CVE-2026-64398 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: add a permission check for FSCTL_SET_ZERO_DATA
FSCTL_SET_ZERO_DATA in smb2_ioctl() destroys file data via ksmbd_vfs_zero_data() -> vfs_fallocate(PUNCH_HOLE/ZERO_RANGE) after checking only the share-level KSMBD_TREE_CONN_FLAG_WRITABLE, with no per-handle access check. A handle opened with only FILE_WRITE_ATTRIBUTES still yields an FMODE_WRITE filp (FILE_WRITE_ATTRIBUTES is part of FILE_WRITE_DESIRE_ACCESS_LE, so smb2_create_open_flags() opens it O_WRONLY), so the vfs_fallocate FMODE_WRITE check does not stop it; only the missing fp->daccess gate would. Reproduced on mainline 7.1-rc7 with KASAN by an authenticated SMB client: a FILE_WRITE_ATTRIBUTES-only handle zeroed 4096 bytes of file data it had no FILE_WRITE_DATA right to (6/6; a FILE_READ_DATA-only handle was correctly denied).
This is the unfixed sibling of commit cc57232cae23 ("ksmbd: fix FSCTL permission bypass by adding a permission check for FSCTL_SET_SPARSE"). Because SET_ZERO_DATA writes data (not an attribute), require FILE_WRITE_DATA.
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Analysis
by VulDB Data Team • 07/25/2026
The vulnerability described represents a critical permission bypass flaw in the Linux kernel's ksmbd implementation that affects SMB2 file system operations. This issue specifically targets the FSCTL_SET_ZERO_DATA function within the smb2_ioctl() handler, which allows unauthorized data modification through improper access control checks. The vulnerability stems from a missing per-handle permission validation that should enforce FILE_WRITE_DATA rights when executing zero data operations, creating a scenario where handles with only FILE_WRITE_ATTRIBUTES permissions can inadvertently destroy file content.
The technical flaw manifests in the ksmbd_vfs_zero_data() function which calls vfs_fallocate() with PUNCH_HOLE/ZERO_RANGE parameters without performing adequate access validation. While the system correctly checks for share-level writability through KSMBD_TREE_CONN_FLAG_WRITABLE, it fails to validate individual handle permissions against the specific file access rights required for data modification operations. This oversight occurs because handles opened with FILE_WRITE_ATTRIBUTES only still receive FMODE_WRITE file descriptors due to the internal mapping of FILE_WRITE_DESIRE_ACCESS_LE flags during smb2_create_open_flags() processing.
The operational impact of this vulnerability is severe as it allows authenticated attackers to corrupt file data through seemingly benign operations that should only modify file attributes. The flaw was demonstrated using kernel version 7.1-rc7 with KASAN enabled, showing that a handle with FILE_WRITE_ATTRIBUTES rights could zero 4096 bytes of data that it should not have access to modify. This behavior directly contradicts Windows SMB semantics where such operations require explicit FILE_WRITE_DATA permissions, creating an inconsistency that can be exploited for data destruction attacks.
Security implications extend beyond simple data corruption as this vulnerability enables attackers with limited file permissions to perform unauthorized data modification operations that could compromise system integrity and confidentiality. The flaw operates at the kernel level through the ksmbd subsystem, making it particularly dangerous since it bypasses standard file system access controls. This type of vulnerability aligns with CWE-284 (Improper Access Control) and could be classified under ATT&CK technique T1070.006 (File Deletion) or T1566.001 (Phishing) when combined with other attack vectors that might lead to unauthorized access.
The vulnerability is related to a previously identified but unfixed sibling issue that was addressed in commit cc57232cae23, which fixed FSCTL permission bypass for FSCTL_SET_SPARSE operations. However, the SET_ZERO_DATA operation was overlooked during this security review, creating an asymmetric permission model where attribute modifications require appropriate rights while data modification operations do not. This creates a dangerous inconsistency that could allow attackers to perform partial data destruction without proper authorization, undermining the fundamental security model of access control in network file systems.
Mitigation strategies should focus on implementing proper per-handle access validation for FSCTL_SET_ZERO_DATA operations, ensuring that FILE_WRITE_DATA permissions are required before allowing zero range operations. The fix must be integrated into the ksmbd subsystem's permission checking logic to validate handle-specific access rights against the operation being performed, rather than relying solely on share-level writability flags. System administrators should also implement monitoring for unauthorized file modification activities and consider restricting SMB shares to minimize exposure risk while awaiting official patches that address this specific access control bypass vulnerability in the kernel's ksmbd implementation.
This vulnerability demonstrates the importance of comprehensive security reviews for file system operations, particularly those involving low-level data manipulation functions that can affect system integrity. The flaw highlights the need for strict adherence to principle of least privilege and proper separation of concerns between attribute modification and data modification operations within network file systems. Organizations should ensure their kernel versions include the relevant security patches and maintain awareness of similar vulnerabilities that might exist in other file system implementations or network protocols.