# CVE-2026-97559: smb: client: fail DACL rewrite when the new DACL exceeds 64K

> An integer overflow in the Linux kernel SMB client allows corrupting DACLs sent to servers, potentially causing data corruption or denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-104691, BELL-CVE-2026-97559, DEBIAN-CVE-2026-97559, GHSA-34vg-4qp7-6958, UBUNTU-CVE-2026-97559

## What is CVE-2026-97559?

CVE-2026-97559 is a vulnerability in Linux, published on 25 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official 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.

## Which versions are affected by CVE-2026-97559?


## How do I fix CVE-2026-97559?

Apply the vendor's patch.

## Is CVE-2026-97559 being exploited?

There is no known exploitation of CVE-2026-97559 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-97559 and fix it if it is affected.

What it is: CVE-2026-97559 is a vulnerability in Linux, published on 25 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-97559 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-97559/ (Markdown: https://insights.sbomify.com/CVE-2026-97559/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-97559
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-97559
- OSV: https://osv.dev/vulnerability/CVE-2026-97559

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-97559/
Summaries are generated and may contain mistakes; the official description above is the record.