CVE-2026-31712

Estado: ModificadaAlta (8.3)—

In 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:

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

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).

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Tighten both loops to require

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.

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.

Detalles técnicos trazas, registros y código del informe original

  if (offsetof(struct smb_ace, access_req) > aces_size)
      break;
  ace_size = le16_to_cpu(ace->size);
  if (ace_size > aces_size)
      break;

  granted |= le32_to_cpu(ace->access_req);               /* upper loop */
  compare_sids(&sid, &ace->sid);                         /* lower loop */

  ace_size >= offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE

parse_sec_desc() already enforces an equivalent check (lines 441-448);
smb_check_perm_dacl() simply grew weaker validation over time.

CVSS

Versión: 3.1
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H
Puntuación base: 8.3

Probabilidad de explotación (EPSS)

Probabilidad de explotación en los próximos 30 días: 0.53%
Percentil entre todas las CVEs puntuadas: 43
Fecha de la puntuación: 5/10/2026

EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).

🎯 Técnicas ATT&CK

Cómo se explota esta vulnerabilidad y qué consigue el atacante, en el lenguaje de MITRE ATT&CK.

ExplotaciónT1210 Exploitation of Remote Serviceslateral movement75 %
Impacto principalT1059 Command and Scripting Interpreterexecution60 %
Impacto secundarioT1565.001 Stored Data Manipulationimpact55 %

Acceso remoto autenticado (AV:N/PR:L) a ksmbd SMB. OOB read en validación DACL permite corrupción de estado del kernel y ejecución indirecta tras crear archivo con ACL manipulado.

Inferido por nuestro agente de análisis a partir de la descripción oficial, el vector CVSS y la CWE, y comprobado por un supervisor. Puede contener errores.

🛡️ Mitigaciones ATT&CK que cubren estas técnicas

Privileged Account Management Disable or Remove Feature or Program Vulnerability Scanning Threat Intelligence Program Restrict Web-Based Content Restrict File and Directory Permissions

Tecnologías afectadas (1)

Linux — Linux Kernel

CWE

CWE-787

Referencias

JSON original (NVD)

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      "value": "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->size` is smaller than the struct it claims to describe:\n\n  if (offsetof(struct smb_ace, access_req) > aces_size)\n      break;\n  ace_size = le16_to_cpu(ace->size);\n  if (ace_size > 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->size == 4 with aces_size == 4, pass both checks, and then\n\n  granted |= le32_to_cpu(ace->access_req);               /* upper loop */\n  compare_sids(&sid, &ace->sid);                         /* lower loop */\n\nreads access_req at offset 4 (OOB by up to 4 bytes) and ace->sid 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 >= 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."
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