# CVE-2026-31712: ksmbd: require minimum ACE size in smb_check_perm_dacl()

> An authenticated SMB user can trigger an out-of-bounds read in the Linux kernel ksmbd module by crafting a specific DACL, potentially causing kernel crashes or corruption.

- **Severity:** High 8.3 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 1 May 2026
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2026-31712, DEBIAN-CVE-2026-31712, GHSA-59cp-8v32-7335, ROOT-OS-DEBIAN-12-CVE-2026-31712, ROOT-OS-UBUNTU-2204-CVE-2026-31712, ROOT-OS-UBUNTU-2404-CVE-2026-31712, UBUNTU-CVE-2026-31712

## What is CVE-2026-31712?

CVE-2026-31712 is a high-severity memory corruption vulnerability in Linux kernel, published on 1 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

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

ksmbd: require minimum ACE size in smb_check_perm_dacl()

Both ACE-walk loops in smb_check_perm_dacl() only guard against an
under-sized remaining buffer, not against an ACE whose declared
`ace->size` is smaller than the struct it claims to describe:

  if (offsetof(struct smb_ace, access_req) > aces_size)
      break;
  ace_size = le16_to_cpu(ace->size);
  if (ace_size > aces_size)
      break;

The first check only requires the 4-byte ACE header to be in bounds;
it does not require access_req (4 bytes at offset 4) to be readable.
An attacker who has set a crafted DACL on a file they own can declare
ace->size == 4 with aces_size == 4, pass both checks, and then

  granted |= le32_to_cpu(ace->access_req);               /* upper loop */
  compare_sids(&sid, &ace->sid);                         /* lower loop */

reads access_req at offset 4 (OOB by up to 4 bytes) and ace->sid at
offset 8 (OOB by up to CIFS_SID_BASE_SIZE + SID_MAX_SUB_AUTHORITIES
* 4 bytes).

Tighten both loops to require

  ace_size >= offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE

which is the smallest valid on-wire ACE layout (4-byte header +
4-byte access_req + 8-byte sid base with zero sub-auths).  Also
reject ACEs whose sid.num_subauth exceeds SID_MAX_SUB_AUTHORITIES
before letting compare_sids() dereference sub_auth[] entries.

parse_sec_desc() already enforces an equivalent check (lines 441-448);
smb_check_perm_dacl() simply grew weaker validation over time.

Reachability: authenticated SMB client with permission to set an ACL
on a file.  On a subsequent CREATE against that file, the kernel
walks the stored DACL via smb_check_perm_dacl() and triggers the
OOB read.  Not pre-auth, and the OOB read is not reflected to the
attacker, but KASAN reports and kernel state corruption are
possible.

Weakness: CWE-787 (Out-of-bounds Write)

## Which versions are affected by CVE-2026-31712?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31712?

Apply the vendor's patch.

## Is CVE-2026-31712 being exploited?

There is no known exploitation of CVE-2026-31712 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31712 and fix it if it is affected.

What it is: CVE-2026-31712 is a high-severity memory corruption vulnerability in Linux kernel, published on 1 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-31712 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-31712/ (Markdown: https://insights.sbomify.com/CVE-2026-31712/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31712
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31712
- OSV: https://osv.dev/vulnerability/CVE-2026-31712

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31712/
Summaries are generated and may contain mistakes; the official description above is the record.