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medium threat

CVE-2026-31712: ksmbd Minimum ACE Size Vulnerability

CVE-2026-31712 is a security vulnerability in ksmbd requiring a minimum ACE size check in smb_check_perm_dacl(), potentially leading to unauthorized access or privilege escalation.

CVE-2026-31712 is a vulnerability affecting ksmbd related to access control list (ACL) entry size validation within the smb_check_perm_dacl() function. The vulnerability arises from a failure to enforce a minimum size requirement for Access Control Entries (ACEs) when evaluating permissions. This oversight could potentially be exploited by a malicious actor to craft specially crafted ACEs, leading to unintended access control behavior. Given the lack of specifics provided by the advisory, the exact exploitation mechanism and impact require further investigation. However, this type of vulnerability can potentially lead to information disclosure, privilege escalation, or denial of service. It is crucial to apply the provided patch to prevent potential exploitation.

Attack Chain

Due to limited information, a detailed attack chain cannot be fully established. However, a potential exploitation scenario could involve the following steps:

  1. An attacker gains initial access to a system with a vulnerable ksmbd implementation.
  2. The attacker crafts a malicious SMB request containing a specially crafted DACL with undersized ACEs.
  3. The SMB server processes the request and calls the smb_check_perm_dacl() function.
  4. The function fails to properly validate the size of the ACEs within the DACL.
  5. The server grants unauthorized access or permissions to the attacker based on the malformed ACEs.
  6. The attacker leverages the gained access to perform unauthorized actions, such as reading sensitive files or executing arbitrary code.
  7. The attacker escalates privileges within the system.
  8. The attacker achieves their objective, such as data exfiltration or system compromise.

Impact

Successful exploitation of CVE-2026-31712 could lead to unauthorized access to sensitive data, privilege escalation, or potentially denial of service. The precise impact depends on the specific configuration and access control policies of the affected system. However, any vulnerability related to ACL validation carries significant risk, potentially undermining the security posture of the entire system. The number of affected systems depends on the adoption rate of the vulnerable ksmbd version.

Recommendation

  • Apply the security update provided by Microsoft to patch CVE-2026-31712 on all affected systems using ksmbd.
  • Monitor SMB traffic for suspicious requests containing malformed DACLs that might attempt to exploit this vulnerability. Use a network intrusion detection system (NIDS) to detect abnormal SMB activity (see rules below).
  • Enable and review SMB server logs for any errors or warnings related to ACL processing, which may indicate exploitation attempts.

Detection coverage 2

Detect CVE-2026-31712 Exploitation Attempt - Malformed SMB DACL

medium

Detects CVE-2026-31712 exploitation attempt — suspicious SMB requests with malformed DACLs potentially exploiting insufficient ACE size validation in ksmbd

sigma tactics: privilege_escalation techniques: T1068 sources: network_connection, windows

Detect CVE-2026-31712 Exploitation Attempt - SMB with suspicious ACE structure

medium

Detects CVE-2026-31712 exploitation attempt — detects unusual SMB traffic with suspicious ACE (Access Control Entry) structures indicating a possible exploit.

sigma tactics: privilege_escalation techniques: T1068 sources: network_connection, windows

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