CVE-2021-47434 is a vulnerability identified in the Linux kernel's xHCI (Extensible Host Controller Interface) USB controller driver. The issue arises from improper handling of the command ring control register (CRCR) during the aborting of a command. The CRCR is a 64-bit register where bits [6:63] represent the command ring pointer and bits [0:3] represent control bits such as command stop and abort. When aborting a command, the kernel reads the CRCR, sets the abort bit, and writes back to the CRCR. However, the read operation returns the command ring pointer as all zeros, causing the write operation to only update the control bits. Because the 64-bit write is split into two 32-bit writes, there is a race condition where the xHCI controller may stop the command ring before the upper 32 bits (which are zeros) are written. This results in the xHCI controller updating its internal command ring pointer upper dword to zero, leading to memory access failures when the command ring is restarted. The fix involves modifying the write operation to only update the lower 32 bits of the CRCR, where the control bits reside, preventing the corruption of the command ring pointer. This vulnerability can cause the USB controller to malfunction, potentially leading to system instability or denial of service due to memory access errors in the USB subsystem. No known exploits are currently reported in the wild, and the vulnerability was published on May 22, 2024.

For European organizations, this vulnerability could have significant operational impacts, especially for those relying heavily on Linux-based systems with USB peripherals. The xHCI controller is critical for USB device management; corruption of its command ring pointer can cause USB devices to malfunction or disconnect unexpectedly, potentially disrupting workflows that depend on USB input devices, storage, or specialized hardware. In environments such as data centers, industrial control systems, or healthcare where Linux servers or workstations are prevalent, this could lead to system crashes or denial of service conditions, affecting availability and operational continuity. Although the vulnerability does not directly expose confidentiality or integrity risks, the resulting instability could indirectly impact business operations and service availability. The lack of known exploits reduces immediate risk, but the vulnerability’s presence in the Linux kernel means it affects a broad range of devices and distributions used across Europe.

European organizations should prioritize applying the official Linux kernel patches that address CVE-2021-47434 as soon as they become available from their Linux distribution vendors. Specifically, ensure kernel versions are updated to include the fix that restricts writes to the lower 32 bits of the CRCR during command aborts. Organizations should also audit their Linux systems to identify those running affected kernel versions and plan timely patch management. For critical systems where immediate patching is not feasible, consider temporarily disabling USB devices or limiting USB usage to reduce exposure. Monitoring system logs for xHCI-related errors or USB device failures can help detect exploitation attempts or instability caused by this vulnerability. Additionally, testing patches in staging environments before deployment can prevent unintended disruptions. Finally, maintaining robust backup and recovery procedures will mitigate potential downtime caused by USB subsystem failures.