Linux Kernel ksmbd SMB server Out-Of-Bounds Write Rated 8.3 CVSS

Status: UPDATED  |  Advisory ID: CVE-2026-31712

Key Details

Affected Products, Subsystems & Sectors

What to Know

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. (NVD)

What to Do

Monitor Linux's and Siemens's web pages for any future patch releases.

References