CVE-2026-64396 ksmbd: fix UAF of struct file_lock in SMB2_LOCK deferred-lock cancellation
A use-after-free in the Linux kernel's ksmbd module allows attackers to execute arbitrary code or escalate privileges via SMB2 lock cancellation.
- 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-2026-64396?
CVE-2026-64396 is a high-severity use after free 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: fix UAF of struct file_lock in SMB2_LOCK deferred-lock cancellation
When a blocking byte-range lock request is deferred in the FILE_LOCK_DEFERRED path, ksmbd registers the asynchronous work into the connection's async_requests list via setup_async_work(). The cancel callback smb2_remove_blocked_lock() holds a reference to the flock.
If the lock waiter is subsequently woken up but the work state is no longer KSMBD_WORK_ACTIVE (e.g., due to a concurrent cancellation), the cleanup path calls locks_free_lock(flock) without dequeuing the work from the async_requests list. Concurrently, smb2_cancel() walks the list under conn->request_lock and invokes the cancel callback, which then dereferences the already freed 'flock'. This leads to a slab-use-after-free inside __wake_up_common.
Fix this by restructuring the cleanup logic after the worker returns from ksmbd_vfs_posix_lock_wait(). Move list_del(&smb_lock->llist) and release_async_work(work) to the top of the cleanup block. This guarantees that the async work is completely dequeued and serialized under conn->request_lock before locks_free_lock(flock) is called, rendering the flock unreachable for any concurrent smb2_cancel().
Weakness: CWE-416
Which versions are affected by CVE-2026-64396?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-64396?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-64396 being exploited?
There is no known exploitation of CVE-2026-64396 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2026-64396, DEBIAN-CVE-2026-64396, GHSA-w2fg-c334-xw3q, ROOT-OS-DEBIAN-12-CVE-2026-64396, ROOT-OS-UBUNTU-2204-CVE-2026-64396, UBUNTU-CVE-2026-64396.