CVE-2021-47396 is a vulnerability identified in the Linux kernel's mac80211-hwsim module, which is responsible for simulating wireless hardware for testing purposes. The issue arises from incorrect handling of high-resolution timers (hrtimers) used to schedule beacon transmissions. Specifically, if the hrtimer fires late—due to factors such as virtual CPU scheduling delays—the current implementation attempts to rearm the timer at the next scheduled deadline, which may already be in the past. This causes the timer to repeatedly try to catch up by firing multiple times in rapid succession until it reaches the current time, potentially leading to prolonged stalls and performance degradation. The root cause is that the timer is not properly aligned to the Target Beacon Transmission Time (TBTT), which is the intended periodic interval for beacon transmissions. The fix involves changing the timer rearming logic to use hrtimer_forward_now(), ensuring that the timer is forwarded to the next valid interval after the current time, thus preventing the accumulation of delayed firings and maintaining proper timing alignment. While this vulnerability primarily affects the simulation environment rather than production wireless hardware, it reveals a flaw in timer management that could cause system stalls or degraded performance in environments using the mac80211-hwsim module for testing or development. There are no known exploits in the wild, and the vulnerability does not appear to directly compromise confidentiality or integrity but can impact system availability through stalls or delays.

For European organizations, the impact of CVE-2021-47396 is generally limited to environments that utilize the mac80211-hwsim module for wireless hardware simulation, such as development labs, testing environments, or research institutions working on wireless technologies. In such contexts, the vulnerability could cause system stalls or degraded performance, delaying testing cycles and potentially impacting development timelines. Although it does not directly threaten production systems or data confidentiality, any disruption in development or testing workflows can indirectly affect operational readiness and product quality. Organizations heavily invested in Linux-based wireless development or simulation, including telecommunications companies, academic research centers, and vendors of wireless equipment, may experience more pronounced effects. However, since this vulnerability does not affect production wireless drivers or hardware directly, the broader enterprise IT infrastructure and critical systems are unlikely to be impacted. The absence of known exploits and the nature of the flaw as a timing misalignment issue further reduce the immediate risk to operational environments.

To mitigate CVE-2021-47396, organizations should ensure that their Linux kernel versions include the patch that corrects the hrtimer handling in the mac80211-hwsim module. Specifically, updating to the latest stable kernel release where this fix is incorporated is essential. Development and testing environments using wireless simulation should verify that their kernel builds are up-to-date and that the mac80211-hwsim module behaves correctly under timer delays. Additionally, monitoring system performance and logs for signs of timer-related stalls or delays can help identify if the issue is present. For environments where updating the kernel promptly is challenging, consider isolating simulation workloads to dedicated systems to minimize impact. Since this vulnerability does not require user interaction or authentication, proactive patch management is the most effective defense. Finally, organizations should maintain good practices around kernel updates and testing to ensure that similar timing-related issues are caught early in development cycles.