GHSA-77X3-GHG7-W5WJ

Vulnerability from github – Published: 2026-07-19 18:31 – Updated: 2026-07-20 15:31
VLAI
Details

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops

Commit d07b26f39246 ("ksmbd: require minimum ACE size in smb_check_perm_dacl()") introduced a transposed bounds check:

if (offsetof(struct smb_ace, sid) + aces_size < CIFS_SID_BASE_SIZE)

Since offsetof(..sid) is 8 and CIFS_SID_BASE_SIZE is 8, this evaluates to aces_size < 0. Because aces_size is always non-negative, this check becomes dead code and never breaks the loop.

Worse, that commit removed the old 4-byte guard, meaning the loop now reads ace->size (offset 2) even when aces_size is 0-3 bytes. This re-opens a 2-byte heap out-of-bounds (OOB) read past the pntsd allocation during subsequent SMB2_CREATE operations.

Fix this by properly transposing the comparison to require at least 16 bytes (8-byte offset + 8-byte SID base), matching the correct form used in smb_inherit_dacl().

Severity
8.1 (High)
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
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{
  "affected": [],
  "aliases": [
    "CVE-2026-63909"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-07-19T16:17:08Z",
    "severity": "HIGH"
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops\n\nCommit d07b26f39246 (\"ksmbd: require minimum ACE size in\nsmb_check_perm_dacl()\") introduced a transposed bounds check:\n\n    if (offsetof(struct smb_ace, sid) + aces_size \u003c CIFS_SID_BASE_SIZE)\n\nSince offsetof(..sid) is 8 and CIFS_SID_BASE_SIZE is 8, this evaluates\nto `aces_size \u003c 0`. Because `aces_size` is always non-negative, this\ncheck becomes dead code and never breaks the loop.\n\nWorse, that commit removed the old 4-byte guard, meaning the loop now\nreads `ace-\u003esize` (offset 2) even when `aces_size` is 0-3 bytes. This\nre-opens a 2-byte heap out-of-bounds (OOB) read past the pntsd allocation\nduring subsequent SMB2_CREATE operations.\n\nFix this by properly transposing the comparison to require at least\n16 bytes (8-byte offset + 8-byte SID base), matching the correct form\nused in smb_inherit_dacl().",
  "id": "GHSA-77x3-ghg7-w5wj",
  "modified": "2026-07-20T15:31:50Z",
  "published": "2026-07-19T18:31:45Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-63909"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/0e60dafe97eca61721f3db456f97d97a80c6c8ae"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/0fe08c5776a798f46df1fd74b331be26bdd644d6"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/4f7c131d2bdd7cd64b96f60d10be5ea72253f520"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/5500ba1d410aed1eded3eb04a76b10cfb4409334"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/94215d55b09445993929f4fc966061d61de74929"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/d333af32e4451285e427f2d9c29de3a39f6f6d48"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/f6324b4240cf0b26a84c33f68a1222d727ff4af2"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H",
      "type": "CVSS_V3"
    }
  ]
}



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