# CVE-2026-31712: ksmbd: require minimum ACE size in smb_check_perm_dacl()

CVE-2026-31712 is fixed in linux on 1 platform, first by DEBIAN-CVE-2026-31712, released May 15, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-31712
- Priority: High
- CVSS: 8.3 (Important)
- EPSS: 0.0032
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: May 1, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-31712

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 6.1.176-1 | [DEBIAN-CVE-2026-31712](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31712.md) | May 15, 2026 |
| Linux | linux Debian 13.5 (trixie) | 6.12.85-1 | [DEBIAN-CVE-2026-31712](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31712.md) | May 15, 2026 |
| Linux | linux Debian 14 | 7.0.3-1 | [DEBIAN-CVE-2026-31712](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31712.md) | May 15, 2026 |

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Data as of 2026-11-09T01:18:40Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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