CVE-2021-47357 is a use-after-free vulnerability identified in the Linux kernel's ATM (Asynchronous Transfer Mode) network subsystem, specifically within the iphase module's exit routine (ia_module_exit). The vulnerability arises because the module's removal function calls del_timer() to delete a timer, but del_timer() does not wait for the timer's handler to complete execution. Consequently, the timer handler may still be running after the module's resources have been freed, leading to a use-after-free condition. This can cause undefined behavior such as kernel crashes, memory corruption, or potentially arbitrary code execution within kernel space. The fix involves replacing del_timer() with del_timer_sync(), which ensures that the timer handler has fully completed and cannot reschedule itself before the module is removed, thus preventing the use-after-free scenario. This vulnerability affects Linux kernel versions containing the specified commit hash (1da177e4c3f41524e886b7f1b8a0c1fc7321cac2) and likely other versions with similar code. No known exploits are reported in the wild as of the publication date (May 21, 2024).

For European organizations, this vulnerability poses a risk primarily to systems running Linux kernels with the affected ATM iphase module. While ATM technology is less common today, it remains in use in some legacy telecommunications and specialized network environments. Exploitation could lead to kernel crashes causing denial of service or potentially privilege escalation if an attacker can trigger the use-after-free condition. This could impact critical infrastructure, telecom providers, and enterprises relying on legacy network equipment or embedded Linux systems. The vulnerability's exploitation requires local code execution or kernel module manipulation, limiting remote attack vectors but still posing a threat in multi-user or shared environments. The instability or compromise of Linux-based systems could disrupt services, data integrity, and availability, which are critical for compliance with European data protection and operational regulations.

European organizations should audit their Linux systems to identify the presence of the ATM iphase module and verify kernel versions against the affected commit. Systems running affected kernels should be updated promptly with the patched kernel containing the fix that replaces del_timer() with del_timer_sync(). For environments where kernel upgrades are not immediately feasible, consider disabling the ATM iphase module if it is not required to reduce attack surface. Additionally, implement strict access controls to prevent unauthorized loading or unloading of kernel modules and restrict local user privileges to minimize the risk of exploitation. Continuous monitoring for unusual kernel behavior or crashes related to timer handlers can help detect attempts to exploit this vulnerability. Finally, maintain up-to-date backups and incident response plans to mitigate potential impacts from exploitation.