CVE-2026-64399 ksmbd: add permission checks for FSCTL_DUPLICATE_EXTENTS_TO_FILE
A missing permission check in the Linux kernel ksmbd module allows clients to overwrite file data on read-only shares or with insufficient write permissions.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-64399?
CVE-2026-64399 is a critical missing authorization vulnerability in Linux kernel, published on 25 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: add permission checks for FSCTL_DUPLICATE_EXTENTS_TO_FILE
The FSCTL_DUPLICATE_EXTENTS_TO_FILE arm of smb2_ioctl() overwrites the destination file's data via vfs_clone_file_range() with neither the share-level KSMBD_TREE_CONN_FLAG_WRITABLE check nor a per-handle fp->daccess check that the other write-bearing arms carry. A client can overwrite destination data on a read-only share, or from a handle opened with only FILE_WRITE_ATTRIBUTES (which still yields an FMODE_WRITE filp). FILE_WRITE_ATTRIBUTES-only destination handle overwrote the file's data via the clone. Add both checks, matching the FSCTL_SET_SPARSE permission fix; require FILE_WRITE_DATA since this writes data.
Which versions are affected by CVE-2026-64399?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-64399?
Apply the vendor's patch.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-64399 being exploited?
There is no known exploitation of CVE-2026-64399 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2026-64399, DEBIAN-CVE-2026-64399, GHSA-mrp6-592v-c3fh, ROOT-OS-DEBIAN-12-CVE-2026-64399, ROOT-OS-UBUNTU-2204-CVE-2026-64399, UBUNTU-CVE-2026-64399.