CVE-2026-64398 ksmbd: add a permission check for FSCTL_SET_ZERO_DATA
An authenticated SMB client can zero file data without FILE_WRITE_DATA permission due to a missing per-handle access check in ksmbd.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-64398?
CVE-2026-64398 is a high-severity missing authorization vulnerability in Linux kernel, published on 25 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2026-64398?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-64398?
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-2026-64398 being exploited?
There is no known exploitation of CVE-2026-64398 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2026-64398, DEBIAN-CVE-2026-64398, GHSA-ch49-xqcg-99vv, ROOT-OS-DEBIAN-12-CVE-2026-64398, ROOT-OS-UBUNTU-2204-CVE-2026-64398, UBUNTU-CVE-2026-64398.