{"id":"CVE-2026-31712","summary":"ksmbd: require minimum ACE size in smb_check_perm_dacl()","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(&sid, &ace-\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.","modified":"2026-05-28T03:55:24.516850871Z","published":"2026-05-01T13:56:08.583Z","related":["CGA-2wg7-hvgm-x7rg","openSUSE-SU-2026:10793-1"],"database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31712.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/151b1799861fde38087c08f613abc2843ef597b0"},{"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":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31712.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-31712"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9"},{"fixed":"325d4ac11f526cb8964cff14548ccf02d8c756d8"},{"fixed":"95e5aa3c3261da8c95b27d7aecf8ee39b9f86a4c"},{"fixed":"90089584b2e25c4510b7b987387b4405f0673ece"},{"fixed":"151b1799861fde38087c08f613abc2843ef597b0"},{"fixed":"d07b26f39246a82399661936dd0c853983cfade7"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-31712.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"5.15.0"},{"fixed":"6.6.140"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.84"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.25"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.0.2"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-31712.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H"}]}