# CVE-2026-64578: ksmbd: validate compound request size before reading StructureSize2

> A remote attacker can trigger a slab-out-of-bounds read in the Linux kernel's ksmbd SMB server by sending a crafted compound request, potentially causing a crash or leaking memory.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 5 Aug 2026
- **Updated:** 19 Aug 2026
- **Also known as:** BELL-CVE-2026-64578, DEBIAN-CVE-2026-64578, GHSA-wrjv-h4c7-cj2f, UBUNTU-CVE-2026-64578

## What is CVE-2026-64578?

CVE-2026-64578 is a high-severity memory corruption vulnerability in Linux, published on 5 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

ksmbd: validate compound request size before reading StructureSize2

When ksmbd validates a compound (chained) SMB2 request,
ksmbd_smb2_check_message() reads pdu->StructureSize2 without first
checking that the compound element is large enough to contain it.
StructureSize2 is a 2-byte field at offset 64
(__SMB2_HEADER_STRUCTURE_SIZE) from the start of each element.

The compound-walking logic only guarantees that a full 64-byte SMB2
header is present for the trailing element: when NextCommand is 0, len is
reduced to the number of bytes remaining after next_smb2_rcv_hdr_off. A
remote client can craft a compound request whose last element has exactly
64 bytes, so the 2-byte StructureSize2 read at offset 64 extends one byte
past the receive buffer, producing a slab-out-of-bounds read.

  BUG: KASAN: slab-out-of-bounds in ksmbd_smb2_check_message (fs/smb/server/smb2misc.c:402)
  Read of size 2 at addr ffff888012ae31ac by task kworker/0:1/14
  The buggy address is located 172 bytes inside of allocated 173-byte region
  Workqueue: ksmbd-io handle_ksmbd_work
  Call Trace:
   ...
   kasan_report (mm/kasan/report.c:595)
   ksmbd_smb2_check_message (fs/smb/server/smb2misc.c:402)
   handle_ksmbd_work (fs/smb/server/server.c:119)
   process_one_work (kernel/workqueue.c:3314)
   worker_thread (kernel/workqueue.c:3397)
   kthread (kernel/kthread.c:436)
   ret_from_fork (arch/x86/kernel/process.c:158)
   ret_from_fork_asm (arch/x86/entry/entry_64.S:245)

Reject any compound element that is too small to hold StructureSize2
before dereferencing it.

## Which versions are affected by CVE-2026-64578?


## How do I fix CVE-2026-64578?

Apply the vendor's patch.

## Is CVE-2026-64578 being exploited?

There is no known exploitation of CVE-2026-64578 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64578 and fix it if it is affected.

What it is: CVE-2026-64578 is a high-severity memory corruption vulnerability in Linux, published on 5 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-64578 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-2026-64578/ (Markdown: https://insights.sbomify.com/CVE-2026-64578/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64578
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64578
- OSV: https://osv.dev/vulnerability/CVE-2026-64578

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64578/
Summaries are generated and may contain mistakes; the official description above is the record.