CVE-2026-98809 Bluetooth: hci_sync: don't drain cmd_sync backlog on unregister
A local attacker can cause a denial of service in the Linux kernel by exhausting the Bluetooth HCI command backlog, leading to system hangs during device unregistration.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-98809?
CVE-2026-98809 is a denial of service vulnerability in Linux, published on 18 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
Bluetooth: hci_sync: don't drain cmd_sync backlog on unregister
hci_unregister_dev() disables cmd_work and cmd_timer, then calls hci_cmd_sync_clear(), whose cancel_work_sync() waits for hci_cmd_sync_work() to return. That worker dequeues and runs every entry on cmd_sync_work_list. With cmd_work disabled nothing reaches the controller, so each queued HCI command waits the full HCI_CMD_TIMEOUT (2s).
The backlog has no bound. Userspace can keep queuing MGMT commands such as MGMT_OP_GET_CLOCK_INFO against a controller that doesn't answer. Closing /dev/vhci then blocks in vhci_release() for backlog * 2s:
INFO: task syz-executor:5749 blocked for more than 143 seconds. cancel_work_sync hci_cmd_sync_clear hci_unregister_dev vhci_release
In a local reproduction the backlog held more than 5000 entries, which comes to hours of hang.
Stop the worker from taking new entries once HCI_UNREGISTER is set, and wake any request still waiting with -ENODEV before cancelling the work. The entries left over are destroyed with -ECANCELED by the existing sweep in hci_cmd_sync_clear(). They stay on the list until the work has stopped, so hci_cmd_sync_dequeue() and friends still see them. A callback already running may still issue another command, which delays unregister by at most one timeout per remaining command, not by the whole backlog.
Which versions are affected by CVE-2026-98809?
How do I fix CVE-2026-98809?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-98809 being exploited?
There is no known exploitation of CVE-2026-98809 in the wild and no public exploit code.
Sources
Also known as AZL-108714, BELL-CVE-2026-98809, DEBIAN-CVE-2026-98809, GHSA-h4f7-ppg7-73c7, UBUNTU-CVE-2026-98809.