# CVE-2026-31706: ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl()

CVE-2026-31706 is fixed in linux on 1 platform, first by DEBIAN-CVE-2026-31706, released May 15, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-31706
- Priority: High
- CVSS: 8.8 (Important)
- EPSS: 0.0049
- 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-2026-31706

## Description

In the Linux kernel, the following vulnerability has been resolved:  ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl()  smb_inherit_dacl() trusts the on-disk num_aces value from the parent directory's DACL xattr and uses it to size a heap allocation:    aces_base = kmalloc(sizeof(struct smb_ace) * num_aces * 2, ...);  num_aces is a u16 read from le16_to_cpu(parent_pdacl->num_aces) without checking that it is consistent with the declared pdacl_size. An authenticated client whose parent directory's security.NTACL is tampered (e.g. via offline xattr corruption or a concurrent path that bypasses parse_dacl()) can present num_aces = 65535 with minimal actual ACE data.  This causes a ~8 MB allocation (not kzalloc, so uninitialized) that the subsequent loop only partially populates, and may also overflow the three-way size_t multiply on 32-bit kernels.  Additionally, the ACE walk loop uses the weaker offsetof(struct smb_ace, access_req) minimum size check rather than the minimum valid on-wire ACE size, and does not reject ACEs whose declared size is below the minimum.  Reproduced on UML + KASAN + LOCKDEP against the real ksmbd code path. A legitimate mount.cifs client creates a parent directory over SMB (ksmbd writes a valid security.NTACL xattr), then the NTACL blob on the backing filesystem is rewritten to set num_aces = 0xFFFF while keeping the posix_acl_hash bytes intact so ksmbd_vfs_get_sd_xattr()'s hash check still passes.  A subsequent SMB2 CREATE of a child under that parent drives smb2_open() into smb_inherit_dacl() (share has "vfs objects = acl_xattr" set), which fails the page allocator:    WARNING: mm/page_alloc.c:5226 at __alloc_frozen_pages_noprof+0x46c/0x9c0   Workqueue: ksmbd-io handle_ksmbd_work    __alloc_frozen_pages_noprof+0x46c/0x9c0    ___kmalloc_large_node+0x68/0x130    __kmalloc_large_node_noprof+0x24/0x70    __kmalloc_noprof+0x4c9/0x690    smb_inherit_dacl+0x394/0x2430    smb2_open+0x595d/0xabe0    handle_ksmbd_work+0x3d3/0x1140  With the patch applied the added guard rejects the tampered value with -EINVAL before any large allocation runs, smb2_open() falls back to smb2_create_sd_buffer(), and the child is created with a default SD.  No warning, no splat.  Fix by:    1. Validating num_aces against pdacl_size using the same formula      applied in parse_dacl().    2. Replacing the raw kmalloc(sizeof * num_aces * 2) with      kmalloc_array(num_aces * 2, sizeof(...)) for overflow-safe      allocation.    3. Tightening the per-ACE loop guard to require the minimum valid      ACE size (offsetof(smb_ace, sid) + CIFS_SID_BASE_SIZE) and      rejecting under-sized ACEs, matching the hardening in      smb_check_perm_dacl() and parse_dacl().  v1 -> v2:   - Replace the synthetic test-module splat in the changelog with a     real-path UML + KASAN reproduction driven through mount.cifs and     SMB2 CREATE; Namjae flagged the kcifs3_test_inherit_dacl_old name     in v1 since it does not exist in ksmbd.   - Drop the commit-hash citation from the code comment per Namjae's     review; keep the parse_dacl() pointer.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 13.5 (trixie) | 6.12.85-1 | [DEBIAN-CVE-2026-31706](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31706.md) | May 15, 2026 |
| Linux | linux Debian 14 | 7.0.3-1 | [DEBIAN-CVE-2026-31706](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31706.md) | May 15, 2026 |

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Data as of 2026-11-08T19:12:23Z 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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