CVE-2026-98696 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
USB: serial: fix driver deregistration order
USB serial driver modules register one driver for the USB bus and one or more drivers for the ports on the USB serial bus.
When unloading a driver module, the USB driver must be deregistered before the USB serial bus drivers so that I/O is stopped before unbinding the ports to avoid use-after-free in completion handlers accessing port data.
Note that the "new_id" attributes must first be removed to prevent new ids from being added and triggering a probe of the USB driver after it has been deregistered.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel's USB serial subsystem manages communication between host systems and various peripheral devices through a layered architecture involving both bus-level drivers and port-specific handlers. This architectural design requires careful synchronization during module initialization and teardown to maintain memory safety and system stability. A critical flaw was identified in the deregistration sequence of these driver modules, where the order of operations did not adequately protect against race conditions that could lead to use-after-free vulnerabilities. The vulnerability stems from an incorrect ordering when unloading a USB serial driver module, which registers one primary driver for the USB bus and potentially multiple drivers for individual ports on the virtual USB serial bus.
During normal operation, input/output requests are processed through completion handlers associated with specific port data structures. When a driver is being unloaded, it must ensure that all ongoing I/O operations are halted before any memory or structural bindings related to those ports are released. In the flawed implementation, the system failed to strictly enforce this sequence, allowing scenarios where completion handlers could still access port data after the underlying structures had been freed. This creates a classic use-after-free condition, where pointers referencing deallocated memory are dereferenced by active kernel threads or interrupt contexts. Such conditions can lead to unpredictable behavior, including kernel panics, privilege escalation if an attacker can control the contents of the freed memory region, or information disclosure through leaked data structures.
The root cause lies in the teardown logic not prioritizing the cessation of I/O streams before unbinding port drivers. Additionally, the handling of new_id attributes was insufficient during this process. These attributes allow dynamic addition of device IDs to a driver at runtime. If these are not removed prior to deregistering the main USB bus driver, there exists a window where a probe operation could be triggered for the already-deregistered driver. This race condition exacerbates the risk by potentially invoking initialization or binding routines on invalid memory contexts, further destabilizing the kernel state. The fix involves reordering these operations so that new_id attributes are removed first to prevent any new device probes from occurring after deregistration has begun. Subsequently, I/O is explicitly stopped across all active ports before proceeding with the unbinding of port drivers and finally unregistering the main USB bus driver.
From a classification perspective, this vulnerability aligns with CWE-416, Use After Free, as it involves accessing memory that has already been released due to improper sequencing in resource management. It also relates to CWE-362, Concurrent Execution using Shared Resource with Improper Synchronization, because the issue arises from race conditions between driver unloading and ongoing I/O completion handlers. In terms of the MITRE ATT&CK framework for enterprise security, this type of kernel-level vulnerability can be leveraged in techniques associated with Defense Evasion or Privilege Escalation, particularly if exploited to gain arbitrary code execution within the kernel space. Attackers targeting embedded systems or servers relying on USB serial connectivity could potentially exploit this flaw during module reloads or hot-plug events to compromise system integrity.
Mitigation strategies primarily involve applying the upstream Linux kernel patch that corrects the deregistration order. System administrators should ensure their kernels are updated with versions containing this fix, particularly in environments where USB serial devices are frequently connected and disconnected or where driver modules are dynamically loaded and unloaded as part of routine operations. For organizations managing large fleets of such systems, automated vulnerability scanning tools configured to detect outdated kernel versions can help identify affected instances. Additionally, enforcing strict module loading policies via SELinux or AppArmor profiles may limit the impact if an exploit is attempted by restricting which users or processes can unload critical USB serial drivers. Regular auditing of kernel logs for warnings related to USB driver unbinding and I/O errors can also serve as an early detection mechanism for potential exploitation attempts targeting this specific race condition.