CVE-2025-21781 is a vulnerability identified in the Linux kernel specifically affecting the batman-adv (Better Approach To Mobile Adhoc Networking advanced) module, which is used for mesh networking. The issue arises from improper handling of reference counting and interface association during asynchronous work related to throughput metric updates. The vulnerability manifests as a kernel panic (crash) triggered during the removal of network interfaces, particularly during system reboot sequences. The root cause is that while reference counting prevents premature freeing of certain data structures (batadv_hardif_neigh_node and batadv_hard_iface), there is no guarantee that the hard interface remains associated with the soft interface until the completion of the asynchronous work (batadv_v_elp_throughput_metric_update). This race condition leads to a use-after-free or invalid memory access scenario, causing the kernel to panic. The crash is reproducible and was confirmed by modifying the metric update work to delayed work, which helped identify the timing issue. The fix involves ensuring that the throughput metric update work does not proceed without a valid soft interface association, preventing the kernel panic. This vulnerability affects specific Linux kernel versions identified by the commit hash c833484e5f3872a38fe232c663586069d5ad9645. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, the impact of this vulnerability primarily concerns systems running Linux kernels with the batman-adv module enabled and actively used for mesh networking. Such environments are common in research institutions, telecommunications, and specialized industrial or IoT deployments that rely on mesh networks for resilient connectivity. The kernel panic triggered by this vulnerability can lead to unexpected system crashes and reboots, resulting in denial of service (DoS). This can disrupt critical network infrastructure, cause data loss in volatile memory, and degrade operational continuity. While the vulnerability does not directly lead to privilege escalation or remote code execution, the availability impact can be significant for organizations relying on mesh networks for communication, especially in sectors like utilities, transportation, and emergency services. Additionally, repeated crashes may increase maintenance costs and reduce trust in Linux-based networking solutions. Since the vulnerability is triggered during interface removal or reboot, it may also affect automated update or maintenance procedures, complicating patch deployment and system management.

1. Immediate patching: Apply the official Linux kernel patch that addresses this issue by ensuring the throughput metric update work does not execute without a valid soft interface association. Monitor Linux kernel updates and backport fixes if necessary for long-term support kernels. 2. Disable batman-adv if not required: For systems not utilizing mesh networking, consider disabling or removing the batman-adv module to eliminate exposure. 3. Controlled reboot procedures: Implement controlled and monitored reboot sequences to minimize the risk of triggering the panic during interface removal. 4. System monitoring: Deploy kernel crash monitoring and alerting to detect and respond quickly to any panic events related to this vulnerability. 5. Testing environments: Before deploying patches in production, test them in staging environments that replicate mesh networking configurations to ensure stability. 6. Network segmentation: Isolate mesh network nodes to limit the impact of potential crashes on broader network infrastructure. 7. Vendor coordination: For commercial Linux distributions, coordinate with vendors to obtain timely patches and guidance tailored to specific kernel versions and deployments.