CVE-2023-53136 is a vulnerability identified in the Linux kernel related to the handling of struct pid references within the af_unix socket implementation, specifically in the out-of-band (OOB) message support. The issue arises because the function queue_oob() calls maybe_add_creds(), which can hold a reference to a process identifier (pid) structure. However, the socket buffer (skb) destructor is not properly set either directly or via the unix_scm_to_skb() function. This improper handling means that when the skb is freed using kfree_skb() or consume_skb(), the reference to the pid structure is leaked, causing a memory leak. The leak was detected by syzbot, an automated kernel fuzzer, which reported unreferenced pid objects remaining in memory, indicating that the kernel is not properly releasing resources associated with these pids. The fix implemented fully supports the scm (socket control message) even for OOB messages, ensuring that the pid references are correctly managed and released, preventing the leak. This vulnerability is a resource management flaw rather than a direct code execution or privilege escalation issue. It affects Linux kernel versions identified by the commit hash 314001f0bf927015e459c9d387d62a231fe93af3 and was published on May 2, 2025. There are no known exploits in the wild at this time, and no CVSS score has been assigned yet.

For European organizations, the primary impact of this vulnerability is related to system stability and resource exhaustion. Memory leaks in the kernel can lead to gradual degradation of system performance, increased memory consumption, and potentially system crashes or denial of service (DoS) conditions if the leak is exploited or triggered repeatedly over time. Organizations running Linux-based servers, especially those using af_unix sockets for inter-process communication, could experience reduced reliability or availability of critical services. While this vulnerability does not directly expose confidentiality or integrity risks, the resulting DoS could disrupt business operations, particularly in environments with high uptime requirements such as financial institutions, telecommunications providers, and critical infrastructure operators. Additionally, embedded Linux systems or IoT devices prevalent in industrial and manufacturing sectors across Europe might also be affected, potentially impacting operational technology environments. Since no active exploitation is known, the immediate risk is moderate, but the vulnerability should be addressed promptly to prevent future exploitation or inadvertent system failures.

To mitigate this vulnerability, European organizations should prioritize applying the official Linux kernel patches that address the struct pid leak in the af_unix OOB message handling. Kernel updates should be tested and deployed promptly in production environments. For systems where immediate patching is not feasible, monitoring tools should be employed to track kernel memory usage and detect abnormal increases that could indicate a leak. Administrators should audit and limit the use of af_unix sockets for OOB messages where possible, or implement additional resource limits to prevent excessive pid references from accumulating. Additionally, organizations should ensure robust system logging and alerting to detect early signs of resource exhaustion or kernel instability. Regular kernel fuzz testing and integration of automated testing tools like syzbot in development pipelines can help identify similar issues proactively. Finally, maintaining an up-to-date inventory of Linux kernel versions in use across the organization will facilitate rapid response to such vulnerabilities.