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medium advisory

CVE-2026-43490: ksmbd inherited ACE SID length validation vulnerability

Microsoft published information about CVE-2026-43490, a vulnerability in ksmbd related to the validation of inherited ACE SID length.

On 2026-05-16, Microsoft disclosed CVE-2026-43490, a security vulnerability affecting ksmbd. The vulnerability relates to the validation of inherited Access Control Entry (ACE) Security Identifier (SID) lengths. Insufficient validation of ACE SID lengths can lead to various security issues. While the advisory provides limited technical details, the nature of the vulnerability suggests a potential for exploitation in scenarios involving file sharing and permissions management within the ksmbd implementation. The lack of specific exploitation details necessitates a focus on defensive measures and monitoring for anomalous behavior related to ksmbd.

Attack Chain

Due to the limited information available, a detailed attack chain is speculative. However, a potential attack chain could involve the following steps:

  1. An attacker gains access to a system where ksmbd is running and configured to share files.
  2. The attacker crafts a malicious ACE containing a malformed or oversized SID.
  3. The attacker attempts to apply this crafted ACE to a shared file or directory.
  4. ksmbd fails to properly validate the length of the SID in the ACE.
  5. This leads to a buffer overflow or other memory corruption issue during ACE processing.
  6. The attacker exploits this memory corruption to achieve code execution.
  7. The attacker escalates privileges and moves laterally within the network.

Impact

Successful exploitation of CVE-2026-43490 could allow an attacker to execute arbitrary code on a system running ksmbd. This could lead to data breaches, system compromise, and lateral movement within the network. The specific impact would depend on the privileges of the ksmbd process and the overall security posture of the affected system.

Recommendation

  • Apply the security updates released by Microsoft to address CVE-2026-43490 in ksmbd.
  • Monitor systems running ksmbd for suspicious file access patterns and ACE modifications.
  • Deploy the Sigma rule to detect suspicious ksmbd process behavior.
  • Review and harden file sharing permissions to minimize the potential attack surface.
  • Enable Sysmon process creation logging to improve detection capabilities for anomalous ksmbd behavior, enabling the Sigma rule.

Detection coverage 2

Detect Suspicious ksmbd Process Name

medium

Detects suspicious process names for ksmbd, which may indicate malicious activity related to CVE-2026-43490

sigma tactics: defense_evasion techniques: T1036 sources: process_creation, linux

Detect suspicious file access by ksmbd

high

Detects suspicious file access patterns by ksmbd, potentially indicating attempts to manipulate ACEs or exploit CVE-2026-43490.

sigma tactics: privilege_escalation techniques: T1068 sources: file_event, linux

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