CVE-2021-47355 is a vulnerability identified in the Linux kernel's ATM nicstar driver module. The issue arises from improper handling of timer deletion during the driver's removal process. Specifically, the nicstar_cleanup() function calls del_timer() to delete a timer, but del_timer() does not wait for the timer handler to finish executing. Consequently, the timer handler may still be running after the driver's remove function completes, leading to a use-after-free condition. This means that the timer handler could access memory that has already been freed, potentially causing memory corruption, system instability, or kernel crashes. The vulnerability is addressed by replacing del_timer() with del_timer_sync(), which ensures that the timer handler has fully completed and cannot reschedule itself before the driver removal proceeds. This fix prevents the use-after-free scenario by synchronizing timer deletion with handler completion. The affected versions are specific Linux kernel commits identified by the hash 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, indicating a particular code state prior to the patch. There are no known exploits in the wild at the time of publication, and no CVSS score has been assigned. The vulnerability is technical and low-level, impacting kernel stability and memory safety within the ATM nicstar driver context.

For European organizations, the impact of CVE-2021-47355 depends largely on the deployment of Linux systems running the vulnerable kernel versions with the ATM nicstar driver enabled. While ATM (Asynchronous Transfer Mode) networking is less common today, some legacy or specialized telecommunications and networking equipment may still rely on this driver. Exploitation of this vulnerability could lead to kernel crashes or denial of service, potentially disrupting critical network infrastructure or services. In environments where Linux servers or embedded devices use this driver, attackers with local access or the ability to trigger driver removal could cause system instability or crashes, impacting availability. Although there is no evidence of remote exploitation or privilege escalation, the use-after-free condition could be leveraged in complex attack chains to escalate privileges or execute arbitrary code if combined with other vulnerabilities. European organizations in sectors such as telecommunications, industrial control systems, or research institutions using specialized Linux kernels might be more affected. The overall impact is moderate but should not be underestimated in critical infrastructure contexts.

To mitigate CVE-2021-47355, organizations should: 1) Apply the official Linux kernel patches that replace del_timer() with del_timer_sync() in the nicstar driver to ensure safe timer deletion. 2) Audit and inventory Linux systems to identify those running affected kernel versions and confirm whether the ATM nicstar driver is in use. 3) For systems where the driver is not required, consider disabling or blacklisting the nicstar module to reduce the attack surface. 4) Implement strict access controls to limit who can trigger driver removal or load/unload kernel modules, minimizing the risk of exploitation. 5) Monitor system logs and kernel messages for unusual crashes or instability that could indicate attempts to exploit this vulnerability. 6) Maintain up-to-date kernel versions and subscribe to Linux security advisories to quickly respond to related vulnerabilities. 7) In environments with legacy or specialized networking equipment, coordinate with vendors to ensure firmware and software are patched accordingly.