citation from the code comment per Namjae's review; keep the parse_dacl() pointer.

Published May 1, 2026 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: Linux

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 0.6% (30-day exploitation probability)+7
Technical severity (high)+6

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

0.63%

ahead of 49% of scored CVEs · Oct 14, 2026

Affects

Linux

Linux and 1 more

CVSS base

8.8

High · 2 source(s)

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

Documented patch — git.kernel.org (NIST National Vulnerability Database)
Documented patch — git.kernel.org (NIST National Vulnerability Database)
Documented patch — git.kernel.org (NIST National Vulnerability Database)
Documented patch — git.kernel.org (NIST National Vulnerability Database)

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Linux Linux, Linux Linux Kernel).
Apply the fixed version or the patch documented under Remediation.
Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Linux	Linux	≥ e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and < 063a7409b0de46d7c770b65bb0338e6fdb3b1f0a≥ e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and < 3e5360b422dd741cb315654a191fa73869a37414≥ e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and < 59c32abaaec9cdd6164811c7e864e72f7554b82d≥ e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and < 3e4e2ea2a781018ed5d75f969e3e5606beb66e48version 5.15	Vulnerable	CVE Program (cve.org) e CISA ADP
Linux	Linux Kernel	≥ 5.15 and < 6.12.84≥ 6.13 and < 6.18.25≥ 6.19 and < 7.0.2	Vulnerable	NIST National Vulnerability Database

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

Low

User interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
8.8	CVSS 3.1	High	2.8	5.9	416baaa9-dc9f-4396-8d5f-8c081fb06d67 via NIST National Vulnerability Database
8.8	CVSS 3.1	High	—	—	Linux via CVE Program (cve.org) e CISA ADP

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H (score shown at the top: 416baaa9-dc9f-4396-8d5f-8c081fb06d67)

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.63%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).

web/proxy/WAF logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first.

2026
Affected products added · linux/linux
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 13, 2026
Fix documented
Oct 13, 2026 · NIST National Vulnerability Database
CVSS changed · 8.8
Oct 13, 2026 · NIST National Vulnerability Database
Source status changed · → Analyzed
Oct 13, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 13, 2026 · NIST National Vulnerability Database
CVE published
May 1, 2026 · Linux

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

4 on the record

git.kernel.org/stable/c/063a7409b0de46d7c770b65bb0338e6fdb3b1f0a
Patch, NIST National Vulnerability Database
git.kernel.org/stable/c/3e4e2ea2a781018ed5d75f969e3e5606beb66e48
Patch, NIST National Vulnerability Database
git.kernel.org/stable/c/59c32abaaec9cdd6164811c7e864e72f7554b82d
Patch, NIST National Vulnerability Database
git.kernel.org/stable/c/3e5360b422dd741cb315654a191fa73869a37414
Patch, NIST National Vulnerability Database

Elsewhere on this site

Linux
every CVE for this vendor

Related CVEs (same products)

Critical CVE-2026-99999 ipv4: validate checksum_start before completing checksum 8h
Unknown CVE-2026-98432 ipv4: free inet_opt and ireq_opt after an RCU grace period 8h
Unknown CVE-2026-98427 net/sched: codel: bound the dropping loop per dequeue call 1d
Unknown CVE-2026-98431 RDMA/uverbs: Fix mmap_lock/disassociation_lock circular dependency 1d
Unknown CVE-2026-98430 RDMA/efa: Keep admin queues alive while IRQ is registered 1d
Unknown CVE-2026-98429 RDMA/efa: Keep EQ resources alive while IRQ is registered 1d

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No CISA SSVC assessment.
No CWE classification.
No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-31706 is "ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl()", rated High. It affects Linux (Linux); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 8.8 describes technical severity. EPSS 0.63% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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