CVE-2021-47118 is a use-after-free vulnerability in the Linux kernel related to the handling of the `cad_pid` variable, which is a struct pid reference used during system boot and later modifiable via sysctl. The issue arises because during the initialization of `cad_pid` in the kernel_init_freeable() function, a reference count increment (get_pid()) is not performed on the init task's struct pid. Later, when `cad_pid` is changed through proc_do_cad_pid(), the kernel decrements the reference count on the old pid via put_pid(), which leads to freeing the init task's struct pid prematurely. This results in dangling pointers and a use-after-free condition, which can cause kernel memory corruption and potentially lead to system instability or privilege escalation. The vulnerability was discovered through fuzz testing with Syzkaller on Linux kernel version 5.13-rc3 but has existed since the conversion of `cad_pid` to struct pid in kernel version 2.6.19. The root cause is a missing reference count increment on the initial assignment of `cad_pid`. The bug manifests during signal delivery and process exit handling, as indicated by the KASAN (Kernel Address Sanitizer) error logs. The fix involves properly incrementing the reference count on the init task's struct pid when assigning it to `cad_pid`. This vulnerability affects all Linux kernel versions from 2.6.19 up to the patched versions after 5.13-rc3, impacting any system using affected kernels. No known exploits are currently reported in the wild, but the nature of the bug could allow local attackers to cause denial of service or potentially escalate privileges by exploiting kernel memory corruption.

For European organizations, this vulnerability poses a significant risk primarily to servers, embedded devices, and infrastructure running affected Linux kernel versions. Since Linux is widely deployed across European enterprises, cloud providers, telecommunications, and critical infrastructure, exploitation could lead to system crashes, denial of service, or privilege escalation attacks. This could disrupt business operations, impact service availability, and compromise sensitive data. Particularly at risk are organizations running custom or outdated Linux kernels without timely patching, including industrial control systems and IoT devices that may not receive regular updates. The vulnerability's exploitation requires local access or the ability to execute code on the system, which means attackers would need some foothold or insider access. However, once exploited, the attacker could gain elevated privileges or destabilize critical systems. The lack of known exploits suggests limited immediate threat, but the long existence of the bug means it could be targeted in the future. European organizations with stringent security and compliance requirements must prioritize patching to mitigate potential operational and reputational damage.

1. Immediate application of the official Linux kernel patches that fix the reference counting issue with `cad_pid`. Organizations should track vendor advisories and update kernels to versions released after the fix (post 5.13-rc3). 2. For systems where kernel upgrades are not immediately feasible, implement strict access controls to limit local user privileges and prevent untrusted users from modifying sysctl parameters or executing arbitrary code. 3. Employ kernel hardening techniques such as Kernel Address Sanitizer (KASAN) and other memory safety tools during development and testing phases to detect similar issues early. 4. Monitor system logs and kernel messages for anomalies related to process exit and signal handling that could indicate exploitation attempts. 5. For embedded and IoT devices, coordinate with vendors to ensure timely firmware updates or consider network segmentation to isolate vulnerable devices. 6. Conduct regular vulnerability assessments and penetration testing focusing on kernel-level vulnerabilities to identify potential exploitation paths. 7. Educate system administrators about the risks of running outdated kernels and the importance of timely patch management, especially in critical infrastructure environments.