CVE-2026-31709

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.

CVSS Details

CVSS Score

8.8

Severity

HIGH

CVSS Vector

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

Configurations (Affected Products)

cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* - VULNERABLE

Linux Kernel (Stable branches prior to patch)

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

# Conceptual PoC for CVE-2026-31709
# This snippet demonstrates crafting the malicious DACL structure.
import struct

def craft_malicious_dacl():
    # SMB_ACL Header: Revision (2), Size (2), Num_ACEs (2), Sbz (2)
    revision = 2
    acl_size = 8  # Total size is just the header (truncated)
    num_aces = 0xFFFF  # Maliciously high number of ACEs claimed
    sbz = 0
    
    # Pack header: <HHHH means 4 unsigned shorts (little-endian)
    dacl_header = struct.pack('<HHHH', revision, acl_size, num_aces, sbz)
    return dacl_header

# This structure would be placed in the Security Descriptor of an SMB response
# When the Linux client parses this, it expects 0xFFFF ACEs but only has 8 bytes.
print(f"Malicious DACL: {craft_malicious_dacl().hex()}")

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-31709
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-31709
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-31709/
[4] VulDB https://vuldb.com/cve/CVE-2026-31709
[5] https://git.kernel.org/stable/c/0a8cf165566ba55a39fd0f4de172119dd646d39a
[6] https://git.kernel.org/stable/c/8e47d297e7cf9a6029a0d38e7b22faba7d7aaf12
[7] https://git.kernel.org/stable/c/b78db9bddc84136f6a0bb49e8883cf200dfb87a8
[8] https://git.kernel.org/stable/c/d92f3f0b22414e7515696a02224d0af55e3004a3

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-31709", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-05-01T14:16:20.950", "lastModified": "2026-05-17T16:16:15.933", "vulnStatus": "Modified", "cveTags": [], "descriptions": [{"lang": "en", "value": "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."}], "metrics": {"cvssMetricV31": [{"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H", "baseScore": 8.8, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH"}, "exploitabilityScore": 2.8, "impactScore": 5.9}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "NVD-CWE-noinfo"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.12", "versionEndExcluding": "7.0.2", "matchCriteriaId": "57E416E8-C706-4061-8BC1-7C61879FD612"}]}]}], "references": [{"url": "https://git.kernel.org/stable/c/0a8cf165566ba55a39fd0f4de172119dd646d39a", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/8e47d297e7cf9a6029a0d38e7b22faba7d7aaf12", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}, {"url": "https://git.kernel.org/stable/c/b78db9bddc84136f6a0bb49e8883cf200dfb87a8", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/d92f3f0b22414e7515696a02224d0af55e3004a3", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}]}}