The Linux kernel vulnerability CVE-2026-64395 involves the ksmbd server's handling of the FSCTL_DUPLICATE_EXTENTS_TO_FILE control code. The source file handle was passed to vfs_clone_file_range() or vfs_copy_file_range() without verifying that the handle had read access (FILE_READ_DATA). Consequently, a handle opened with only attribute access could be exploited to copy file contents into a destination readable by an attacker. The fix enforces a requirement that the source handle must have FILE_READ_DATA permission before these VFS operations proceed, aligning with other ksmbd data-copy paths.

An attacker with a handle opened with attribute-only access could copy file contents to an attacker-readable destination, potentially exposing sensitive data. This bypasses intended access controls on the source file, leading to unauthorized data disclosure.

A fix has been implemented in the Linux kernel to require FILE_READ_DATA permission on the source handle before performing duplicate extent copy operations in ksmbd. Users should apply the official kernel updates that include this fix. Patch status is not explicitly confirmed in the provided data; therefore, check the vendor advisory or Linux kernel release notes for the specific patch and update accordingly.