GHSA-59CP-8V32-7335
Vulnerability from github – Published: 2026-05-01 15:30 – Updated: 2026-06-19 15:33In the Linux kernel, the following vulnerability has been resolved:
ksmbd: require minimum ACE size in smb_check_perm_dacl()
Both ACE-walk loops in smb_check_perm_dacl() only guard against an
under-sized remaining buffer, not against an ACE whose declared
ace->size is smaller than the struct it claims to describe:
if (offsetof(struct smb_ace, access_req) > aces_size) break; ace_size = le16_to_cpu(ace->size); if (ace_size > aces_size) break;
The first check only requires the 4-byte ACE header to be in bounds; it does not require access_req (4 bytes at offset 4) to be readable. An attacker who has set a crafted DACL on a file they own can declare ace->size == 4 with aces_size == 4, pass both checks, and then
granted |= le32_to_cpu(ace->access_req); / upper loop / compare_sids(&sid, &ace->sid); / lower loop /
reads access_req at offset 4 (OOB by up to 4 bytes) and ace->sid at offset 8 (OOB by up to CIFS_SID_BASE_SIZE + SID_MAX_SUB_AUTHORITIES * 4 bytes).
Tighten both loops to require
ace_size >= offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE
which is the smallest valid on-wire ACE layout (4-byte header + 4-byte access_req + 8-byte sid base with zero sub-auths). Also reject ACEs whose sid.num_subauth exceeds SID_MAX_SUB_AUTHORITIES before letting compare_sids() dereference sub_auth[] entries.
parse_sec_desc() already enforces an equivalent check (lines 441-448); smb_check_perm_dacl() simply grew weaker validation over time.
Reachability: authenticated SMB client with permission to set an ACL on a file. On a subsequent CREATE against that file, the kernel walks the stored DACL via smb_check_perm_dacl() and triggers the OOB read. Not pre-auth, and the OOB read is not reflected to the attacker, but KASAN reports and kernel state corruption are possible.
{
"affected": [],
"aliases": [
"CVE-2026-31712"
],
"database_specific": {
"cwe_ids": [
"CWE-787"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-05-01T14:16:21Z",
"severity": "HIGH"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nksmbd: require minimum ACE size in smb_check_perm_dacl()\n\nBoth ACE-walk loops in smb_check_perm_dacl() only guard against an\nunder-sized remaining buffer, not against an ACE whose declared\n`ace-\u003esize` is smaller than the struct it claims to describe:\n\n if (offsetof(struct smb_ace, access_req) \u003e aces_size)\n break;\n ace_size = le16_to_cpu(ace-\u003esize);\n if (ace_size \u003e aces_size)\n break;\n\nThe first check only requires the 4-byte ACE header to be in bounds;\nit does not require access_req (4 bytes at offset 4) to be readable.\nAn attacker who has set a crafted DACL on a file they own can declare\nace-\u003esize == 4 with aces_size == 4, pass both checks, and then\n\n granted |= le32_to_cpu(ace-\u003eaccess_req); /* upper loop */\n compare_sids(\u0026sid, \u0026ace-\u003esid); /* lower loop */\n\nreads access_req at offset 4 (OOB by up to 4 bytes) and ace-\u003esid at\noffset 8 (OOB by up to CIFS_SID_BASE_SIZE + SID_MAX_SUB_AUTHORITIES\n* 4 bytes).\n\nTighten both loops to require\n\n ace_size \u003e= offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE\n\nwhich is the smallest valid on-wire ACE layout (4-byte header +\n4-byte access_req + 8-byte sid base with zero sub-auths). Also\nreject ACEs whose sid.num_subauth exceeds SID_MAX_SUB_AUTHORITIES\nbefore letting compare_sids() dereference sub_auth[] entries.\n\nparse_sec_desc() already enforces an equivalent check (lines 441-448);\nsmb_check_perm_dacl() simply grew weaker validation over time.\n\nReachability: authenticated SMB client with permission to set an ACL\non a file. On a subsequent CREATE against that file, the kernel\nwalks the stored DACL via smb_check_perm_dacl() and triggers the\nOOB read. Not pre-auth, and the OOB read is not reflected to the\nattacker, but KASAN reports and kernel state corruption are\npossible.",
"id": "GHSA-59cp-8v32-7335",
"modified": "2026-06-19T15:33:08Z",
"published": "2026-05-01T15:30:33Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-31712"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/151b1799861fde38087c08f613abc2843ef597b0"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/282cbbb476b9f35793452bc461934af4c7eca169"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/325d4ac11f526cb8964cff14548ccf02d8c756d8"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/90089584b2e25c4510b7b987387b4405f0673ece"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/95e5aa3c3261da8c95b27d7aecf8ee39b9f86a4c"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/d07b26f39246a82399661936dd0c853983cfade7"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/f20adc4ef7428bc485ee83fd1a592252fb87718b"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H",
"type": "CVSS_V3"
}
]
}
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