CVE-2026-64392 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: use opener credentials for delete-on-close
Delete-on-close can be completed by deferred or durable handle teardown, where no request work is available. Both the base-file unlink and the ADS xattr removal consequently run with the ksmbd worker credentials and can bypass filesystem permission checks.
Run both operations with the credentials captured in struct file when the handle was opened. This preserves the authenticated user's fsuid, fsgid, supplementary groups and capability restrictions at final close.
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Analysis
by VulDB Data Team • 07/26/2026
The vulnerability in question affects the linux kernel's ksmbd implementation which provides smb2/smb3 server functionality for linux systems. This issue represents a privilege escalation weakness that occurs during file deletion operations when handles are closed, particularly in scenarios involving delete-on-close functionality. The flaw stems from improper credential handling during deferred or durable handle teardown processes where the system executes cleanup operations without maintaining proper user context.
The technical root cause involves the ksmbd subsystem executing both base-file unlink operations and alternate data stream xattr removal using worker thread credentials rather than preserving the original opener's authentication context. When handles are closed, whether through normal termination or deferred cleanup mechanisms, the system should maintain the security context established when the file handle was initially opened. However, this implementation fails to capture and utilize the correct user credentials at the time of final close operations.
This vulnerability enables attackers to bypass filesystem permission checks by executing deletion operations with elevated privileges that belong to the ksmbd worker processes rather than the authenticated user who originally opened the file. The security implications extend beyond simple file deletion as the compromised operations can potentially access or modify files that should otherwise be restricted based on the original user's permissions and capabilities.
The operational impact of this vulnerability manifests when legitimate users with valid smb sessions attempt to perform delete-on-close operations on files they have access to, but the underlying system executes these operations with reduced security context. This creates a scenario where unauthorized file modifications or deletions could occur through indirect means, particularly in multi-user environments where proper access controls should prevent such behavior.
The fix implemented addresses this by capturing and preserving the opener credentials within the struct file structure when handles are initially established. This ensures that both base-file unlink operations and ADS xattr removal execute with the authenticated user's fsuid, fsgid, supplementary groups, and capability restrictions intact at the final close event. The solution aligns with security best practices by maintaining proper privilege separation and ensuring that cleanup operations honor the original user's security context.
From a cybersecurity perspective, this vulnerability maps to CWE-284 Access Control Issues and potentially relates to ATT&CK techniques involving privilege escalation through service manipulation. The fix demonstrates proper credential handling practices that should be implemented in all kernel subsystems dealing with user authentication contexts and file system operations, particularly those involving deferred execution paths where the original security context might otherwise be lost.
This issue highlights the importance of maintaining security context throughout complex operation lifecycles in kernel space implementations. The resolution ensures that even during asynchronous cleanup operations, the fundamental security principles of least privilege and proper authentication are maintained, preventing potential abuse scenarios where system resources could be accessed or modified beyond what the original authenticated user should have permission to perform.