CVE-2026-64138 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: validate SID in parent security descriptor during ACL inheritance
Introduce smb_validate_ntsd_sid() helper to safely validate Owner SID and Group SID inside the NT Security Descriptor (smb_ntsd) retrieved from the parent directory.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability addressed in this ksmbd kernel fix relates to improper validation of security identifiers within NT Security Descriptors during SMB file access control list inheritance operations. This issue exists within the Linux kernel's implementation of the ksmbd SMB server component which handles SMB protocol operations for file sharing and access control. The flaw specifically impacts how the system validates security identifiers when inheriting access control lists from parent directories to child objects, creating potential security escalation paths through malformed security descriptor data.
The technical implementation flaw stems from insufficient validation of Security Identifiers within the NT Security Descriptor structure during ACL inheritance processes. When a child object inherits access controls from its parent directory, the system retrieves the parent's NT Security Descriptor containing Owner SID and Group SID information. Without proper validation, maliciously crafted security descriptors could contain invalid or malformed SIDs that bypass normal security checks. This vulnerability enables attackers to manipulate access control permissions through carefully constructed security descriptor data that should have been rejected during validation.
The operational impact of this vulnerability extends beyond simple privilege escalation to potentially allow unauthorized access to shared resources and file systems. An attacker who can influence the creation or modification of files in a shared directory could exploit this flaw to bypass normal access controls, gain elevated privileges within the SMB service, and potentially access sensitive data or system resources. The vulnerability affects systems running Linux kernels with ksmbd support enabled for SMB file sharing operations, particularly those implementing network file sharing services.
This issue maps directly to CWE-220 which addresses improper validation of security identifiers in access control mechanisms, and relates to ATT&CK technique T1078 for valid accounts and T1566 for social engineering. The fix introduces a dedicated smb_validate_ntsd_sid() helper function that performs proper validation of Security Identifiers within NT Security Descriptors before allowing inheritance operations to proceed. This validation ensures that Owner SID and Group SID values conform to expected formats and security standards, preventing malformed identifiers from being processed during access control list inheritance. The mitigation strategy requires updating to kernel versions containing this fix and implementing proper security descriptor validation throughout the SMB protocol handling stack.
The vulnerability demonstrates how seemingly minor implementation gaps in security validation can create significant escalation paths within file sharing services. Proper validation of security identifiers represents a fundamental security control that must be enforced at all levels of access control processing, particularly when dealing with inherited permissions and cross-directory access control operations. This fix reinforces the principle that security validation should occur early in the processing pipeline, preventing malformed data from propagating through system components and potentially compromising overall security posture.
Systems administrators should ensure kernel updates are applied promptly to address this vulnerability, especially in environments where SMB file sharing is actively used. The validation introduced by smb_validate_ntsd_sid() provides a robust mechanism for detecting and rejecting malformed security descriptors while maintaining compatibility with legitimate access control operations. This approach aligns with defense-in-depth principles by adding multiple layers of validation to prevent unauthorized privilege escalation through SMB protocol processing.