CVE-2026-52944
MediumCVSS 5.5Exploitation Probability (EPSS)
Low risk2th percentile - higher than 2% of all known CVEs
Summary
In the Linux kernel's ksmbd (SMB) filesystem, a missing access control for FSCTL_SET_SPARSE was found. The fsctl_set_sparse() function modifies the sparse attribute of a file without checking if the share is read-only or if the client has proper permissions (FILE_WRITE_DATA or FILE_WRITE_ATTRIBUTES).
Risk Assessment
The organization is at risk of unauthorized file attribute modification by users with read-only access or insufficient rights, potentially leading to data integrity breaches and security policy violations.
Recommendation
Immediately update the Linux kernel to a version containing the fix (commit adding permission checks for FSCTL_SET_SPARSE). Until the update, restrict SMB share access to trusted clients.
Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: ksmbd: fix FSCTL permission bypass by adding a permission check for FSCTL_SET_SPARSE FSCTL_SET_SPARSE in fsctl_set_sparse() modifies the file's sparse attribute and saves it through xattr without any permission checks. This exposes two issues: 1) A client on a read-only share can change the sparse attribute on files it opened, even though the share is read-only. Other FSCTL write operations already check test_tree_conn_flag(work->tcon, KSMBD_TREE_CONN_FLAG_WRITABLE), but FSCTL_SET_SPARSE does not. 2) Even on writable shares, clients without FILE_WRITE_DATA or FILE_WRITE_ATTRIBUTES access should not modify the sparse attribute. Similar handle-level checks exist in other functions but are missing here. Add both share-level writable check and per-handle access check. Use goto out on error to avoid leaking file references.