CVE-2026-89738 usb: gadget: at91_udc: drain polled-VBUS timer/work before udc is freed
A use-after-free vulnerability in the Linux kernel's at91_udc USB gadget driver allows attackers to cause a denial of service or potentially execute code during driver unbind.
- 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-2026-89738?
CVE-2026-89738 is a high-severity use after free vulnerability in Linux, published on 11 Sep 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:
usb: gadget: at91_udc: drain polled-VBUS timer/work before udc is freed
In polled-VBUS mode (board.vbus_pin && board.vbus_polled), probe arms a self-restarting cycle: at91_vbus_timer() schedules vbus_timer_work, and at91_vbus_timer_work() calls at91_vbus_update() and re-arms the timer via mod_timer(). Both recover the same udc through container_of and dereference it on every iteration.
Neither teardown path cancels this cycle. udc is devm-allocated, so it is freed after at91udc_remove() returns, and is likewise freed when probe fails and devres runs. A timer callback or work item that is pending or running at either point dereferences the freed udc.
Add at91_udc_shutdown_vbus_timer() and call it from at91udc_remove() and from the usb_add_gadget_udc() failure path in probe; the remaining probe error paths fail before the timer is armed. timer_shutdown_sync() waits for a running callback and clears timer->function, which makes the work handler's mod_timer() a permanent no-op; cancel_work_sync() then drains any pending or running work whose re-arm attempt now does nothing. The timer must be shut down first, since cancelling the work alone would let the timer re-queue it. The guard mirrors probe: in IRQ mode the timer and work_struct are never initialized.
This does not require a fault; a normal driver unbind can interleave with an already queued work item.
This issue was found by an in-house static analysis tool.
Which versions are affected by CVE-2026-89738?
How do I fix CVE-2026-89738?
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-2026-89738 being exploited?
There is no known exploitation of CVE-2026-89738 in the wild and no public exploit code.
Sources
Also known as AZL-100542, BELL-CVE-2026-89738, DEBIAN-CVE-2026-89738, GHSA-3f57-g7j5-hj3q, UBUNTU-CVE-2026-89738.