CVE-2021-47123 is a vulnerability identified in the Linux kernel's io_uring subsystem, which is a high-performance asynchronous I/O interface introduced to improve I/O efficiency. The vulnerability arises from improper handling of linked timeouts within the io_uring completion path. Specifically, the issue involves a double free condition on a linked timeout object during a race condition in the completion handling function io_link_timeout_fn(). The root cause is the failure to remove a linked timeout from the master request link list before freeing it, which can lead to a use-after-free scenario. This occurs when the first invocation of io_link_timeout_fn() frees the linked timeout in a failure path, but the timeout remains referenced in the master's free list, potentially causing subsequent accesses to freed memory. Such use-after-free vulnerabilities can lead to memory corruption, which attackers might exploit to cause denial of service (system crashes) or potentially execute arbitrary code with kernel privileges. The vulnerability affects specific Linux kernel versions identified by commit hashes, and it was publicly disclosed in March 2024. No known exploits are reported in the wild at this time, and no CVSS score has been assigned yet. The fix involves ensuring that linked timeouts are properly removed from the master request list before being freed, preventing the use-after-free condition.

For European organizations relying on Linux-based systems, especially those using recent kernel versions with io_uring enabled, this vulnerability poses a risk of system instability or compromise. Since io_uring is increasingly adopted in high-performance server environments, cloud infrastructures, and containerized applications, exploitation could lead to denial of service through kernel crashes or privilege escalation attacks if an attacker can trigger the race condition. This is particularly critical for sectors with high availability and security requirements such as finance, telecommunications, government, and critical infrastructure. The lack of known exploits reduces immediate risk, but the potential for exploitation remains significant due to the kernel-level nature of the flaw. Organizations running Linux kernels with io_uring support should consider this vulnerability a serious threat to system integrity and availability.

Organizations should promptly apply the official Linux kernel patches that address CVE-2021-47123 once available from their distribution vendors or kernel maintainers. Until patches are applied, mitigating risk includes disabling io_uring functionality if feasible, especially in environments where it is not required. Monitoring kernel logs for unusual crashes or anomalies related to io_uring can help detect attempted exploitation. Additionally, employing kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR), Control Flow Integrity (CFI), and using security modules like SELinux or AppArmor can reduce exploitation likelihood. For environments using container orchestration or cloud platforms, ensure that underlying host kernels are updated and that container runtimes do not expose io_uring interfaces unnecessarily. Regular vulnerability scanning and maintaining an up-to-date inventory of kernel versions in use will aid in timely remediation.