smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2

{
  "id": "30895decd6472800eba36d5f1fa63fb285a5650bd45ad6c1a3d4d74fdc9d8bbb",
  "url": "https://osv.dev/vulnerability/CVE-2026-89637",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2",
    "summary": "In the Linux kernel, the following vulnerability has been resolved: smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2 When a valid primary TRANSACT2 response has been received (mid->resp_buf set, mid->multiRsp true) and a subsequent secondary response causes cifs_check_trans2() to return false -- either because the SMB header is invalid (malformed != 0) or because check2ndT2() rejects the PDU -- handle_mid() overwrites mid->resp_buf with the new buffer (leaking the primary buffer) and, because mid->multiRsp is set, skips the server->smallbuf/bigbuf NULL-out. When the user thread frees mid->resp_buf, server->smallbuf or server->bigbuf is left dangling; the demux thread reuses it for the next packet, resulting in a use-after-free. Combine both early-exit conditions and, when mid->multiRsp is already set, abort the pending transaction inline: set multiEnd, call dequeue_mid() with malformed=true, and return true so handle_mid() exits without touching mid->resp_buf or the server buffer pointers.",
    "evidence": "osv:CVE-2026-89637",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "6.18.51",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "7.2.4",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "critical",
    "structured": {
      "id": "CVE-2026-89637",
      "aliases": [
        "CVE-2026-89637"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
                {
                  "introduced": "316cf94a910f6f93d43cc574359d163ccae098a3"
                },
                {
                  "fixed": "9eed72e9534b10a6d9f8f5146feff3db53aebdba"
                },
                {
                  "fixed": "5e6533a683f6a851158d9f33fb4ea8f4f25d7f84"
                },
                {
                  "fixed": "730d0bb19507b9e19c2fe5343109ac618e2fbce5"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89637.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "3.6.0"
                },
                {
                  "fixed": "6.18.51"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.19.0"
                },
                {
                  "fixed": "7.2.4"
                }
              ]
            }
          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89637.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:35.581668930Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/5e6533a683f6a851158d9f33fb4ea8f4f25d7f84",
        "https://git.kernel.org/stable/c/730d0bb19507b9e19c2fe5343109ac618e2fbce5",
        "https://git.kernel.org/stable/c/9eed72e9534b10a6d9f8f5146feff3db53aebdba",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/89xxx/CVE-2026-89637.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-89637",
        "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/2026/89xxx/CVE-2026-89637.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-89637",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 503014,
  "published_at": "2026-09-11T19:45:31.048+00:00"
}