Vulnerability-Lookup

GHSA-Q27P-5553-XHRR

Vulnerability from github – Published: 2026-09-11 21:31 – Updated: 2026-09-13 09:32

Details

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

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{
  "affected": [],
  "aliases": [
    "CVE-2026-89637"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-09-11T20:19:49Z",
    "severity": "CRITICAL"
  },
  "details": "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": "GHSA-q27p-5553-xhrr",
  "modified": "2026-09-13T09:32:26Z",
  "published": "2026-09-11T21:31:34Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-89637"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/5e6533a683f6a851158d9f33fb4ea8f4f25d7f84"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/730d0bb19507b9e19c2fe5343109ac618e2fbce5"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/9eed72e9534b10a6d9f8f5146feff3db53aebdba"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ]
}

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

Vulnerability from cvelistv5 – Published: 2026-09-11 19:45 – Updated: 2026-09-13 06:32

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

Linux mitre root

References

3 references

URL	Tags
https://git.kernel.org/stable/c/9eed72e9534b10a6d…
https://git.kernel.org/stable/c/5e6533a683f6a8511…
https://git.kernel.org/stable/c/730d0bb19507b9e19…

Impacted products

2 products

Vendor	Product	Version
Linux	Linux	Affected: 316cf94a910f6f93d43cc574359d163ccae098a3 , < 9eed72e9534b10a6d9f8f5146feff3db53aebdba (git) Affected: 316cf94a910f6f93d43cc574359d163ccae098a3 , < 5e6533a683f6a851158d9f33fb4ea8f4f25d7f84 (git) Affected: 316cf94a910f6f93d43cc574359d163ccae098a3 , < 730d0bb19507b9e19c2fe5343109ac618e2fbce5 (git)
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)

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{
  "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": [
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          "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"
}

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.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

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