smb: client: restrict implied bcc[0] exemption to responses without data area

{
  "id": "700950ea34a7144ddc2d9cc457e892e1e4156ae8bc70f8653cbc7b2af127b6ca",
  "url": "https://osv.dev/vulnerability/CVE-2026-64448",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "smb: client: restrict implied bcc[0] exemption to responses without data area",
    "summary": "In the Linux kernel, the following vulnerability has been resolved: smb: client: restrict implied bcc[0] exemption to responses without data area smb2_check_message() has a long-standing quirk that accepts a response whose calculated length is one byte larger than the bytes actually received (\"server can return one byte more due to implied bcc[0]\"). This was introduced to accommodate servers that omit the trailing bcc[0] overlap byte when no data area is present. However, the exemption is applied unconditionally, regardless of whether the command actually carries a data area (has_smb2_data_area[]). When a response with a data area is subject to the +1 exemption, the reported data can extend one byte beyond the bytes actually received, yet smb2_check_message() still accepts it. The subsequent decoder then reads past the end of the receive buffer. This is reachable during NEGOTIATE and SESSION_SETUP, before the session is established. The resulting out-of-bounds reads are visible under KASAN when mounting against a non-conforming server; both the SPNEGO/negTokenInit and the NTLMSSP challenge decoders are affected: BUG: KASAN: slab-out-of-bounds in asn1_ber_decoder+0x16a7/0x1b00 Read of size 1 at addr ffff8880084d67c0 by task mount.cifs/81 CPU: 1 UID: 0 PID: 81 Comm: mount.cifs Not tainted 7.1.0-rc6 #1 Call Trace: dump_stack_lvl+0x4e/0x70 print_report+0x157/0x4c9 kasan_report+0xce/0x100 asn1_ber_decoder+0x16a7/0x1b00 decode_negTokenInit+0x19/0x30 SMB2_negotiate+0x31d9/0x4c90 cif",
    "evidence": "osv:CVE-2026-64448",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "5.10.261",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "5.15.212",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.1.178",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.6.145",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.12.96",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.18.39",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "7.1.4",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-64448",
      "aliases": [
        "CVE-2026-64448"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
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              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
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                {
                  "fixed": "8d0bbc78046d264bbf6a574ea6f9072258a43e35"
                },
                {
                  "fixed": "b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0"
                },
                {
                  "fixed": "31c6312608c60b72a1feb99a5afb680645a3e8a3"
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                {
                  "fixed": "573e502d14714d2947e22e7eff40ec20a6a44a42"
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                {
                  "fixed": "419ec1b604d7fb60c10aec2dc062371f9fcd4940"
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                {
                  "fixed": "ceb875a375dedbf51c9425c1d13a2d7a8435c08c"
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                {
                  "fixed": "6e9d10f62773b99bd927940fd9cbdfe7207e23ff"
                },
                {
                  "fixed": "53b7c271f06be4dd5cfc8c6ef552a8355c891a7f"
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              ]
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          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-64448.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "3.6.0"
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                {
                  "fixed": "5.10.261"
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              ]
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              "type": "ECOSYSTEM",
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                  "introduced": "5.11.0"
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                  "introduced": "6.7.0"
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          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-64448.json"
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        }
      ],
      "modified": "2026-10-28T02:30:41.565045353Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/31c6312608c60b72a1feb99a5afb680645a3e8a3",
        "https://git.kernel.org/stable/c/419ec1b604d7fb60c10aec2dc062371f9fcd4940",
        "https://git.kernel.org/stable/c/53b7c271f06be4dd5cfc8c6ef552a8355c891a7f",
        "https://git.kernel.org/stable/c/573e502d14714d2947e22e7eff40ec20a6a44a42",
        "https://git.kernel.org/stable/c/6e9d10f62773b99bd927940fd9cbdfe7207e23ff",
        "https://git.kernel.org/stable/c/8d0bbc78046d264bbf6a574ea6f9072258a43e35",
        "https://git.kernel.org/stable/c/b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0",
        "https://git.kernel.org/stable/c/ceb875a375dedbf51c9425c1d13a2d7a8435c08c",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/64xxx/CVE-2026-64448.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-64448",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/64xxx/CVE-2026-64448.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-64448",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 509240,
  "published_at": "2026-07-25T08:51:18.796+00:00"
}