CVE-2021-47646 is a high-severity use-after-free (UAF) vulnerability in the Linux kernel's block layer, specifically related to the Budget Fair Queueing (BFQ) I/O scheduler. The vulnerability arises from improper handling of queue merges in the BFQ scheduler, where a commit intended to fix a crash inadvertently triggered a use-after-free condition due to a different underlying bug. The issue was introduced by a previous commit that did not correctly reset the 'last_bfqq_created' pointer on group changes, leading to potential dereferencing of freed memory. This vulnerability can cause kernel crashes and potentially allow an attacker with limited privileges (local, with low complexity) to execute arbitrary code or escalate privileges by exploiting the corrupted memory state. The vulnerability does not require user interaction but does require local access with some privileges. The CVSS v3.1 score is 7.8, reflecting high impact on confidentiality, integrity, and availability. The Linux kernel is widely used across many distributions and devices, making this a significant concern for systems relying on affected kernel versions. The vulnerability has been addressed by reverting and correcting the problematic commits to restore safe queue merge handling in the BFQ scheduler.

For European organizations, this vulnerability poses a significant risk, especially for those running Linux-based servers, workstations, or embedded devices using affected kernel versions. Exploitation could lead to system crashes, denial of service, or privilege escalation, potentially allowing attackers to gain unauthorized access to sensitive data or disrupt critical services. Sectors such as finance, healthcare, telecommunications, and government, which heavily rely on Linux infrastructure, could face operational disruptions and data breaches. Additionally, the vulnerability could be leveraged in targeted attacks against critical infrastructure or industrial control systems that use Linux kernels with the BFQ scheduler enabled. The local attack vector means that attackers would need some form of access to the system, which could be achieved through compromised user accounts or insider threats. The absence of known exploits in the wild reduces immediate risk but does not eliminate the threat, especially as patches are not universally applied.

European organizations should prioritize patching affected Linux kernel versions as soon as updates become available from their distribution vendors. Specifically, ensure that kernel versions include the fix that resets 'last_bfqq_created' on group changes and properly handle BFQ queue merges. System administrators should audit their environments to identify systems running vulnerable kernels and apply vendor-supplied security patches promptly. For environments where immediate patching is not feasible, consider disabling the BFQ I/O scheduler or switching to alternative schedulers like CFQ or deadline, if compatible with workload requirements, to mitigate risk. Implement strict access controls and monitoring to detect and prevent unauthorized local access, as exploitation requires local privileges. Employ kernel integrity monitoring and logging to detect anomalous behavior indicative of exploitation attempts. Regularly update and harden Linux systems following best practices, including minimizing user privileges and employing multi-factor authentication to reduce the likelihood of local compromise.