CVE-2025-21722 is a high-severity vulnerability affecting the Linux kernel's NILFS2 (New Implementation of a Log-structured File System version 2) filesystem implementation. The issue arises from improper handling of buffer head states when the filesystem detects corruption and falls back to read-only mode. Specifically, the vulnerability involves forcibly clearing folio (page cache) states even when buffer heads are still referenced, leading to state inconsistencies and use-after-free conditions. Two main problematic scenarios are described: first, when NILFS2 calls mark_buffer_dirty() on a buffer not in an up-to-date state, triggering kernel warnings and potential instability; second, during the propagation of dirty states in the NILFS2 b-tree structure, where buffers expected to be dirty are incorrectly identified as clean, causing further inconsistencies. These problems stem from callbacks that handle page/folio write requests forcibly clearing buffer states at inappropriate times, particularly when the filesystem switches to read-only mode due to corruption. The patch resolves the issue by adding checks to ensure buffers are not referenced before clearing their states, preventing use-after-free and buffer state corruption. The vulnerability is classified under CWE-416 (Use After Free) and has a CVSS 3.1 score of 7.8, indicating high severity. Exploitation requires local privileges with low complexity and no user interaction, and it impacts confidentiality, integrity, and availability of the system. Although no known exploits are reported in the wild, the vulnerability could lead to system crashes, data corruption, or privilege escalation if exploited.

For European organizations, this vulnerability poses significant risks, especially those relying on Linux servers using the NILFS2 filesystem, which is often employed in environments requiring high data integrity and continuous data logging. Exploitation could lead to system instability, data corruption, or denial of service, impacting critical infrastructure, enterprise servers, and cloud environments. Confidentiality and integrity of data stored on affected filesystems could be compromised, potentially exposing sensitive information or causing loss of critical business data. The requirement for local privileges means that attackers who gain initial access (e.g., via phishing or other means) could escalate their privileges or disrupt services. This is particularly concerning for sectors like finance, healthcare, and government agencies in Europe, where data integrity and availability are paramount. Additionally, the vulnerability could affect embedded Linux devices used in industrial control systems, increasing the risk of operational disruptions.

European organizations should prioritize patching Linux kernels to versions that include the fix for CVE-2025-21722. Since the vulnerability is in the NILFS2 filesystem code, organizations should audit their systems to identify any use of NILFS2 and consider migrating critical data to more widely used and actively maintained filesystems if NILFS2 is not essential. Implement strict access controls to limit local user privileges and reduce the risk of exploitation by unprivileged users. Employ kernel hardening techniques such as SELinux or AppArmor to restrict filesystem operations and monitor kernel logs for warnings related to buffer state inconsistencies. Regularly back up data stored on NILFS2 filesystems to mitigate data loss risks. For environments where patching is delayed, consider isolating affected systems or running them with reduced privileges and monitoring for anomalous behavior. Finally, incorporate this vulnerability into incident response plans to quickly detect and respond to potential exploitation attempts.