smb: client: validate the whole DACL before rewriting it in...

8.8 / 10

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Description

In the Linux kernel, the following vulnerability has been resolved:

smb: client: validate the whole DACL before rewriting it in cifsacl

build_sec_desc() and id_mode_to_cifs_acl() derive a DACL pointer from a
server-supplied dacloffset and then use the incoming ACL to rebuild the
chmod/chown security descriptor.

The original fix only checked that the struct smb_acl header fits before
reading dacl_ptr->size or dacl_ptr->num_aces. That avoids the immediate
header-field OOB read, but the rewrite helpers still walk ACEs based on
pdacl->num_aces with no structural validation of the incoming DACL body.

A malicious server can return a truncated DACL that still contains a
header, claims one or more ACEs, and then drive
replace_sids_and_copy_aces() or set_chmod_dacl() past the validated
extent while they compare or copy attacker-controlled ACEs.

Factor the DACL structural checks into validate_dacl(), extend them to
validate each ACE against the DACL bounds, and use the shared validator
before the chmod/chown rebuild paths. parse_dacl() reuses the same
validator so the read-side parser and write-side rewrite paths agree on
what constitutes a well-formed incoming DACL.

AI Analysis

A vulnerability in the Linux kernel's SMB client allows a malicious server to return a truncated DACL, potentially leading to unauthorized access or data modification.

Basic Information

ID CVE-2026-31709

Source Linux

Published May 1, 2026 at 13:56

Modified May 3, 2026 at 05:45

Affected Product

Vendor Linux

Product Linux

Version bc3e9dd9d104ca1b75644eab87b38ce8a924aef4

Affected Versions Linux Linux bc3e9dd9d104ca1b75644eab87b38ce8a924aef4
Linux Linux bc3e9dd9d104ca1b75644eab87b38ce8a924aef4
Linux Linux 5.12

AI Assessment

AI Score 8.8 / 10

AI Severity High

Vendor Linux Foundation

Product Linux Kernel

Version 5.12, bc3e9dd9d104ca1b75644eab87b38ce8a924aef4

References

{
    "lastseen": "",
    "description": "In the Linux kernel, the following vulnerability has been resolved:\n\nsmb: client: validate the whole DACL before rewriting it in cifsacl\n\nbuild_sec_desc() and id_mode_to_cifs_acl() derive a DACL pointer from a\nserver-supplied dacloffset and then use the incoming ACL to rebuild the\nchmod/chown security descriptor.\n\nThe original fix only checked that the struct smb_acl header fits before\nreading dacl_ptr->size or dacl_ptr->num_aces.  That avoids the immediate\nheader-field OOB read, but the rewrite helpers still walk ACEs based on\npdacl->num_aces with no structural validation of the incoming DACL body.\n\nA malicious server can return a truncated DACL that still contains a\nheader, claims one or more ACEs, and then drive\nreplace_sids_and_copy_aces() or set_chmod_dacl() past the validated\nextent while they compare or copy attacker-controlled ACEs.\n\nFactor the DACL structural checks into validate_dacl(), extend them to\nvalidate each ACE against the DACL bounds, and use the shared validator\nbefore the chmod/chown rebuild paths.  parse_dacl() reuses the same\nvalidator so the read-side parser and write-side rewrite paths agree on\nwhat constitutes a well-formed incoming DACL.",
    "published": "2026-05-01T13:56:06.522Z",
    "modified": "2026-05-03T05:45:33.109Z",
    "type": "cve",
    "title": "smb: client: validate the whole DACL before rewriting it in cifsacl",
    "source": "Linux",
    "references": "https://git.kernel.org/stable/c/b78db9bddc84136f6a0bb49e8883cf200dfb87a8\nhttps://git.kernel.org/stable/c/0a8cf165566ba55a39fd0f4de172119dd646d39a",
    "id": "CVE-2026-31709",
    "bulletinFamily": "",
    "cwe": null,
    "cvelist": null,
    "sourceData": "Linux Linux bc3e9dd9d104ca1b75644eab87b38ce8a924aef4\nLinux Linux bc3e9dd9d104ca1b75644eab87b38ce8a924aef4\nLinux Linux 5.12",
    "sourceHref": "",
    "cvss": {
        "score": 8.8,
        "severity": "HIGH",
        "vector": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H",
        "version": "3.1"
    },
    "cvss2": [],
    "cvss3": {
        "version": "",
        "vectorString": "",
        "baseScore": 0,
        "baseSeverity": "",
        "attackVector": "",
        "attackComplexity": "",
        "privilegesRequired": "",
        "userInteraction": "",
        "scope": "",
        "confidentialityImpact": "",
        "integrityImpact": "",
        "availabilityImpact": "",
        "cvssV3": {
            "version": "",
            "vectorString": "",
            "baseScore": 0,
            "baseSeverity": "",
            "attackVector": "",
            "attackComplexity": "",
            "privilegesRequired": "",
            "userInteraction": "",
            "scope": "",
            "confidentialityImpact": "",
            "integrityImpact": "",
            "availabilityImpact": ""
        }
    },
    "href": "",
    "category_name": "CVE",
    "post_link": "",
    "product": "Linux",
    "version": "bc3e9dd9d104ca1b75644eab87b38ce8a924aef4",
    "vendor": "Linux",
    "ai_description": "A vulnerability in the Linux kernel's SMB client allows a malicious server to return a truncated DACL, potentially leading to unauthorized access or data modification.",
    "ai_severity": "High",
    "ai_vendor": "Linux Foundation",
    "ai_product": "Linux Kernel",
    "ai_version": "5.12, bc3e9dd9d104ca1b75644eab87b38ce8a924aef4",
    "ai_score": 8.8
}

smb: client: validate the whole DACL before rewriting it in cifsacl_CVE-2026-31709