GCVE Workshop - 22 September 2026 (14:00-18:00), Luxembourg Before The Vulnopticon Conference - Registration

CVE-2026-89637 (GCVE-0-2026-89637)

Vulnerability from cvelistv5 – Published: 2026-09-11 19:45 – Updated: 2026-09-13 06:32
VLAI
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.
Severity
9.8 (Critical)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 316cf94a910f6f93d43cc574359d163ccae098a3 , < 9eed72e9534b10a6d9f8f5146feff3db53aebdba (git)
Affected: 316cf94a910f6f93d43cc574359d163ccae098a3 , < 5e6533a683f6a851158d9f33fb4ea8f4f25d7f84 (git)
Affected: 316cf94a910f6f93d43cc574359d163ccae098a3 , < 730d0bb19507b9e19c2fe5343109ac618e2fbce5 (git)
guessed Create a notification for this product.
Linux Linux Affected: 3.6
Unaffected: 0 , < 3.6 (semver)
Unaffected: 6.18.51 , ≤ 6.18.* (semver)
Unaffected: 7.2.4 , ≤ 7.2.* (semver)
Unaffected: 7.3-rc1 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
Show details on NVD website
To clipboard 

{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/smb/client/smb1transport.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "9eed72e9534b10a6d9f8f5146feff3db53aebdba",
              "status": "affected",
              "version": "316cf94a910f6f93d43cc574359d163ccae098a3",
              "versionType": "git"
            },
            {
              "lessThan": "5e6533a683f6a851158d9f33fb4ea8f4f25d7f84",
              "status": "affected",
              "version": "316cf94a910f6f93d43cc574359d163ccae098a3",
              "versionType": "git"
            },
            {
              "lessThan": "730d0bb19507b9e19c2fe5343109ac618e2fbce5",
              "status": "affected",
              "version": "316cf94a910f6f93d43cc574359d163ccae098a3",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "fs/smb/client/smb1transport.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "3.6"
            },
            {
              "lessThan": "3.6",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.51",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.2.*",
              "status": "unaffected",
              "version": "7.2.4",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.3-rc1",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.18.51",
                  "versionStartIncluding": "3.6",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.2.4",
                  "versionStartIncluding": "3.6",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.3-rc1",
                  "versionStartIncluding": "3.6",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nsmb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2\n\nWhen a valid primary TRANSACT2 response has been received (mid-\u003eresp_buf\nset, mid-\u003emultiRsp true) and a subsequent secondary response causes\ncifs_check_trans2() to return false -- either because the SMB header is\ninvalid (malformed != 0) or because check2ndT2() rejects the PDU --\nhandle_mid() overwrites mid-\u003eresp_buf with the new buffer (leaking the\nprimary buffer) and, because mid-\u003emultiRsp is set, skips the\nserver-\u003esmallbuf/bigbuf NULL-out.  When the user thread frees\nmid-\u003eresp_buf, server-\u003esmallbuf or server-\u003ebigbuf is left dangling; the\ndemux thread reuses it for the next packet, resulting in a use-after-free.\n\nCombine both early-exit conditions and, when mid-\u003emultiRsp is already\nset, abort the pending transaction inline: set multiEnd, call\ndequeue_mid() with malformed=true, and return true so handle_mid() exits\nwithout touching mid-\u003eresp_buf or the server buffer pointers."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 9.8,
            "baseSeverity": "CRITICAL",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:N - cifs_check_trans2() runs in cifs_demultiplex_thread on SMB1 TRANS2 responses received over TCP/445; a malicious or compromised SMB1 server (or MITM on an unsigned session) delivers the crafted primary and malformed secondary PDUs entirely over the network.\nAC:L - The attacker fully controls both fragments: a valid primary with TotalDataCount greater than DataCount sets mid-\u003emultiRsp, then a secondary with WordCount != 10, inconsistent counts, or a malformed SMB header makes cifs_check_trans2() return false. The resulting UAF is deterministic with no race outside the attacker\u0027s control.\nPR:N - The attacker is the remote SMB peer and needs no account or privileges on the victim; cifs_check_trans2() runs in the demux thread on server-supplied frames, and session credentials belong to the client mount, not the attacking server.\nUI:N - Once an SMB1 CIFS mount exists (fstab, autofs, cifsroot, embedded/industrial NAS; CONFIG_CIFS_ALLOW_INSECURE_LEGACY defaults on), reconnect and ordinary lookup/readdir/getattr plus cifs_qfs_tcon QUERY_FS_INFO issue TRANS2 automatically, so the server can trigger the UAF with no further user action.\nS:U - The use-after-free corrupts the client kernel\u0027s own CIFS receive-buffer mempool (cifs_req_poolp/cifs_sm_req_poolp) within the same host security authority; this is not a VM escape, IOMMU bypass, or sandbox boundary crossing.\nC:H - handle_mid() overwrites mid-\u003eresp_buf without detaching server-\u003ebigbuf/smallbuf, so after the user thread frees the buffer the demux thread reuses the dangling mempool object for the next packet; that use-after-free enables heap reuse and arbitrary kernel memory disclosure.\nI:H - The same dangling server receive buffer is written by cifsd and later freed by the user thread, yielding a kernel heap use-after-free. Attacker-controlled subsequent SMB payloads and heap grooming can turn this into an arbitrary write or control-flow hijack.\nA:H - Reusing a freed CIFS small/large response buffer can fault on poisoned or unmapped slab objects and oops or panic the demux thread, and a malicious server can retrigger the primary-then-malformed-secondary sequence on every multi-part TRANS2."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-09-13T06:32:16.928Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/9eed72e9534b10a6d9f8f5146feff3db53aebdba"
        },
        {
          "url": "https://git.kernel.org/stable/c/5e6533a683f6a851158d9f33fb4ea8f4f25d7f84"
        },
        {
          "url": "https://git.kernel.org/stable/c/730d0bb19507b9e19c2fe5343109ac618e2fbce5"
        }
      ],
      "title": "smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-89637",
    "datePublished": "2026-09-11T19:45:31.048Z",
    "dateReserved": "2026-09-11T19:38:34.739Z",
    "dateUpdated": "2026-09-13T06:32:16.928Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-89637",
      "date": "2026-09-20",
      "epss": "0.00588",
      "percentile": "0.46815"
    },
    "microsoft_vex": {
      "current_release_date": "2026-09-15T01:53:37.000Z",
      "cve": "CVE-2026-89637",
      "id": "msrc_CVE-2026-89637",
      "initial_release_date": "2026-09-13T01:02:46.000Z",
      "product_status:known_not_affected": "1",
      "source": "Microsoft CSAF VEX",
      "status": "final",
      "title": "smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2",
      "url": "https://msrc.microsoft.com/csaf/vex/2026/msrc_cve-2026-89637.json",
      "version": "3"
    },
    "nvd": {
      "cve": {
        "affected": [
          {
            "affectedData": [
              {
                "defaultStatus": "unaffected",
                "product": "Linux",
                "programFiles": [
                  "fs/smb/client/smb1transport.c"
                ],
                "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                "vendor": "Linux",
                "versions": [
                  {
                    "lessThan": "9eed72e9534b10a6d9f8f5146feff3db53aebdba",
                    "status": "affected",
                    "version": "316cf94a910f6f93d43cc574359d163ccae098a3",
                    "versionType": "git"
                  },
                  {
                    "lessThan": "5e6533a683f6a851158d9f33fb4ea8f4f25d7f84",
                    "status": "affected",
                    "version": "316cf94a910f6f93d43cc574359d163ccae098a3",
                    "versionType": "git"
                  },
                  {
                    "lessThan": "730d0bb19507b9e19c2fe5343109ac618e2fbce5",
                    "status": "affected",
                    "version": "316cf94a910f6f93d43cc574359d163ccae098a3",
                    "versionType": "git"
                  }
                ]
              },
              {
                "defaultStatus": "affected",
                "product": "Linux",
                "programFiles": [
                  "fs/smb/client/smb1transport.c"
                ],
                "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                "vendor": "Linux",
                "versions": [
                  {
                    "status": "affected",
                    "version": "3.6"
                  },
                  {
                    "lessThan": "3.6",
                    "status": "unaffected",
                    "version": "0",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "6.18.*",
                    "status": "unaffected",
                    "version": "6.18.51",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "7.2.*",
                    "status": "unaffected",
                    "version": "7.2.4",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "*",
                    "status": "unaffected",
                    "version": "7.3-rc1",
                    "versionType": "original_commit_for_fix"
                  }
                ]
              }
            ],
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
          }
        ],
        "cveTags": [],
        "descriptions": [
          {
            "lang": "en",
            "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nsmb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2\n\nWhen a valid primary TRANSACT2 response has been received (mid-\u003eresp_buf\nset, mid-\u003emultiRsp true) and a subsequent secondary response causes\ncifs_check_trans2() to return false -- either because the SMB header is\ninvalid (malformed != 0) or because check2ndT2() rejects the PDU --\nhandle_mid() overwrites mid-\u003eresp_buf with the new buffer (leaking the\nprimary buffer) and, because mid-\u003emultiRsp is set, skips the\nserver-\u003esmallbuf/bigbuf NULL-out.  When the user thread frees\nmid-\u003eresp_buf, server-\u003esmallbuf or server-\u003ebigbuf is left dangling; the\ndemux thread reuses it for the next packet, resulting in a use-after-free.\n\nCombine both early-exit conditions and, when mid-\u003emultiRsp is already\nset, abort the pending transaction inline: set multiEnd, call\ndequeue_mid() with malformed=true, and return true so handle_mid() exits\nwithout touching mid-\u003eresp_buf or the server buffer pointers."
          }
        ],
        "id": "CVE-2026-89637",
        "lastModified": "2026-09-13T07:17:28.883",
        "metrics": {
          "cvssMetricV31": [
            {
              "cvssData": {
                "attackComplexity": "LOW",
                "attackVector": "NETWORK",
                "availabilityImpact": "HIGH",
                "baseScore": 9.8,
                "baseSeverity": "CRITICAL",
                "confidentialityImpact": "HIGH",
                "integrityImpact": "HIGH",
                "privilegesRequired": "NONE",
                "scope": "UNCHANGED",
                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
                "version": "3.1"
              },
              "exploitabilityScore": 3.9,
              "impactScore": 5.9,
              "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
              "type": "Secondary"
            }
          ]
        },
        "published": "2026-09-11T20:19:49.370",
        "references": [
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/5e6533a683f6a851158d9f33fb4ea8f4f25d7f84"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/730d0bb19507b9e19c2fe5343109ac618e2fbce5"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/9eed72e9534b10a6d9f8f5146feff3db53aebdba"
          }
        ],
        "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "vulnStatus": "Received"
      }
    },
    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-09-18T22:00:04+00:00",
      "cve": "CVE-2026-89637",
      "id": "CVE-2026-89637",
      "initial_release_date": "2026-09-11T00:00:00+00:00",
      "product_status:known_affected": "185",
      "product_status:known_not_affected": "91",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89637.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "important",
      "current_release_date": "2026-09-17T16:41:53Z",
      "cve": "CVE-2026-89637",
      "id": "CVE-2026-89637",
      "initial_release_date": "2026-09-12T16:24:34Z",
      "product_status:known_affected": "349",
      "product_status:known_not_affected": "4",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-89637",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-89637.json",
      "version": "6"
    }
  }
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.