CVE-2026-98773 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
USB: cdc-acm: skip URB restart in port_shutdown if disconnected
acm_port_shutdown() is called from the tty-port machinery whenever the last user closes the port. acm_disconnect() sets acm->disconnected and calls tty_port_tty_vhangup(), which eventually triggers port_shutdown(). At that point acm_disconnect() has already called acm_poison_urbs() and is about to free the control and read URBs.
However, acm_port_shutdown() unconditionally calls acm_unpoison_urbs() and, when the ALWAYS_POLL_CTRL quirk is set, immediately re-submits the ctrlurb and all read URBs. If the concurrent acm_disconnect() then frees those URBs and their DMA buffers while they are in-flight, the result is a use-after-free in acm_ctrl_irq() or acm_read_bulk_callback() accessing the freed DMA memory.
Fix by checking acm->disconnected after draining the delayed anchor and returning early: if we are in a disconnect path the URBs must remain poisoned and must not be resubmitted.
The early return is placed after the delayed-anchor drain loop because that loop only cancels write-side accounting (wb->use = false and autopm counter) and does not touch the hardware path.
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Analysis
by VulDB Data Team • 10/19/2026
This vulnerability represents a critical use-after-free condition within the Linux kernel's USB CDC ACM driver, specifically affecting the interaction between port shutdown procedures and device disconnection logic. The flaw originates in the acm_port_shutdown function, which is invoked by the tty-port machinery when the last user closes a serial port interface. Under normal circumstances, this routine manages resource cleanup or re-initialization for active ports. However, a race condition emerges when a physical USB disconnect occurs simultaneously with the port closure sequence. During a disconnection event, the acm_disconnect function sets an internal disconnected flag and initiates the teardown of control and read Uniform Request Blocks (URBs), including calling acm_poison_urbs to mark them as invalid before freeing their associated DMA memory buffers.
The core technical flaw lies in the unconditional execution path within acm_port_shutdown. Despite the device being in a state where URBs are being freed by the concurrent disconnect routine, this shutdown function proceeds to call acm_unpoison_urbs and, if specific quirks such as ALWAYS_POLL_CTRL are enabled, immediately resubmits these control and read URBs back into the USB subsystem. Because the disconnection process has already initiated or completed the freeing of the underlying DMA buffers for these URBs, any subsequent access to this memory by the kernel's interrupt handlers results in a use-after-free vulnerability. Specifically, when the hardware triggers an interrupt upon receiving data on the control endpoint, the acm_ctrl_irq callback is executed. Similarly, bulk read completions trigger acm_read_bulk_callback. Both of these callbacks attempt to process or inspect the URB structures and their associated DMA buffers that have already been freed by the disconnect routine, leading to memory corruption, kernel panics, or potential arbitrary code execution if an attacker can influence the contents of the reclaimed memory.
From a classification perspective, this vulnerability aligns with CWE-416, Use After Free, as it involves accessing memory after it has been returned to the system for reuse without ensuring that no further references exist. In terms of adversarial tactics, this scenario relates to ATT&CK technique T1059, Command and Scripting Interpreter, if exploited via kernel privilege escalation, or more broadly to exploitation techniques targeting driver-level stability such as those found in T1203, Exploitation for Defense Evasion, where system instability is induced. The vulnerability highlights the complexity of managing asynchronous hardware events within the Linux kernel's USB subsystem, particularly when multiple threads handle device lifecycle changes concurrently without sufficient synchronization barriers around critical resource deallocation points.
The operational impact of this flaw includes severe system instability, manifesting as kernel oopses or full panics upon unplugging a CDC ACM compliant device while its port is being closed by an application. In more sophisticated attack scenarios, the ability to trigger use-after-free conditions in kernel space allows for potential privilege escalation from user mode to ring 0. An attacker could potentially craft a sequence of operations that forces the race condition reliably, allowing them to overwrite function pointers or critical data structures within the freed DMA memory regions before they are reallocated for other purposes. This undermines the integrity and availability guarantees provided by the operating system's kernel layer.
To mitigate this vulnerability, the primary remediation involves modifying the acm_port_shutdown logic to explicitly check the device's disconnected status after draining any delayed anchors but before attempting to unpoison or resubmit URBs. By returning early if the disconnected flag is set, the driver ensures that no new I/O requests are submitted for resources that are in the process of being torn down by the disconnect routine. This synchronization mechanism prevents the race condition by ensuring that once a device begins its disconnection sequence, subsequent shutdown calls from tty-port machinery do not interfere with or re-activate hardware interactions. System administrators and developers should apply kernel updates that include this patch to restore safe handling of USB CDC ACM devices during hot-unplug scenarios.