# CVE-2026-63909: ksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops

> An out-of-bounds read in the Linux kernel ksmbd module allows attackers to read heap memory or cause a crash during SMB2 operations.

- **Severity:** High 8.1 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/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:** 19 Jul 2026
- **Updated:** 5 Aug 2026
- **Also known as:** BELL-CVE-2026-63909, DEBIAN-CVE-2026-63909, GHSA-77x3-ghg7-w5wj, UBUNTU-CVE-2026-63909

## What is CVE-2026-63909?

CVE-2026-63909 is a high-severity memory corruption vulnerability in Linux, published on 19 Jul 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: OOB read regression in smb_check_perm_dacl() ACE-walk loops

Commit d07b26f39246 ("ksmbd: require minimum ACE size in
smb_check_perm_dacl()") introduced a transposed bounds check:

    if (offsetof(struct smb_ace, sid) + aces_size < CIFS_SID_BASE_SIZE)

Since offsetof(..sid) is 8 and CIFS_SID_BASE_SIZE is 8, this evaluates
to `aces_size < 0`. Because `aces_size` is always non-negative, this
check becomes dead code and never breaks the loop.

Worse, that commit removed the old 4-byte guard, meaning the loop now
reads `ace->size` (offset 2) even when `aces_size` is 0-3 bytes. This
re-opens a 2-byte heap out-of-bounds (OOB) read past the pntsd allocation
during subsequent SMB2_CREATE operations.

Fix this by properly transposing the comparison to require at least
16 bytes (8-byte offset + 8-byte SID base), matching the correct form
used in smb_inherit_dacl().

## Which versions are affected by CVE-2026-63909?


## How do I fix CVE-2026-63909?

Apply the vendor's patch.

## Is CVE-2026-63909 being exploited?

There is no known exploitation of CVE-2026-63909 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63909 and fix it if it is affected.

What it is: CVE-2026-63909 is a high-severity memory corruption vulnerability in Linux, published on 19 Jul 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-2026-63909 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-2026-63909/ (Markdown: https://insights.sbomify.com/CVE-2026-63909/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63909
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63909
- OSV: https://osv.dev/vulnerability/CVE-2026-63909

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63909/
Summaries are generated and may contain mistakes; the official description above is the record.