Description
In the Linux kernel, the following vulnerability has been resolved:

smb: client: fail DACL rewrite when the new DACL exceeds 64K

replace_sids_and_copy_aces() and set_chmod_dacl() accumulate the size of
the DACL they build in a u16. That accumulator can wrap.

validate_dacl() caps num_aces at (dacl_size - sizeof(struct smb_acl)) /
20, i.e. 3276 for a maximally sized DACL, while each rewritten ACE can
grow to sizeof(struct smb_ace) (76 bytes) once its SID is replaced with
one carrying SID_MAX_SUB_AUTHORITIES sub-authorities. The worst case is
therefore sizeof(struct smb_acl) + 3276 * 76 = 248984 bytes, far beyond
what a u16 can hold. A wraparound is reached with 863 ACEs.

After the wraparound, ndacl_ptr->size becomes meaningless and the offset
will point anywhere in the ACE array. As a result, we will see
corruption of the DACL, which then gets sent to the server. This is not
an out-of-bounds write as the allocation now covers the worst-case
expansion, so writes will always go into the buffer.

Adjust the code to use a u32 internally and return -EOVERFLOW in the
overflow case. The operation must be refused, because a DACL can only
hold 2^16-1 bytes on the wire and larger DACLs cannot be represented.

set_chmod_dacl() carries the same pattern and is fixed the same way. It
only wraps once the source DACL comes within roughly 380 bytes of the
64K ceiling, but the failure mode is identical.
Published: 2026-09-25
Score: n/a
EPSS: n/a
KEV: No
Impact: Denial of Service via corrupted DACL
Action: Upgrade kernel
AI Analysis

Impact

This vulnerability occurs in the Linux kernel SMB client when rewriting a DACL larger than 64KB. The code that computes the size of the new DACL uses a 16‑bit accumulator that can wrap, causing the internal size field to become meaningless and the offset pointer to reference arbitrary locations within the ACE array. The resulting buffer corruption is sent to the SMB server, which may process the malformed DACL and become unstable. This flaw enables a remote attacker that can control the DACL content to force a denial of service or, if the server incorrectly interprets the corrupted data, could lead to further compromise. The weakness is an integer overflow that results in a buffer overflow (CWE‑680) and an unchecked buffer write (CWE‑120).

Affected Systems

The flaw affects the Linux kernel SMB client component. Any Linux distribution shipping the Linux kernel version that contains this vulnerable SMB client code is affected. The list of impacted products is represented by the CPE string cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*, indicating all Linux kernel releases that have not yet had the patch applied.

Risk and Exploitability

No EPSS score is available and the vulnerability is not listed in the CISA KEV catalog, implying that no widespread exploitation is documented as of now. The CVSS score is not supplied, so the severity cannot be quantified precisely. The risk of exploitation remains moderate because the flaw requires a client to send a specially crafted DACL to a target SMB server, but the consequence—corruption of ACL data and potential denial of service—could be significant for environments that rely on SMB for file sharing.

Generated by OpenCVE AI on September 25, 2026 at 13:06 UTC.

Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

  • Apply a Linux kernel update that includes the CIFS/SMB client patch from commit 2ef532a01f44687f5b95001e88247cf05bff9892 or the most recent stable release.
  • If an immediate kernel upgrade is not possible, restrict SMB client traffic to trusted hosts or block SMB traffic from untrusted networks using firewall rules.
  • Configure the SMB client to limit the size of DACLs it sends, ensuring that no ACE list exceeds 64KB, or verify that the DACL size is capped before sending.
  • Continuously monitor SMB logs for errors or abnormal DACL parsing failures that might indicate exploitation attempts.

Generated by OpenCVE AI on September 25, 2026 at 13:06 UTC.

Tracking

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Advisories

No advisories yet.

History

Fri, 25 Sep 2026 13:30:00 +0000

Type Values Removed Values Added
Weaknesses CWE-120
CWE-680

Fri, 25 Sep 2026 10:30:00 +0000

Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: smb: client: fail DACL rewrite when the new DACL exceeds 64K replace_sids_and_copy_aces() and set_chmod_dacl() accumulate the size of the DACL they build in a u16. That accumulator can wrap. validate_dacl() caps num_aces at (dacl_size - sizeof(struct smb_acl)) / 20, i.e. 3276 for a maximally sized DACL, while each rewritten ACE can grow to sizeof(struct smb_ace) (76 bytes) once its SID is replaced with one carrying SID_MAX_SUB_AUTHORITIES sub-authorities. The worst case is therefore sizeof(struct smb_acl) + 3276 * 76 = 248984 bytes, far beyond what a u16 can hold. A wraparound is reached with 863 ACEs. After the wraparound, ndacl_ptr->size becomes meaningless and the offset will point anywhere in the ACE array. As a result, we will see corruption of the DACL, which then gets sent to the server. This is not an out-of-bounds write as the allocation now covers the worst-case expansion, so writes will always go into the buffer. Adjust the code to use a u32 internally and return -EOVERFLOW in the overflow case. The operation must be refused, because a DACL can only hold 2^16-1 bytes on the wire and larger DACLs cannot be represented. set_chmod_dacl() carries the same pattern and is fixed the same way. It only wraps once the source DACL comes within roughly 380 bytes of the 64K ceiling, but the failure mode is identical.
Title smb: client: fail DACL rewrite when the new DACL exceeds 64K
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References

Subscriptions

Linux Linux Kernel
cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published:

Updated: 2026-09-25T10:21:48.866Z

Reserved: 2026-09-24T16:01:01.155Z

Link: CVE-2026-97559

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Received

Published: 2026-09-25T11:17:06.430

Modified: 2026-09-25T11:17:06.430

Link: CVE-2026-97559

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-09-25T13:15:07Z

Weaknesses
  • CWE-120

    Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')

  • CWE-680

    Integer Overflow to Buffer Overflow