CVE-2026-63909 (GCVE-0-2026-63909)
Vulnerability from cvelistv5
Published
2026-07-19 14:55
Modified
2026-08-05 12:37
Severity ?
8.1 (High) - CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
Summary
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().
References
Impacted products
Vendor Product Version
Linux Linux Version: 282cbbb476b9f35793452bc461934af4c7eca169
Version: f20adc4ef7428bc485ee83fd1a592252fb87718b
Version: 325d4ac11f526cb8964cff14548ccf02d8c756d8
Version: 95e5aa3c3261da8c95b27d7aecf8ee39b9f86a4c
Version: 90089584b2e25c4510b7b987387b4405f0673ece
Version: 151b1799861fde38087c08f613abc2843ef597b0
Version: d07b26f39246a82399661936dd0c853983cfade7
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   Linux Linux Version: 6.6.140   ≤
Version: 6.12.84   ≤
Version: 6.18.25   ≤
Version: 7.0.2   ≤
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/smb/server/smbacl.c"
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            }
          ]
        },
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/smb/server/smbacl.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
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              "status": "affected",
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              "versionType": "semver"
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              "status": "affected",
              "version": "6.18.25",
              "versionType": "semver"
            },
            {
              "lessThan": "7.0.12",
              "status": "affected",
              "version": "7.0.2",
              "versionType": "semver"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.6.143",
                  "versionStartIncluding": "6.6.140",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.12.93",
                  "versionStartIncluding": "6.12.84",
                  "vulnerable": true
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                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
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                  "versionEndExcluding": "7.0.12",
                  "versionStartIncluding": "7.0.2",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "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()."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 8.1,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:N - The bug is reachable through ksmbd\u0027s in-kernel SMB server on TCP port 445; a remote attacker sends crafted SMB2 packets that traverse the normal network receive path to smb_check_perm_dacl().\nAC:L - The attacker fully controls the crafted NTSD/DACL planted via SMB and the subsequent SMB2_CREATE that triggers the walk; no race or victim-specific memory layout is required beyond standard heap behavior.\nPR:L - Exploitation requires an authenticated SMB2 session (post SESSION_SETUP and TREE_CONNECT) with permission to create files and set security descriptors (WRITE_DAC) on a share with ACL xattr support; this is a low-privileged authenticated user, not an unauthenticated attacker.\nUI:N - No end-user interaction is required beyond the attacker operating their own SMB client against an exposed ksmbd server.\nS:U - The vulnerability is a kernel heap out-of-bounds read within ksmbd\u0027s own security context; it does not cross a VM, sandbox, or IOMMU security boundary.\nC:H - The bug is an out-of-bounds heap read of ace-\u003esize past the pntsd kmalloc allocation; per kernel guidance OOB reads warrant High confidentiality impact as adjacent heap metadata or pointers may be disclosed.\nI:N - This is a read-only out-of-bounds access with no memory write, type confusion, or demonstrated ability to modify kernel data or achieve code execution through the corrupted read path.\nA:H - Reading beyond the kmalloc boundary can trigger a kernel page fault, oops, or panic; the original fix commit also notes KASAN reports and possible kernel state corruption from the OOB access."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-05T12:37:02.949Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/5500ba1d410aed1eded3eb04a76b10cfb4409334"
        },
        {
          "url": "https://git.kernel.org/stable/c/f6324b4240cf0b26a84c33f68a1222d727ff4af2"
        },
        {
          "url": "https://git.kernel.org/stable/c/0fe08c5776a798f46df1fd74b331be26bdd644d6"
        },
        {
          "url": "https://git.kernel.org/stable/c/d333af32e4451285e427f2d9c29de3a39f6f6d48"
        },
        {
          "url": "https://git.kernel.org/stable/c/94215d55b09445993929f4fc966061d61de74929"
        },
        {
          "url": "https://git.kernel.org/stable/c/4f7c131d2bdd7cd64b96f60d10be5ea72253f520"
        },
        {
          "url": "https://git.kernel.org/stable/c/0e60dafe97eca61721f3db456f97d97a80c6c8ae"
        }
      ],
      "title": "ksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63909",
    "datePublished": "2026-07-19T14:55:15.060Z",
    "dateReserved": "2026-07-19T07:54:57.020Z",
    "dateUpdated": "2026-08-05T12:37:02.949Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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