CVE-2025-21909 is a vulnerability identified in the Linux kernel's wireless networking subsystem, specifically within the nl80211 interface that manages Wi-Fi devices. The issue arises when userspace attempts to set both MONITOR_FLAG_COOK_FRAMES and MONITOR_FLAG_ACTIVE flags simultaneously on the same monitor interface. The MONITOR_FLAG_COOK_FRAMES flag indicates that the monitor interface operates in 'cooked' mode, where the frames are processed and stripped of some headers, while MONITOR_FLAG_ACTIVE indicates an active monitoring state. Setting both flags together causes the kernel to create a sub-interface lacking the IEEE80211_SDATA_IN_DRIVER bit, which is critical for proper driver-level data handling. This inconsistency leads to a kernel warning (WARN_ONCE) during interface deletion, triggered by the check_sdata_in_driver() function. Although the vulnerability does not directly indicate a memory corruption or privilege escalation, the kernel warning suggests a potential stability issue or kernel panic under certain conditions. The flaw was discovered by the Linux Verification Center using Syzkaller, a kernel fuzzing tool, and has been addressed by rejecting the simultaneous setting of MONITOR_FLAG_COOK_FRAMES with other flags. The affected versions are identified by a specific commit hash, indicating the vulnerability is present in certain kernel builds prior to the patch. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, the impact of CVE-2025-21909 primarily concerns system stability and reliability of Linux-based Wi-Fi monitoring interfaces. Organizations relying on Linux servers, network appliances, or embedded systems with wireless monitoring capabilities could experience kernel warnings or crashes if the vulnerability is triggered, potentially leading to denial of service (DoS) conditions. This could disrupt network monitoring, security tools, or wireless infrastructure management. While the vulnerability does not appear to allow privilege escalation or remote code execution, the potential for kernel instability could affect critical infrastructure, especially in sectors like telecommunications, finance, and government where Linux is widely deployed. Additionally, organizations using custom or third-party tools that manipulate monitor interfaces in advanced ways may be more susceptible. The absence of known exploits reduces immediate risk, but the vulnerability's presence in the kernel means that any exploitation could have widespread effects due to Linux's prevalence in European IT environments.

To mitigate CVE-2025-21909, European organizations should promptly update their Linux kernels to versions that include the patch rejecting the simultaneous setting of MONITOR_FLAG_COOK_FRAMES with other flags. Kernel updates should be tested in staging environments to ensure compatibility with existing wireless monitoring tools. Network administrators should audit and review any custom scripts or applications that configure monitor interfaces to avoid setting conflicting flags. Additionally, organizations should implement monitoring for kernel warnings related to nl80211 interfaces to detect attempts to trigger this condition. For critical systems, consider isolating wireless monitoring functions or using dedicated hardware appliances that do not rely on vulnerable kernel versions. Finally, maintain close coordination with Linux distribution vendors and subscribe to security advisories to receive timely updates and patches.